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Flashcards in CPL Met Deck (915):
1

What is carbon dioxide made up of

It is a combined gas made up of components carbon and oxygen, 13 parts of carbon and 32 parts of oxygen by weight

2

What does carbon dioxide do in the atmosphere

Carbon dioxide in the atmosphere absorbs terrestrial (earth) radiation, which in turn has an effect on the temperature of the air surrounding earth. This greenhouse effect, as it is known, is a serious treat to life if co2 amounts are not kept within certain limits

3

What is atmospheric ozone (O3)

Ozone molecules, consisting of three atoms of oxygen, are formed in the upper levels of the stratosphere and the lower levels of the mesosphere. The main importance of this constituent is its ability to absorb ultra violet radiation from the sun

4

How much UV radiation from the sun is absorbed by the ozone layer

60% of the harmful UV radiation from the sun is absorbed by the ozone layer

5

How much nitrogen is in the atmosphere

78.08%

6

How much oxygen is in the atmosphere

20.94%

7

How much argon is in the atmosphere

0.94%

8

How much carbon dioxide is in the atmosphere

0.03%

9

How much neon is in the atmosphere

0.001%

10

How much helium, hydrogen, xenon, ozone etc is in the atmosphere

0.009%

11

What is the troposphere

The lowest layer of the atmosphere is the troposphere which consists of almost all the water vapour and approximately 75% of all the molecular mass of the atmosphere. This is they layer in which weather and turbulence phenomena are most marked

12

What are the heights of the tropopause

The tropopause varies in mean height between some 18km over the equator to just about half of that, 8km, over the poles

13

What are jet streams often associated with

Jet streams (high velocity horizontal wind flows) are often associated with the breaks between the main tropopauses. These jet streams have a profound influence on the development of depressions and fronts

14

What part of the atmosphere is an isothermal layer

The lower part of the stratosphere above the tropopause is a large isothermal layer

15

What is the overriding influence on the height of the tropopause

It can be said that the over riding influence on the height of the tropopause, and thus its temperature, is the temperature lapse rate

16

What's the main reason for differences in height of the tropopause

Lies in the fact that density of air is less over the equator and regathering over the poles. In turn, the main reason for this dissimilarity is the difference in surface temperature due to varying angles of insulation i.e solar input

17

What is the stratosphere

This layer in the atmosphere able the troposphere has a thickness of some 35km. Initially above the tropopause, the layer is isothermal for some 8-10km. Above this height, the temperature slowly increases and towards the top quarter of the stratosphere, temperatures increase rather strongly with height. The main reason for this is the action of the ozone layer, which absorbs large amounts of short-wave ultraviolet solar radiation

18

What is the main reason for the weather in the stratosphere being generally devoid of cloud and turbulence

Since the lower stratosphere is largely isothermal and its top half associated with negative lapse rates, any up draughts caused by low level instability in the troposphere will soon be eliminated in the stratosphere. This is the main resin for the weather in the stratosphere being generally devoid of cloud no turbulence

19

Is the stratosphere over the poles colder or warmer than over the equator

The stratosphere above the poles is less cold than over the equator

20

How high is the mesosphere

The stratopause, at approximately 50km, heralds the beginning of the mesosphere, which is a layer some 30km (about 90,000ft) thick

21

What's the temperature of the mesosphere

The mesosphere is found at the top of the layer and has the lowest temperature of the upper atmosphere, approaching -90°C. The pressure in the mesosphere is generally less that 1hPa and becomes almost unmeasurable towards the mesosphere

22

Where can noctilucnet clouds be found

Noctilucnet clouds can be found near the mesopause, most commonly over polar regions in summer. Their origin is believed to be in meteoric dust particles acting as nuclei for ice particles

23

What is the homosphere

Troposphere, stratosphere and the mesosphere

24

What alters the structure of the gas molecules in the thermosphere

The structure of gas molecules in the thermosphere alerts dramatically under the influence of ultraviolet and solar X-rays which cause ionisation (electrical charging). This process leaves oxygen atoms and nitrogen molecules with a net positive charge (ions). The ionosphere generally lies between 90km and 300km (although thus height may vary considerably)

25

What is the ionosphere have importance to

The ionosphere is of importance to radio wave propagation in that it has the ability to reflect or absorb radio waves depending on their frequency

26

What do temperatures do in the thermosphere

Temperatures within the thermosphere rise rather steadily, especially towards the higher levels, due to very high ultraviolet radiation

27

Near the earth surface the atmosphere exerts a pressure of what

About 10(5) (100,000) pascals

28

What is the standard atmospheric pressure

The standard assumes the temperature to be 0°C (melting ice) while the force of gravity (g) is assumed to be 9.806m/sec2 at a latitude 45 at mean sea level

29

What is the main type of mercury barometer

The Kew type

30

What are the errors in the mercury barometer

The location where the barometer is installed must have its 'g' established and incorporated in correction cards for complete accuracy to be obtained

31

What's the index error on the mercury barometer

Index error is associated with the behaviour of mercury and the manufacture of the instrument

32

What is the advantage of the barograph in the aneroid barometer

An advantage of the aneroid barometer is its application in the barograph. Thus instrument not only only reads the atmospheric pressure at any time but also records it on a graph

33

What formula can be used to calculate the number of feet per hectopascal for any altitude change

Feet per hectopascal = 96T / P
T is absolute temperature, P is the ambient pressure in hPa and 96 is a mathematical factor

34

What is absolute temperature

Expressed in degrees kelvin, avoids negative values in temperature. The relationship between it an Celsius is that the freezing point of pure water under standard conditions, 0°C, is equal to 273 kelvin. Since one degree kelvin equals 1°C, it follows that the boiling point of water under standard condition, 100°C, equals 373 kelvin

35

What is absolute zero

Is zero degrees kelvin; this temperature has never been reached yet although scientists have come close to it. It is said that absolute zeros, molecular movement stops

36

My way of figuring out feet per hectopascal

96 x (273 +/- temp) / hPa

37

How can you obtain total and complete accuracy on weather charts

The reduction to sea level should take into account the height of the station above sea level, the ambient pressure there and the mean temperature within the column down to sea level

38

What are used for accurate measurement of mean column temperature

Most countries use the station temperature as being representative of the column temperature. Although approximate, it is sufficiently accurate and acceptable for stations at elevations below 1,500ft ASL (about 500m). When stations are higher than this ISA values are used

39

Where to highs and lows tend to migrate

Except in global areas where pressure systems stagnate. Highs and lows and their associated extensions, tend to migrate to the east especially so in mid latitudes of both hemispheres

40

What is the semi diurnal variation of pressure

One change, which is caused not by travelling weather systems but by temperature influences, is the semi diurnal variation of pressure. A diurnal cycle means a daily or 24 hour cycle

41

When is the pressure drop at its maximum during the semi diurnal variation of pressure

At approx 1600 hours, the pressure drop is at its maximum and a barograph would record this as the bottom of the semi diurnal pressure curve. Pressures increase beyond this time and peak around 2200 hours. A further decrease then begins cumulating in another bottom of the curve at around 0400 beyond which time a second peak is formed around 1000 hours

42

What is meant by the term pressure gradient

The term pressure gradients which is the change of pressure with horizontal distance measured from high to low

43

What way does pressure gradient always act to the isobars

The pressure gradient always acts at right angles to the isobars

44

What is air said to be in the ISA

Air is dry and of constant composition at all altitudes

45

What is the force of gravity in ISA

Is constant at 9.8m/sec

46

What is the jet standard atmosphere

Is based on the same values as those in the ISA except that the JSA does not include a tropopause and assumes a temperature lapse rate of 2°C per 1,000ft (no decimal point)

47

What is absolute altitude

Sea level pressure correctly set on the sub scale of an altimeter, whether the aircraft is airborne or on the ground, will produce an altimeter reading above sea level. This reading is known as the absolute altitude

48

What is QNH

This is the atmospheric pressure obtained from stations and reduced to mean sea level using ISA values. When QNH is set on the sub scale of an altimeter the instrument will read absolute altitude, more commonly referred to as just altitude

49

What is QFE

This is the ambient atmospheric pressure at a given datum. With the sub scale set to QFE, the altimeter will indicate vertical distance (height) above a datum

50

What is QNE

This is the altimeter sub scale setting of 1013.25 hPa. With this (ISA) values set, the altimeter will read pressure altitude which means the atmospheric pressure, expressed in terms of altitudes which corresponds to that pressure in the ISA atmosphere

51

What is QFF

This is the atmospheric pressure from stations, reduced to mean sea level using existing atmospheric values. QFF is used only for stations of elevation 1,500ft AMSL and lower

52

What is the relationship between QNH and QFF

When the temperature is higher than ISA, QNH is higher than QFF and when the temperature is lower than ISA, QNH is lower than QFF

53

What will happen when the altimeter sub scale is turned down

When the sub scale is changed to a value less than QNH I.e turn the sub scale down, the altimeter will read down - it reads lower than actual height above sea level

54

What will happen when the altimeter sub scale is turned up

When the sub scale is changed to a value greater than QNH, I.e turn the sub scale up, the altimeter will read up - it reads higher than actual height above sea level

55

What are the three main units used for temperature

The three main units of measurement of temperature are Celsius (formerly referred to as centigrade), Fahrenheit and kelvin (also referred to as absolute)

56

What does the mercury thermometer consist of

This consists of a glass tube within which another tube containing mercury. This mercury will expand with warmer and contract with cooler temperatures. A small reservoir for the surplus mercury is at the bottom of the inside tube. The top of the mercury will indicate the temperature being measured

57

When does the mercury thermometer ceases to be totally accurate

This type is most commonly used but it ceases to be totally satisfactory when temperatures are below -36°C, the approximate freezing point of mercury

58

What is the use of the modified mercury thermometer

A modified mercury thermometer is used to measure and retain, for the time being, the maximum temperature. The modification consists of a restriction within the mercury holding glass tube, just above the reservoir. When the highest temperature for the day has occurred and cooling begins, the mercury contracts but the restriction within the glass tube prevents it from returning to its reservoir and the thin column of mercury breaks at the restriction

59

What does the restriction in the mercury thermometer do

The restriction traps the mercury above it and the highest day temperature can be read even hours after its occurrence

60

When is the alcohol thermometer usually used

This type is normally used when low temperatures are involved because alcohol has a low freezing point

61

How does the alcohol thermometer operate

The grass minimum temperature is usually measured with this type of thermometer. Within the thin column of alcohol is an index, which moves down with the retreating alcohol when air temperature drops. The index is designed so that when temperature increases, the expanding alcohol passes it without affecting its position. Thus the minimum temperature for the night can be read at any time after daybreak

62

How does the fluid in metal thermometer work

This type incorporates a two chamber design, one of which contains a special type of fluid. As temperature changes, the fluid expands or contracts, activating a small piston, which controls the indicating needle. This type of thermometer is often used in engines to measure and display operating temperatures

63

How does the thermocouple thermometer work

Metals of different make up contract or expand differently with temperature changes. By joining each end of two dissimilar metals, an EMF (electromotive force) is produced when temperatures changes. This EMF can then be used, provided it is suitably amplified, to activate the indicating needle

64

How does the bi-metallic thermometer work

Two dissimilar metals are joined together but instead of using the EMF when temperatures change, the actual movement between the two metals is used to activate the indicating needle

65

How does the thermistor work

This type of thermometer is normally used in radiosondes (balloon carried meteorological measuring equipment) and modern electronic temperature gauges. Electrical circuit resistance changes as the temperature alters when the radiosonde ascends and the changed radio signals produced are received and analysed by a station receiver. The information can be recorded so that a total picture of temperatures with height can be produced

66

Hoe does a thermograph work

When there is a requirement for a continuous reading of temperatures, a thermograph is used. In essence this is a similar device to the barograph, using a clockwork mechanism to rotate the graph drum. A pen, activated by a thermometer (often a ni-metallic type), traces the temperatures throughout a certain time period

67

How is the measurement of surface temperature done

Surface air temperature is measured at approximately 1.5 meters above the ground so that free flowing air can influence the thermometer reading. The thermometer is packed in a Stevenson screen, a louvered box arrangement, which ensures that sunlight does not fall on the thermometer

68

What is radiation

Radiation is the movement, passage or transmission of heat from one body to another without the use of intermediary matter such as air. Thus radiation can travel through space where there is no air. The transmission of radiation energy is in the form of electromagnetic waves

69

What is solar energy

The solar energy produced by the sun falls within a wavelength range of 0.5u to 4.0u (u is a micrometer, also known as a micron). Because of the very high source temperatures involved, solar energy is shortwave radiation

70

What is solar energy made up of

Ultraviolet approx 9%
Visible light 45%
Infrared 46%

71

What obstructs solar radiation

Ultraviolet radiation is largely absorbed by the ozone contained in the stratosphere while a large part of the visible radiation is absorbed by water vapour. Also, co2 absorbs a very small percentage of incoming solar radiaiotn

72

How much of the solar radiation does the earth receive

Because of the large distance from the sun, earth receives only about 0.0005% of the total solar radiation emitted into space. Of this minuscule amount, some radiation is reflected back into space by cloud tops while particles within the atmosphere prevent some solar radiation reaching the earth surface due to the effect of scattering. Although this scattering process expels some radiation into space again, some if redirected to earth, this is known as sky radiation

73

What is the total solar radiation received by the earth the sum of

The total solar radiation received by the earth is the sum of sky radiation and direct radiation, called global solar radiation

74

What are the three main factors that influence the amount of solar energy received by earth

The distance between sun and earth, the altitude of the sun and the length of day

75

What are the solstices

Due to the tilted spin axis, the altitude where solar input is perpendicular shifts north and south. The vertical sun is at its most northern latitude (23 degrees north) in June and at its most southern latitude (23 degrees south) in December - these dates are known as solstices

76

When is the solar energy strongest

When the angle of sun rays to the earth surface is perpendicular or nearly so, the solar energy pre square meter is more than when the angle to the surface is oblique I.e the radiation is then spread over a wider area

77

What are the equinoxes

Hours of daylight and dark are equal all over the world when the perpendicular sun is exactly over the geographical equator. This occurs twice a year, on or about 22 March and 22 September

78

What are the instruments that measure solar radiation called

Pyranometers

79

How does the pyranometer work

Most modern pyranometers use a sensing element consisting of alternating black and white wedge shaped thin copper sectors. The white sectors have a coating of highly reflective white paints whereas the black sectors have a coating of highly absorbent black paint. When the unit is exposed to solar radiation, the temperature difference created between the white and black sectors is a function of radiation intensity and can be recorded

80

What is terrestrial radiation

Compared to the temperature inherent in the sun, earth temperature is far colder. Due to this, the radiation given off by earth is in the long wave, low frequency band, between 4u and 80u. This radiation is called terrestrial radiation and involves infrared radiation

81

What are strong absorbers of terrestrial radiation

Water vapour, cloud droplets, carbon dioxide and ozone are strong absorbers of infrared radiation, which means that those substances become transmitters of energy themselves and through that, they will air warming of the atmosphere to some extent

82

What is the atmospheric window

Not all infrared radiation is absorbed by the three atmospheric constituents. Between 8.5u and 13.0u, terrestrial radiation readily escapes

83

What is the energy budget

The energy budget which essentially means that the atmospheric temperature, resulting from radiation, has a relatively steady annual global average of around 15°C

84

How is the earths atmosphere warmed

From below

85

What is the hot plate effect from solar and terrestrial radiation

Incoming solar radiation excites earth surface molecules, which as a consequence cause the surface to heat up. Thus a hot plate effect results, which warms the air above the surface

86

What does the release of terrestrial radiation tend to cause

Although terrestrial radiation is a major factor in warming the atmosphere, its release from earth tends to cool the surface

87

How does the warmed earth surface transfers its heat to the atmosphere above it

Through conduction and convection

88

What is conduction

Conduction is the transfer of heat through inter molecular action. The excited (warmed) earth molecules mix with the air molecules that are in touch with the surface and through this process sensible heat is transferred from one to the other. By sensible heat is meant heat that can actually be felt as distinct from latent heat

89

What happens when solar energy has excited and heated the surface

When solar energy has excited and heated the surface and conduction has warmed a thin layer of air above the surface, convection now transfers this heat to layers of air molecules aloft

90

When does the days max temp occur

Around 2pm

91

When does the days minimum temp occur

Just before sunrise

92

What soil types have greater depth of warming

Soil types containing amounts of water such as peat, can have considerably greater depth of warming. However, too much water will have the opposite effect

93

What will any surface that heats quickly and cools just as quickly involve

Any surface that heats quickly and cools just as quickly will involve a widely undulating diurnal variation of temperature curve whereas a surface that either resists or retains insolation will have a mildly or nil fluctuating curve

94

What's the relationship between solar insolation and the oceans

Provided the angle of insolation is large enough, solar radiation is readily accepted by oceans. However, at angles of 15 degrees and less (shallow insolation), reflection of solar radiation back into the atmosphere is 50% and possibly more. At angles of input 60 degrees and more (steep insolation) the reflection is barely 3%

95

What are the temperature extremes of a maritime climate

Maritime climates rarely have extremes of temperature

96

What is specific heat

This means the amount of thermal heat units (expressed in joules) required to raise the temperature of a unit mass by 1°C

97

What are the reasons why sea air temperatures are relatively steady

Specific heat, mixing, evaporation and wind

98

How does mixing of the oceans make temperatures relatively steady

Water is rarely in a static state. There is invariably some wave action and a certain amount of vertical current flow. Any solar energy received is spread through a far deeper layer of water than on land. Even a substantial solar input, which might have had a dramatic effect on land, will have relatively little effect in water. Mixing tends to slow slow down changes in water temperature

99

How does evaporation from oceans keep temperatures relatively steady

Solar input will try to evaporate some water, and this demands latent heat. Therefore, even if the air temperature over the ocean had heated up a little after sunrise it soon would cool again due to this demand for latent heat

100

How does wind over oceans make temperature relatively steady

Over the land the roughness of the terrain interfered with this wind flow and as a result the surface wind will be somewhat lighter than it might have been had the land been totally flat. Thus, over the oceans where frictional effects are less, winds are generally stronger

101

How does cloud effect the diurnal temperature changes

Considering solar radiation first, the amount reflected back into space depends on cloud types cloud cover (in oktas) and cloud thickness. Any cloud (depending on type) will cause less reception of insolation at the surface and, as a result, surface air temperatures will not alter greatly and the graph will tend to straighten out. Add to this the effect of cloud on absorption of outgoing terrestrial radiation, its own emission of radiation and its blanketing effect, and a barely fluctuating graph will result

102

How is albedo measured

Albedo can be measured by an albedometer, which consists of two pyranometers: one facing upwards and the other facing downwards. The former measures global radiation (direct and refracted) while the latter measures reflected solar radiation, their combined readings being a function of albedo

103

What is condensation

Condensation is the return of water vapour into liquid (or solid) water, the main method being cooling of the atmosphere. The most common methods is cooling through ascent when adiabatic processes produce the desired effect

104

When can the condensation process not take place

The condensation process cannot take place unless water vapour can cling around small particles in the atmosphere such as soot, salt particles and so on (dust particles have shown to be rather ineffective). These are collectively called condensation nuclei

105

What are hygroscopic particles

Sea salt is particularly effective in condensation formation since it has a great affinity for water

106

What is super saturation

When air of great purity such as found in the Antarctic, the lack of condensation particles may prevent condensation at a temperature when it should have occurred

107

What is the saturation vapour pressure of moist air

As evaporation takes place, water vapour adds to the partial pressure of air and a stage will be reached where water vapour content cannot increase any further. This is referred to as the saturation vapour pressure of moist air

108

Does evaporation add to the atmospheric pressure

Yes

109

What does the saturation content graph show above 0°C with respect to ice

It shows that over ice surfaces, the saturation value is less than had the surface been water, which means that water vapour condenses more readily over ice than over water. At temperatures approaching 40°C, it is almost impossible to saturate air

110

Does latent heat change the temperature of the substance

Latent heat is the energy required or given off to make the change - it does not affect the temperature of the substance that is changing

111

What happens to latent heat when it is absorbed

Latent heat of evaporation is absorbed or stored by the water vapour do that when the water vapour returns to the liquid state again (condensation) thus stored latent heat is released

112

When does water have the greatest density

Water has greatest density and therefore least volume at a temperature of +4°C. At temperatures either side of that, density decreases and volume increases

113

What is the greatest producer of precipitation

By far the greatest producer of precipitation however, is not the evaporation that may occur from local water sources, but the horizontal movement of moisture in the atmosphere, known as moisture advection. The average annual precipitation on a global scale is approximately 85cm (33 inches) but this amount varies greatly from place to place around the globe

114

What is absolute humidity

The mass or weight of water vapour per unit volume of air. Alternatively, it may be expressed as the humidity mixing ratio may which is a percentage expression of the mass of water vapour in grams per kilogram of dry air

115

What is relative humidity

The ratio of the weight of water vapour in a given volume of air (absolute humidity) to the maximum weight of water vapour the same volume could hold at the same temperature, expressed as a percentage

116

What is the saturation content

The maximum weight of water vapour the air can hold is generally referred to as the saturation content

117

What is the formula for relative humidity

Relative humidity œ absolute humidity / saturation content %

118

What is meant by absolute humidity

By absolute humidity is meant the amount of water vapour in the air, let it simply be called water vapour. By saturation content is meant the maximum amount of water that air can hold as a gas and the temperature was the over riding factor

119

What's the relationship between the three factors in the relative humidity formula

Relative humidity is directly proportional to water vapour but it is inversely proportional to temperature

120

What is dew point defined as

Dew point is defined as the temperature at which a parcel of air saturates under constant pressure - it is a temp!

121

What does the definition of dew point include

Constant pressure. As a general rule it may be assumed that the dew point will decrease by half a degree Celsius per thousand feet of height due to the associated reduction in pressure

122

What's the difference between relative humidity and dew point

Relative humidity is affected by changes in water content and air temperature. Dew point is affected only by changes in water content (ignoring pressure)

123

What are the instruments called Thant measure relative humidity

Hygrometers, commonly used are the wetbulb/dry bulb, hair hydrometer and the lithium chloride element

124

What is the wet bulb/dry bulb hydrometer

This hydrometer should more accurately be referred to as a psychrometer, which consists of two thermometers mounted side by side. The dry bulb is a simple mercury thermometer, which measures the temperature of the ambient air

125

How does the wet bulb/dry bulb show low humidity

The wet bulb depression will be large, I.e the two thermometer readings are far apart which indicate a low relative humidity

126

How does the wet bulb/dry bulb hydrometer show high relative humidity

The wet bulb depression is less, that is, the two thermometer readings are close together, which indicates a high relative humidity

127

How does the lithium chloride element hydrometer operate

This element changes in its electrical resistance due to vapour pressure differences, which enables it to be used for relative humidity readings. Since its construction is very light, this element is often used for radiosondes (upper air) observations

128

What is the density of moist air

Assume a parcel of air is totally dry - the molecular weight of this air will have a values of 28.96. Water vapour has a molecular weight value of 18. If follows therefore that the mass of a molecule of water vapour is less than the mass of a molecule of dry air

129

What is the principle reason for movement of air

The principle reason for movement of air is that variations in temperature produce fluctuations in pressure and the associated pressure gradients cause air to flow (a classic situation where heat energy due to insolation is converted to kinetic energy)

130

Is horizontal or vertical flows stronger

The horizontal flow is by far the stronger of the two, somewhere in the order of a factor of 100 for horizontal wind to one for vertical currents. The exceptions to this rule are violent convective storms such as hurricanes, also known as tropical cyclones or typhoons

131

What are the four forces which have a fundamental influence on the strength and often direction of the wind

Pressure gradient
Coriolis force (also known as geostrophic force)
Centripetal force
Friction forces

132

How is the pressure gradient responsive to the density of air

Density is inversely proportional to pressure gradient. Ignoring other factors therefore, in general, that for a given isobar spacing the wind becomes stronger with height

133

How fast does the earth spin when viewed from space

At the equator, the west to east speed (sideways speed) is approximately 1,000mph. Since at the axis of rotation, the poles, this speed is zero it follows that at latitudes between the equator and the poles, the west to east sideways varies

134

What is the coriolis force formula

Coriolis force = 2 w p v sinLat

135

What is the w in the coriolis formula

Is the angular velocity of earth spin, simply its rpm of 360 degrees per 24 hours

136

What is the p (rho) in the coriolis formula

Air density

137

What is the v in the coriolis force formula

The speed of the airflow, the wind speed

138

What is the sinLat in the coriolis formula

Is the sine of the latitude, which means that at the equator where the latitude is nil, the sine of the latitude must be nil thus coriolis force there must be nil. At the poles, sin90 is 1; coriolis force must be at its maximum

139

What is the interaction between coriolis force and pressure gradient

When a flow begins its escape from a high (to a low pressure system) under the influence of the pressure gradient, as soon as it obtains a velocity at all, coriolis force will act upon it at right angles and force it to its left in the Southern Hemisphere

140

What is the geostrophic wind

When the wind blows along straight isobars

141

What is a gradient wind

When the wind follows curved isobars

142

What are the three fundamental forces affecting the air within pressure systems

Pressure gradient, coriolis force and centripetal (cyclostrophic) forc. These three determine the strength of the wind and the circulation around the systems

143

Is pressure gradient constant

There is a certain amount of leakage from a high towards a low it follows that the value of a given pressure gradient will likely decay after some time. Thus pressure gradient is rarely a constant

144

Are pressure systems stationary for long

Pressure systems are seldom stationary for long, they tend to migrate generally from west to east and at the same time often move north or south

145

When will the wind be geostrophic

When pressure gradient and coriolis force are equal, isobars will be straight and the wind will be geostrophic

146

When will the wind be a gradient wind

When pressure gradient and coriolis force are not equal, one or the other will be dominant, isobars will be curved, the wind will be a gradient wind and the stronger force will provide the centripetal force

147

What is the wind strength fundamentally determined by

The pressure gradient force (PG) I.e the stronger the pressure gradient the stronger the wind

148

What is the coriolis force determined by

The coriolis force is determined by the strength of the wind and by latitude, both directly proportional and when the coriolis force is greater than the PG the gradient wind is anticlockwise around a high and conversely when the coriolis force is smaller that the PG the gradient wind is clockwise around a low (both in the Southern Hemisphere)

149

What happens between isobar spacing and different latitudes

At any given latitude and with identical isobar spacing, the strength of the wind is stronger around a high than around a low and given identical isobar spacing, as a pressure system moves towards lower latitudes, the wind strength increases. The opposite applies when moving to higher latitudes

150

What are cyclostrophic winds

Where strong winds following tightly spaced curved isobars are encountered near the equator, indicating balanced flow. These are tropical cyclones and tornadoes. Such systems require a very large centripetal force, which is produced by a very strong pressure gradient. The hurricane strength winds associated with these systems are referred to as cyclostrophic winds

151

What's the depth of the friction layer

The depth of the layer is generally assumed to be about 2,000ft this value fluctuates depending on a number of factors such as the nature of the surface and it must therefore be remembered that the 2,000ft commonly used is only a global average

152

Land areas and particularly those involving rough surfaces with their deep friction layers will accordingly be associated with a surface wind that

Is weaker than the wind above the friction layer and flows at a relatively large angle across the surface isobars to the centre of the low

153

Areas that have a shallow friction layer such as oceans will therefore be associated with a surface wind that

Is almost as strong as the wind above the friction layer and flows at a fairly small angle across the surface isobars to the centre of the low

154

Which way does the surface wind move after sunset and daybreak

In the Southern Hemisphere, the surface wind backs and increases after daybreak and veers and decreases after sunset

155

What does the degree of diurnal variation depend on

The intensity and extent of the friction layer which, in turn, depends on the type of surface, cloud cover, season, latitude and the characteristic of pressure systems involved (e.g presence of inversions)

156

How is geostrophic wind measured

The instrument used for this purpose is the geostrophic scale - a plastic ruler which, when placed at right angles to isobars, indicates wind strength. The scale uses a reciprocal graduation meaning the smaller the distance between graduations, the stronger the wind speed and vice versa

157

How is surface wind measured

There are a number of instruments collectively called anemometers, which are used to give an accurate measure of the surface wind. In order to minimise ground interference as much as possible, the wind is usually measured 10m above the ground, approximately 30ft

158

How long are winds averaged over in take off and landing reports and synoptic reports

For take off and landing reports, the wind is averaged over two minutes. For synoptic reports such as METARS it is customary to use a mean wind velocity over a period of ten minutes

159

How does the rotating cup anemometer work

The rotation can be used mechanically to activate an indicating needle through a speedometer-type cable system or it may be used to drive a dynamo or similar generator to produce electric current to activate an indicating needle. This instrument only measures wind speed

160

What is one of the disadvantages of using the rotating cup anemometer

It's inaccuracy when wind speeds are low, or high and gusting. The inertia of the cups plus the friction in the driving mechanism, no matter how well constructed, will invariably cause some inaccuracy when the wind is light whereas at high wind speeds, the momentum of the revolving cups will resist rapidly changing speeds

161

What are the diameters of the 25 knot windsock

It has a length of not more than 3.6m and an open end at the mast of not more than 0.9m (many standard windsocks are 2.82m long x 0.66m wide)

162

What is buys ballots law

In the Southern Hemisphere, one stands with ones back to the wind, the area of low pressure is to ones right

163

What happens if you descend through a shear zone

If an aircraft descends through a shear zone below which the wind is lighter, the aircrafts airspeed will reduce, its angle of descent will steepen and its rate of descent will increase. These changes can be brought under control of course but it takes a certain amount of height to do so

164

How can you avoid shear at altitude

When shear is experienced at altitude it is normally possible to vacate the region by climbing a few thousand feet or by turning away from it

165

What's the big problem with wind shear

The big problem with wind shear is that when it presents itself at low heights such as within the airspace surrounding the take off and climb out sector or the approach to land sector, it may affect the aircraft unexpectedly and cause it to descend steeper and at a greater rate towards the ground

166

What are the characteristics of stable air

Stable air will likely have fair to poor visibility, if cloud is present it will be layer type and if precipitation occurs it is likely to be rain or drizzle

167

What are the characteristics of unstable air

Cloud will be in the form of cumulus (heap type) cloud that can tip at a mere few thousand feet or reach higher than 60,000ft in tropical regions. Precipitation will be showers, heavy at times and occasionally accompanied by thunderstorms. Outside cloud, visibility can be very good, indeed brilliant if the air originates from cold polar regions

168

What are the main factors that determine if air is stable or unstable

The temperature lapse rate of the environment air and the adiabatic change in temperature of a vertically moving parcel of air

169

How is the environmental lapse rate measured

The temp lapse rate that applies in a given locality at a given time is known as the environmental lapse rate, which can be measured by sending a thermometer aloft. A balloon carried instrument (radiosonde) can be used for the purpose, or can be carried aboard an aircraft. Either way, as various heights are passed, the thermometer readings are transmitted to a ground station where they are recorded and plotted on a graph

170

What happens when a rising parcel of air enters reduced atmospheric pressure

A rising parcel of air, metering reducing atmospheric pressure and increasing its volume, will cool and thus cooling process will be an adiabatic one

171

What is an adiabatic process

An adiabatic process (of temperature change) is simply one where no energy (heat) is gained from or lost to the surroundings

172

What is an non adiabatic process

One where energy (heat) is lost to or gained from the surroundings

173

What is an isobaric process

One where a change in temperature occurs when the pressure remains constant

174

What is super adiabatic

ELR's in excess of 3°C/1,000ft

175

What is the rising condensation level (RCL)

The level at which condensation takes place and cloud forms when the cause of rising is through mechanical means, such as mountains, is known as the rising condensation level (RCL), also referred to as the lifting condensation level (LCL)

176

How can you calculate the RCL

Difference between the surface temperature and the dew point, divide answer by three = RCL

177

What is the convective condensation level (CCL)

When the ELR is steeper than the DALR, the dry are is unstable. If this rising air cools to its dew point, further ascent will produce condensation and cloud will form. The height where this occurs is referred to as the convective condensation level (CCL)

178

When will convective rising of air not produce cloud

Convective rising of air will not produce cloud if the DALR intersects the ELR before dew point has been reached

179

How do you calculate the minimum surface temperature to produce convective cloud

Plot a vertical line up from the dew point temperature. Where the ELR and this vertical line cross, multiply this by three and add to the dew point value (on a altitude/temp graph)

180

What does high and low dew points create with convective rising

If air is dry (consistent with a low dew point) the convective cloud base is high. If the dew point is very low, such as found in desert areas, convective cloud may not form at all

181

What is convective stability

When a parcel of air is dry on the bottom and moist on the top it begins as unstable and the air is risen up a hill. The dry air will cool at the DALR while the moist air will cool at the SALR. The ELR is then decreased between the layers and becomes more shallow and instability has changed to stability

182

What is convective instability

When there is a parcel of air with moist air on the bottom and dry air above, it begins as stable but as it is risen up a hill, the lower level will cool at the SALR and the higher level at the DALR, the ELR is then increased from the original and the layer which way stable initially has become unstable during the ascent

183

What is diurnal instability

Refers to the behaviour of air as the surface temperature varies from day to night. As the temperature increases after sunrise the ELR steepens and instability may occur. The ELR will go through a phase where conditional stability exists. In the afternoon, when the surface temperature peaks, the instability will be at its maximum. The reverse happens during the night

184

What is latent instability

This involves having a stable surface layer with a high moisture content and air above this layer is unstable. If the lower of air is now physically forced through the unstable layer above by some means, all the air becomes unstable. Thus the stable layer will initially produce a sheet of stratus cloud but as it enters the unstable layer aloft, the stratus turns into cumulus, which under certain conditions can be quite formidable. Thus both layer and cumulus types of cloud will be present in this type of stability

185

How is a sea breeze created

After sunrise, the land heats while water in the sea retains its temperature. Thus the air over the warming land will rise through conduction and convection. As it warms, the air expands and so then pressure falls off less quickly with altitude than it did during the night. As the warm air rises the pressure will be higher over the land than at the same level over the sea. This upper air will start to flow according to the pressure gradient from land to sea. As it does so the pressure falls over the land and rises over the sea due to the transfer of mass. This sets up a pressure gradient from sea to land at the surface and initiates a wind flow, the sea breeze

186

When does a sea breeze usually set in

The sea breeze sets in at around 10am and peaks at around 2 to 4pm

187

What is the usual strength of the sea breeze

A sea breeze measures between 10 and 15kts but this strength can vary somewhat. In temperate regions where the land behind the coast is sandy and prone to heating, the breeze can be quite strong. This is very common in spring when the sea temperature is still relatively cool while the land heats up quite substantially

188

What is the horizontal and vertical extents of the sea breeze

This may vary from place to place, inland travel of the sea air is generally about 25 to 40km. The vertical development in temperate regions is approximately 2,000 to 3,000ft. In lower latitudes, especially on regions where sandy areas behind the coast produce very hot surfaces, the vertical extent can be many thousands of feet high

189

What is the cloud formation in a general sea breeze

The presence of cloud may involve a few oktas of cumulus characterised by some turbulence and reduced visibility from the land surface up to the cloud tops. Above it, conditions can be very smooth and clear. These clouds are often referred to as fair weather cumulus and they are generally confined to land

190

What cloud formation is found from a sea breeze in tropical regions

In tropical regions, the warm sea will evaporate moisture into the air readily, the hot land will produce some very intense updraughts and the stage may be set for quit substantial cloud formations including cumulonimbus with associated thunderstorm activity. Many places that fall in this category experience these cloud developments and thunderstorms at a regular time of the day, normally around 3pm

191

What precipitation is expected from a sea breeze

This depends largely on the degree of heating and the supply of moisture of the sea. In tropical regions it is not uncommon to have shower activity at the peak of sea breeze activity but in most other cases precipitation is not common

192

What temperatures are expected during a sea breeze

Over the warm land, the onset of the sea breeze transports cold sea air inland, which can often produce a fairly sudden reduction in temperature around mid morning. This factor is again influenced by season in that, during the summer months, the temperature of the sea air is not as cold as during springtime and the fall in temperature is then not quite so dramatic

193

What kind of pressure gradient is found from a sea breeze

Where substantial differences exist between sea and land temperatures, the action of the entire sea breeze process is intense and a relatively steep pressure gradient can occur. This sets the stage for a strong sea breeze with the result that the coriolis effect becomes significant. A consequence of this is that the circulation of air over the land becomes cyclonic as a heat low develops. This in turn affects the direction of the sea breeze as it crosses the coast, in some cases almost causing the wind to become nearly parallel with the coast

194

What kind of windshear can develop from a sea breeze

The prevailing wind in a coastal area may occasionally be from land to the sea. Provided the sea breeze is strong enough, it may oppose this prevailing wind and force itself underneath it. At the top of the sea breeze a shear level will be found separating the upper sea breeze from the prevailing wind. This can sometimes be associated with moderate turbulence.
In other cases, the opposition between an offshore prevailing wind and a sea breeze can be experienced right down to ground level a few miles inland so that a convergence type boundary is produced between the winds (straight up and down wind shear)

195

When can turbulence be found from a sea breeze

Sand dunes, large tree plantations, hills and cliffs on the sea breeze will often produce moderate turbulence. It will be quite turbulent from the beach and inland up to about 2,000ft to 3,000ft whereas from the beach and out to sea, conditions are totally smooth from sea level and up

196

What is the pseudo sea breeze

When conditions are calm and cloud or fog covers a given area of reasonably flat countryside, it is possible for a wind similar to the sea breeze to start blowing when the ground not covered by cloud is warmed by insolation. The wind travels from the cloudy region towards the sunny area and some of the cloud or fog can actually move with the airflow. Thus a sudden drop in temperature and unexpected overcast can take place at an unusual time of the day if this has developed towards the early afternoon

197

What is the land breeze

When insolation becomes insufficient to replenish the departing terrestrial radiation, generally towards evening, there is a net loss of heat energy. Although this loss will affect the land, especially sand soils often found in coastal regions, the effect on the sea is minimal. Therefore, as the land continues to cool it will cause the air above it to become denser so that subsidence and increasing surface pressures develop

198

Which has the higher pressure in the land breeze

Pressure is less at sea level and greater at the surface over the land. A closed circuit is formed with colder land air replacing warmer, and rising sea air

199

How strong are leaned breezes usually

Generally the breeze is only some 3-4kts but local topography such as sloping land towards the sea can increase this value

200

When is a land breeze most common

Autumn is the most likely time of the year for land breezes when the sea has still retained much of its summer warmth but the occasional inland frost starts to occur over the land

201

What are some factors with the land breeze

Factors such as cloud, turbulence, precipitation and so on do not readily occur with the land breeze but in tropical regions the rising moist air can often produce substantial cloud development at the end of the night so that around dawn cumulonimbus cloud can form accompanied by thunderstorm activity

202

What are katabatic winds

Descending air in the hills and mountains. The air close to the valley floor will become cold and therefore denser, which encourages it to flow down the valley. The air touching the walls will cool through conduction and again, as the air becomes denser it starts a down slope travel, which is assisted by gravity.

203

Why are clear winter nights most likely to form katabatic winds

Radiation cooling is enhanced whenever terrestrial radiation is freely allowed to escape. Thus clear winter nights almost certainly produce this type of wind in mountainous terrain

204

What is the air said to be doing in a katabatic wind

At any given spot on a valley wall, the air can be said to be diverging when it flows away from the location, which means that the airs pressure at a common horizontal level is greater over the centre of the valley that nearer the slopes where diverging has decreased the pressure slightly

205

What is the normal speed of the katabatic wind

The speed of the katabatic wind is normally light and non turbulent but stronger winds can develop when valley walls are relatively steep. Another factor in katabatic wind strength is the nature of slope cladding e.g snow covered slopes or bare rock walls, which can encourage substantial speeds. It can be concluded therefore that the katabatic wind is generally of light strength and smooth but exceptions do occur

206

How is valley fog formed from katabatic winds

Since most valleys have rivers or streams, it is to be expected that the air contained within the valley confines can be quite moist. Thus if descending katabatic wind brings additional cloud air it may well assist in the production of valley fog

207

What is the anabatic wind

After sunrise, valley walls and especially the high level east facing rocks are warmed by solar radiation. Through conduction, the air touching the walls will warm and convection will cause it to rise. Replacement air is drawn up from the valley floor and also from higher levels away from the walls. This means that at a given horizontal level, the airs pressure becomes less over the centre of the valley than nearer the walls. The overall result is a general rising of air

208

What is the strength of the anabatic wind

The anabatic wind is almost always rather light due mainly to the influence of gravity

209

What is a gust

Gusts are momentary increases in wind speed. The term gustiness includes both gusts and lulls. Gusts are caused by turbulence, which may be small scale, as the result of trees and buildings, or on a much larger scale due to rough or mountainous terrain

210

What are squalls

Squalls are rapid increases in wind speed, which last for some minutes, then die away again. They may be produced by the passage of individual cumulus clouds or thunderstorms or by the passage of frontal systems

211

When might a fohn wind be experienced

A fohn wind may be experienced when a mountain range forms a barrier to wind flow; the orientation of the mountains determines the direction from which the fohn wind blows

212

How does the fohn wind work

At the approaching air arrives at the mountain range, it will be forced upward and initially cool at the DALR while from the RCL (rising condensation level) onwards the SALR takes over. Cloud of various types will form depending on stability and precipitation is almost inevitable. As well, water droplets will adhere to rock faces, trees and similar objects so that the overall result is a decrease in the water content of the rising air

213

What is the dry fohn

Although condensation on the windward side of the mountain range is very commonly associated with the fohn wind, it is not an essential requirement. Some fohn winds develop without precipitation on the windward side when high level air is drawn down to the lee side, warming at the DALR all the way down.

214

What is visibility like on the lee of the mountain in a fohn wind

A strong fohn wind can transport dust and even rock particles of substantial size many miles to the lee of the mountain range and thus is particularly likely vis gorges associated with major river valleys. The resulting reduced visibility on the lee side can be quite dramatic and from a technical point of view, the presence of dust can have a serious effect on engines and aircraft components that rely on external air intakes

215

What are the effects of small scale interference to wind flow such as shelter belts or buildings

A pressurised area forms on the windward side of the obstruction and can extend horizontally up to five times the height of the obstruction. As the airflow bends over the obstruction, a Venturi effect will accelerate the flow above and slightly to the lee of the obstruction. This effect is often greatest up to three times the height of the obstruction. The flow can be disturbed downhill as far as 50 times the height of the obstruction. Substantial turbulence can take place in the lee horizontally up to 20 times the height of the obstruction

216

What does a fohn wind need

A substantial mountain range, a wind blowing more or less at right angles to the mountain range and a high moisture content of the approaching air

217

What is large scale interference to wind flow also known as

Mountain waves, standing waves or lee waves

218

What might mountain waves cause

May cause severe turbulence under certain conditions

219

What do mountain waves need to form

A mountain range of substantial dimension, a wind more or less at right angles to the range, the low level wind must be at least 15kts and increase in strength with height and a generally unstable atmosphere at low levels with a stable layer at altitude, normally slightly above mountain crests. The air above this layer should be less stable or slightly unstable

220

What happens to mountain waves if the mountain range is too small

If a mountain range is too small, the wind will most likely channel around the edges rather than flow over the tops and this will be enhanced if the air in question is very stable. A similar result is likely if the wind direction onto the range is rather oblique. It is generally accepted that the wind must be at right angles to the mountains or within 30 degree of this

221

What limit does the wind direction need to stay in during a gain in height of mountain waves

The increasing wind with height must retain a fairly constant direction, generally within 20 to 25 degrees throughout a deep layer

222

After crossing mountain tops with mountain waves, what should be expected

After crossing mountain tops, the air flows in an oscillating pattern which can have a wavelength of some 3 to 25km or more and an amplitude of some thousands of feet. Vertical currents can involve speeds of around 1,000 to 1,500ft/min. In extreme cases, vertical speeds of 3,500ft/min have been recorded. The oscillation axis may lean into the wind so that crests of waves at high altitude may be directly above troughs at low altitudes

223

What does the wavelength of mountain waves mean

The horizontal distance between two identical parts of successive waves such as the distance between each crest or between each trough

224

What does mountain wavelength depend on

Mountain wave length depends largely on the speed of the wind in that, the stronger the wind the larger the wave length and vice versa

225

Which side of the mountain has an effect on the wavelength of mountain waves

It has been shown that the upwind shape or height of mountains has relatively little effect on wavelength but the lee slope has considerable influence

226

What is the amplitude of mountain waves mean

The vertical distance between the crest and the trough of a wave. Amplitude is strongly governed by the shape and size of mountains in that the higher the mountains the greater the amplitude potential and vice versa. The largest amplitudes are gained when the width of ridges is equal to the wavelength.

227

When will the amplitude of mountain waves be greatly increased

When a series of ridges obstruct the flow of air, amplitude will be greatly increased if the distance between ridges equals the wavelength. When these distances are greater or smaller, the descending wave flow will meet rising ground with the result that successive waves are likely to be dampened

228

When is turbulence present from mountain waves

Generally, the wave flow is smooth except when windshear or rotor zones exist. Thus in the absence of cloud which may give visible evidence of mountain waves, the indication of an altimeter or VSI in an aircraft will provide the only evidence that wave action prevails; turbulence can not always be expected to be a sure sign

229

What is clear air turbulence found in mountain waves

When a mountain wave, extending to great heights, is associated with a jet stream, windshear can be strong. I'm those circumstances, turbulence within the shear zone can be moderate to severe

230

Where are rotor zones usually found

These can often be found near the lee of mountains at more or less mountain top height when the amplitude of waves is large. The waves steep down slope can cause a strong tumbling motion directly beneath the crest of the wave and the resulting instability and turbulence can be severe. When cloud is present, it will show as ragged and rapidly rotating streaks with the tumbling axis parallel to the mountain range. In the absence of cloud, there is nothing to indicate the presence of rotor action

231

How can you cross mountains when rotor zones are present

It is vital that more than adequate height is in hand above the mountains prior to crossing the tops, especially when flying into the wind. In the latter case, the heading across the ridges should be at least 90 degrees so that if an unexpected downdraught is experienced, a relatively small turn allows the pilot to vacate the zone. If sufficient height cannot be achieved and if the power available is rather limited, then an alternative route around the mountains would be advisable

232

What cloud is likely to form from mountain waves

The rising action on the leading edge of a wave causes adiabatic cooling of air and, through that, formation of cloud provided sufficient moisture is present. On the down wind side of the wave, the descending action with its associated adiabatic warming will evaporate the cloud. Thus a very smooth shaped cloud is formed known as lenticulars

233

When do lenticular clouds not drift down wind

Lenticular clouds do not drift down wind when the pattern of the oscillations is constant in both frequency and amplitude

234

How are mountain waves dissipated

A substantial change in wind direction, a change in the characteristics of the oncoming flow in terms of different stability, or a lessening of the wind speed. These changes are very often provided by the arrival of major weather systems such as fronts or depressions

235

When is rotor streaming likely to develop

When a strong wind blows onto a mountain range and the speed of this wind decreases in strength from crest level upward, it is likely that rotor streaming will develop

236

What is rotor streaming associate with

It's often hidden within deep cloud and precipitation

237

How is rotor streaming formed

The layer of air above the mountain tops, moving at considerably slower speed, acts like a huge damper. Thus enhances a Venturi effect between the damper and the mountain range so that the speed of the low level wind is very high as it emerges on the lee side. A suction effect then develops in the shadow of the range below ridge height and the wind curls down in similar fashion as rotors

238

Where is turbulence least in rotor streaming

Whereas turbulence will be severe to extreme in the lee below ridge height winder these conditions, flight above ridge height (in the weaker wind zone) is likely to be less disturbed

239

What causes a radiation inversion

After sunset, terrestrial radiation over the land continues to escape while there is no replenishment of energy from solar radiation. Thus a net loss of heat takes place so that the surface cools which has the effect of cooling a layer of air adjacent to the ground through conduction

240

What size radiation inversion will form in calm conditions

The layer of air affected is quite thin because the effect of cooling is not carried aloft greatly since air is a poor conductor of heat

241

What size radiation inversion will form with a light wind

If a light wind blows, the resultant mixing will cause the cooling process to be felt up to a greater height so that a layer of air is formed (immediately above the surface) where the temperature is colder than the air above

242

What causes a stronger radiation inversion

The drier the air the more intense the surface cooling, the greater the likelihood of a strong radiation inversion developing

243

What are the positive and negative effects from wind on radiation inversions

If the wind is light, in the region of 2-8kts, mixing will enhance the formation of an inversion but if the wind is stronger than that, the effect is to even out the temperature distribution throughout the lower layers and a radiation inversion is then invariably present

244

What will happen if the reduction in air temp during a radiation inversion cools the air to its dew point

The reduction in air temp within the layer governed by the inversion may well cool air to its dew point. Should this happen when conditions are calm, so that only a thin surface layer cools, dew point will form but if satisfactory mixing has developed low cloud of fog may form within the deeper inversion layer

245

What will happen during a radiation inversion if surface temps fall below 0°C

Should the surface temperature fall below 0°C when condensation has taken palace, frost will form. This will result in very cold surface air conditions especially in inland and sheltered areas during cloudless winter nights

246

Does the radiation inversion increase or decrease wind speed

It should be readily understood that when a radiation inversion is present, the increased density of the air will reduce the wind speed to a greater extent than it otherwise might have done

247

What will happen when climbing out of a radiation inversion

An aircraft climbing out into the air above the inversion may experience a sudden increase in wind speed and possibly also a change in wind direction. This involves vertical windshear found between layers of air having different wind velocity values; this shear can often be accompanied by some unexpected turbulence

248

What happens when descending into a radiation inversion

If an aircraft descends into the inversion layer it will experience a sudden decrease in wind speed, the effect of which will be a steepening of the descent angle and an increase in the rate of descent

249

What happens to visibility from a radiation inversion

Visibility will deteriorate rather abruptly and this can involve distortion or discolouration of lights, looked at in the near horizontal plane

250

What are the other causes of radiation inversions

Although radiation inversions are usually associated with surface conditions, these are by no means the only causes. In certain circumstances it is possible that the top of a layer of cloud during the night may cause sufficient radiation to produce a radiation inversion there

251

Can a radiation inversion form over water

Since water does not warm or cool quickly enough, it is not possible for a radiation inversion to form over the sea or over a lake. However, very shallow marshlands or mudflats at low tide can cool sufficiently quickly for an inversion to form and considering the high moisture content in the environment, fog may readily develop

252

Why does a turbulence inversion often develop

This inversion often develops as the consequence of a surface radiation inversion

253

What are the two main requirements for the formation of a turbulence inversion

The presence of a shallow ELR involving the layers of air near the surface and the presence of a wind that is not too light and not too strong

254

What will cause the turbulence for the turbulence inversion

When surface heating begins after sunrise there is an increase in mixing due to thermal currents and increased wind strength. This will cause turbulence to set in which will move parcels of air up and down in random fashion

255

What is the effect of wind on a turbulence inversion

The effect of wind is that if its strength is stronger than some 8-10kts, the mixing within the layer becomes too severe and the effect on the general ELR will be an overall steepening without the development of a turbulence inversion

256

What clouds can form in the turbulence inversion

If the moisture content of the surface air is sufficiently high, it is likely that the rising parcels will cool below their dew point and condensation will take place. On the other hand, the descending parcels will likely evaporate their water content. Thus a layer of cloud is formed towards the top part of the maxing layer with the inversion providing the lid. Initially this cloud will be of the layer type variety (stratus), having reasonably level base and top. As mixing increases and the ELR within the affected layer steepens further, instability develops, the layer type cloud will change into the cumulus variety, a layer of stratocumulus forms. Should the average ELR above the mixing layer be greater than the SALR, the onset of instability below the inversion will be carried on above it and larger developments of cumulus appear

257

When is the most common time of day for a turbulence inversion to take place

The most common time of day when the development of a turbulence inversion takes place is after sunrise. As solar radiation becomes more effective, the ELR below the inversion will steepen. The difference between the surface temperature and dew point will widen and thus has the effect of raising the cloud base (CCL)

258

What happens after the cloud begins to rise in th turbulence inversion

The inevitable gaps in cloud will appear, local hot spots on the surface will provide extra warming and the entire process now in place will cause further raising of the cloud base and fracturing of the stratocumulus. Provided no moisture content is carrie into the area (such as moisture transported inland when a sea breeze develops) the cloud will finally dissipate around mid morning

259

What happens if a sea breeze does appear in a turbulence inversion

If a sea breeze should appear the development of cloud will be enhanced and the original stratocumulus may then turn into fair weather cumulus

260

What are the process for both the radiation and turbulence inversions

When a stable air mass within an anticyclone overlies an area of flat lands especially during autumn and winter. The shallow ELR , the light wind and clear skies often associated with this type of air encourages the escape of terrestrial radiation during the night and through that, invariably produce a strong surface inversion

261

Can turbulence inversions form over water

Is is possible to see turbulence inversions over oceans

262

What happens to the air during a subsidence inversion

Considering that air at lower levels has a higher pressure and greater density than aloft when air subsides from great heights, the rate of subsidence decreases as it enters the lower, more dense, altitudes. In addition, the more dense air near the surface is not as readily moved outwards through divergence as the high level thinner air moves inwards through convergence aloft

263

What form a subsidence inversion

When air subsides, it is warmed adiabatically and since the higher air can descend though a greater depth and at a faster rate, due to its lower density, its degree of warming is greater than the lower air, which can only descend a smaller depth. Thus at some altitude, the subsiding air is warmer than the air below it and an inversion must therefore have formed

264

What is anticyclonic gloom associated with subsidence inversions

In some situations,a layer of stratiform cloud can spread out over many square miles just below the inversion layer. This cloud is referred to as anticyclonic gloom. Its thickness is not extensive but its horizontal spread can almost cover the entire anticyclone

265

How does the characteristics of the friction layer have a great influence on subsidence inversions

Generally, the friction layer is shallow and consistent over ocean areas and therefore the rate at which subsiding air can escape outwards at the surface is restricted. The opposite happens over mountainous terrain. Thus the type of surface, oceanic or mountainous, can influence the formation or elimination of a subsidence inversion

266

What areas encourage the development of a subsidence inversion

In general, oceanic areas, where the friction layer is shallow but relatively dense, encourages the development of subsidence inversions but as the air mass in which the inversion has formed, moves over adjoining land, the inversion tends to break up due to a number of effects over the land such as greater diurnal change in temperature and local wind flows, often orographically affected

267

What causes a frontal inversion

Since warm air wishes to rise and cold air wishes to descend it follows that when cold air moves towards warm air, the warm air will rise above the cold air. Similarly, if warm air travels towards cold air, the warm air again will over ride the cold air. Thus a warm sector will invariably exist above a cold sector whenever a front between air streams has formed. This might mean that during a vertical ascent from the surface into an over lying front, there must come a height where the cold sector is left and the warm sector entered

268

What front is a frontal inversion more common in

The warm front, due to its characteristics, is more likely to be associated with a frontal inversion

269

What is an isothermal layer

When temperature remains constant with height

270

What are the effects of isothermal layers

The effects and consequences of isothermal layers are often the same as inversion i.e les cloudy, less moist and better visibility above than below the layer. As well, flight conditions are invariably smooth above the layer

271

How can cloud be defined

Cloud can be defined as suspended water in liquid droplet or solid ice crystal form. Thus whenever cloud is present it must mean that at some stage, water vapour has condensed but it must be remembered that not all the water vapour has changed its state into liquid or solid form

272

What does super saturation require to condense

What're vapour will not condense at the saturation temperature but requires a higher relative humidity than 100% and a lower temperature than its calculated dew point

273

Does sea air or land air have more hygroscopic particles

In spite of the presence of salt in sea air, land has a far greater concentration of particles than sea air and in some cases it is shown that hygroscopic particles can be six times more numerous in land air

274

How can relatively humidity be increased

Either increasing the water vapour content or lowering the airs temperature, relative humidity can be increased

275

What is the most common way RH is raised

Through cooling of air

276

What type of cloud is formed in stable air

Layer type cloud

277

What type of cloud if formed in unstable air

Cumulus or heaped type cloud

278

What are the main factors that determine the degree of evaporation and height loss involved in water droplets

The RH of the air beneath the cloud, the state of the falling water and the size of the drop

279

What does the internationally adopted system consider cloud under

Shape and structure, vertical extent and altitude

280

High clouds in tropical latitudes

6km+
20,000ft+

281

Middle cloud in tropical latitudes

2-6km
7-20,000ft

282

Low cloud in tropical latitudes

Below 2km
Below 7,000ft

283

High clouds in mid latitudes

5km+
17,000ft+

284

Middle cloud in mid latitudes

2-5km
7-17,000ft

285

Low cloud in mid latitudes

Below 2km
Below 7,000ft

286

High cloud in polar latitudes

3km+
12,000ft

287

Middle cloud in polar latitudes

2-3km
7-17,000ft

288

Low cloud in polar latitudes

Below 2km
Below 7,000ft

289

What does high cloud consist of

High cloud consists of cirrostratus, cirrocumulus and cirrus. High cloud of the layer type is called cirrostratus while cloud of the heap type is called cirrocumulus

290

What might altostratus be an indication of

Like cirrostratus, altostratus may be an indication of approaching deteriorating weather

291

What sort of icing can be expected in altostratus

The thickness and altitude of altostratus means that icing can be a severe problem. If the freezing level lies within the layer of altostratus, the first 4,000-6,000ft above the freezing level will most likely produce glaze ice, which can accumulate rapidly. Above this height, rime ice at a considerable accretion rate can be expected. Any precipitation to sea level is generally light. In mountainous terrain however precipitation may be moderate to heavy (depending on the freezing level) may involve rain, snow or sleet

292

What icing is present in altocumulus

In global terms icing is usually considered to be light in altocumulus. Precipitation of any significance is not common

293

What degree of turbulence can be found in stratocumulus

Stratocumulus may cover large parts of the sky when resulting from early morning mixing in the friction layer. Turbulence can, on occasions, be moderate especially if a turbulence inversion is involved. Otherwise, turbulence will be light

294

What icing should be expected from stratocumulus

Except in latitudes where the freezing level is at or near ground level, icing is not considered a problem. However, if icing does occur, it can be light to moderate rime ice. Precipitation may be light showers or drizzle patches but will rarely be moderate to heavy

295

What usually causes stratus

This cloud is typical of early morning mixing of air when a low level inversion has formed. Therefore, stratus can sometimes be seen as a turbulence cloud and it often is the forerunner to stratocumulus

296

What icing is found from stratus

In situations where the freezing level is at ground level, very light rime ice may be experienced. Precipitation may involve light drizzle, which accentuates the reduced viability that is sometimes associated with stratus

297

What is cumulus cloud often caused by

Surface heating and/or orographic ascent

298

What precipitation can be found in cumulus

If development is not very extensive, precipitation is most unusual but if development is substantial, a brief period of showers may take place when the clouds reach their maximum extent. This downpour has the effect of cooling the surface, which was the trigger that started the cloud forming in the first place and as a result, the clouds tend to diminish shortly after the shower activity

299

Is turbulence found in cumulus cloud

Turbulence is likely not only within this cloud but also outside of it because the surface heating associated with the phenomenon will produce an unstable lower atmosphere which encourages bumpy and turbulent flying conditions

300

What is the most common type of ice found in nimbostratus

Rime ice but within a shallow height band immediately above the freezing level it is possible to experience clear ice. In either case, the rate of ice accretion can be substantial due to the high concentration of water

301

What does nimbus mean

Large water content

302

What are the artificial clouds

Pyrocumulus and condensation trails (referred to as contrails), both formed as th result of the combustion process

303

When will pyrocumulus form

When large scale bush fire occurs and thermal heating from the fire is sufficient to carry the rising air up high enough for a cloud base to form, pyrocumulus may form. The process is aided by the injection of vast numbers of condensation nuclei and water. When the ELR becomes steep enough, the stage is set for large scale rising of saturated air

304

What cloud does pyrocumulus form

When sufficient moisture is available, cumulus will develop as the result of the heating source

305

When will contrails develop

The combustion process in jet engines produces a large quality of water, which are ejected via the exhaust into the atmosphere. In spite of the very cold ambient temperatures at altitudes where contrails normally form, the high exhaust temperature ensures that the water enters the atmosphere in liquid state but it soon becomes supercooled as the influence of the exhaust heat diminishes. When the supercooled water is mixed with the surrounding air, a contrail will only develop if glaciation takes place - supercooled water turning into ice crystals

306

What process must occur for contrails to persist

Only if glaciation has occurred will the contrail persist because the change from ice to water vapour, sublimation (which causes the contrail to disappear), is a very slow process

307

What is meant by fracto when cloud amounts are referred to

Sometimes the prefix fracto is used. This term is usually restricted to ragged or broken low level patches of cloud of either cumulus or stratus variety

308

How is the reporting of cloud usually done

The lowest layer of cloud is reported regardless of its coverage, i.e even 1 oktas would be reported. The next higher layer will be included in a report if its coverage is 3 oktas or more. The highest layer will be reported if its cover is 5 oktas or more. In all circumstances, the presence of CB cloud, regardless of the amount, will be reported

309

How is the assessment of cloud base done

At secondary aerodromes the cloud base is determined by a ceilometer, which forms part of an automatic weather station (AWS). During hours of darkness, when cloud cannot usually be seen or when accurate cloud base readings are required, it is possible to assess the cloud base with a searchlight or ceilometer

310

How does the cloud searchlight measure cloud base

A searchlight shining vertically onto the base of the cloud. An alidade is used to measure the angle at which the spot of light appears and the height of the base can be calculated using tables

311

How does the ceilometer measure cloud base

A photoelectric cell, facing up, is positioned a certain distance from the base of a near vertically scanning searchlight. Assuming a cloud exists, the light from the searchlight will scatter to some extent from the cloud and some of this scattering will be received by the vertically sensing photoelectric cell. Since its position from the searchlight is known, a triangular calculation can be made instantaneously

312

When can the ceilometer be used

The system can be used during the day or night and it can also be used to provide a ceilogram record when the information is automatically plotted on a graph

313

What is the most modern ceilometer

The vaisala lasted ceilometer, which is installed at most airports in NZ. The equipment continuously measures cloud base by transmitting and receiving the echo from a reflected laser diode. The ceilometer analyses the echo reflected back from the cloud. The system can detect multiple cloud layers as well as the occurrence of rain, snowfall and fog. The measurement range is from 0 to 12,000ft and displays readings at 50ft intervals

314

What are the advantages of the ceilometer

The advantages of an instrumental reading of cloud height is that it removes the uncertainty inherent in visual estimations. The height measured will be very accurate

315

What is the disadvantage of the ceilometer

It doesn't scan the whole sky. The best observations will be those that combine visual and instrumental sources of information

316

What does this extent and type of cloud depend on due to orographic rising

The extent and type of cloud due to orographic rising depends on the RH of the air and its stability

317

What's happens if air is sufficiently moist during orographic rising

If air is sufficiently moist, the lifting condensation level will be below the mountain tops and the cloud will be cumulus if the ELR is greater than 1.5°C/1,000ft or layer type if the ELR is less than that

318

When can cloud develop from orographic rising if the air is dry

If cloud is to develop, the ELR must be steeper than 3°C/1,000ft because dry air will only continue to rise above mountain tops if it is unstable dry air (absolute instability)

319

What does the term orographic cloud refer to

The term orographic cloud is not restricted to cloud in the immediate vicinity of mountains-lenticular clouds in standing waves of the lee of a mountain range can also be considered orographic

320

What is mechanical rising

It involves an uplift of air due to interference from small features such as buildings, tree lines, small hills etc. unless air is very unstable the type of cloud associated with this kind of rising is generally limited to low level development of stratus or stratocumulus

321

When does convective rising result

This type of rising results when the ELR exceeds the DALR value, i.e when warm parcels of dry air expand and rise. There will be a requirement for a minimum surface temperature if convective cloud is to appear and this can be calculated using suitable graphs

322

What happens during convective rising if no moisture is carried along

If no moisture is carried along and the dew point is not altered, the height of the CCL will rise as the surface temperature increases and vice versa

323

What cloud will form from convective rising

Cloud forming as a result of thermal rising will be cumulus cloud its height depending on the amount of moisture available and the instability of the air

324

What are the most common systems in which widespread ascent is involved

Depressions and their troughs are of course the most common system in which widespread scent is involved but it must be remembered that these are not only ones that can produce widespread vertical motion. For instance, the inter-tropical convergence zone (ITCZ), when the trade winds of each hemisphere meet, can be seen as a huge system of widespread ascending air

325

What does large scale ascent involve

Large scale ascent invariably involves slow rates of vertical motion. As a comparison, the ascent rate is most often only half the rate of that experienced in cumulus build ups

326

What is the initial cause of widespread ascent

High level divergent when air near the top of the tropopause has a net outflow. This causes the pressure near the surface to decrease and a depression to form. In well developed depressions the growth of cloud through condensation will cause a substantial release of latent heat and stability will be adversely affected as a result. Thus, while the air may have been stable prior to and shortly after widespread ascent started, the warmth released in cloud will soon encourage sustained rising-a characteristic of unstable air

327

What is the warm front

When warm air overrides cold air it creates a shallow slope. Cumulus cloud can be associated with warm fronts but the predominant cloud type is stratiform including nimbostratus. It can last for many hours, even days. They travel much slower than the cold fronts. Precipitation can be persistent and heavy; when the freezing level is low enough, snow to low levels can be encountered

328

What is the cold front

Slopes fairly steeply. If the rising air is conditionally unstable, as is often the case, cumulus cloud of varying intensity will develop. The cloud base can be very low and in some cases can be accompanied by a squall line, which is then responsible for the sudden increase in wind strength as the front approaches. Cumulonimbus cloud, thunderstorms and hail showers are frequently encountered under those conditions

329

How big is the width of the cloud band of a cold front

Due to the generally steep slope of the frontal interface, the width of the cloud band is not very great, typically in the order of 100 to 150km. Thus if the speed at which the front travels is its usual 25-30kts, the weather associated with the cold front passing a certain locality will only affect it for some two to three hours

330

What is the most common cause for the formation of clouds

Adiabatic cooling of air through rising

331

What is the most common cause for dispersal of cloud

Adiabatic warming of air through subsidence

332

When is large scale dispersal of cloud often experienced

Large scale dispersal of cloud is often experienced when air is forced down through subsidence in an anticyclone

333

How is cloud dispersed from radiation

Warming of clouds through absorbti n of either solar or terrestrial radiation

334

How is cloud burnt off

Warming of low level air through surface heating so that the difference between the airs temp and dew point increases. As this happens, the cloud base (CCL) will rise and, provided no additional water vapour is introduced, the rising cloud base will soon equal the cloud top and cloud is said to have burned off

335

How does a decrease in convective activity disperse cloud

It will reduce the buoyancy of air and thereby cause subsidence of air, which may contain turbulence cloud such as fair weather cumulus. This is a common sequence of events towards the evening of fine summer days

336

How does mixing of unsaturated air disperse cloud

Mixing of unsaturated air outside of cloud with the saturated air within the cloud. The relative humidity overall may then well go below 100% and cloud dispersal will follow

337

What's is the Bergeron theory

That the saturation vapour pressure of air over ice is less than that over liquid water. This can be rephrased by saying that air that is saturated over liquid water droplets will be more than saturated over ice crystals. In other words, water is condensed out more readily over ice than over liquid

338

What is the process of the Bergeron process

If ice crystals are present in the company of supercooled water droplets, the air may be saturated relative to the ice surfaces but unsaturated relative to the water and therefore the water drops may evaporate. Consequently the released water vapour off the evaporating water drops will be attracted to the ice crystals and through the process of deposition (water vapour changing directly to ice) it will cause the ice crystal to grow at the expense of the liquid water drops. Additionally, if the ice crystals are in motion, their collision with supercooled droplets will also cause them to grow

339

When is the rate of deposition most efficient

At temperatures near -15°C the growth rate through deposition seems to be most efficient

340

What nuclei do ice crystals require

The formation of ice crystals requires nuclei different from those needed for the formation of liquid drops, i.e condensation nuclei. Ice nuclei which are required for ice crystal birth, are far less numerous and restricted to only few constituents. Clay seems to be a ready source for ice nuclei, while fine soil particles and volcanic dust are also popular

341

What is the fusion of coalescence

The fusion of coalescence of drops when a large drop has formed and commenced its descent. For the large drop to be formed in the first place requires a large or giant nuclei which are not numerous in the atmosphere. Research has confirmed that oceanic air is more likely to have these giant nuclei and for that reason it is not uncommon for heavy precipitation to occur from clouds that have their origins over ocean areas

342

What are the factors affecting the rate at which a water droplet falls

The speed at which the drop falls depends largely on their mass in that the larger the drops, the higher their terminal velocity. Other factors affecting the rate at which drops fall must be the force of gravity and drag on the drop itself. If the gravitational force is larger than buoyancy and drag, drops will continue to fall

343

What type of precipitation will form if ice crystals have been formed

If ice crystals have been formed, it is relatively certain that growth will take place and the type of precipitation then depends on the dryness and temperature of the air below. If ice crystals are not present, the onset of precipitation is then dependent on the availability of some large drops to start the coalescence process

344

What does hail consist of

Hailstones consist of layered ice of both the clear and rime variety around a cor ice crystal. The size of hail stoned can vary from some 3mm radius up to 40mm and even more

345

What is continuous precipitation

(Rain, drizzle, snow) implies an extensive, thick cloud sheet, which is not likely to clear in the short term

346

What is intermittent precipitation

Temporary improvements are likely

347

What are showers

(Rain, hail, or snow) can only fall from cumuliform cloud and are characterised by much brighter periods, possibly even large blue sky patches between the periods of precipitation. Showers are also characterised by sudden onset and cessation

348

How is the rate of precipitation measured

In terms of light, moderate and heavy. The amount of water falling on a flat surface or into a rain gauge per unit time, will determine the rate

349

What size drop will split into smaller drops

Once a drop reaches a smaller size (approx 5mm) it splits into smaller drops

350

What are the two types of weather radar currently used by the meteorological service of NZ

Weather surveillance for precipitation detection over a wider area and wind finding radar

351

What is the weather surveillance radar

A network of five radar installations is required to cover NZ- three have been installed to date at AU,WL, and Rakaia

352

What is the main requirements of the weather surveillance radar

Unobstructed radar coverage, access to electricity and access for installation and maintenance

353

What are the two modes of the weather surveillance radar

Intensity for detection and identification of precipitation and Doppler to provide speed and direction of travel of the precipitation

354

What is the most commonly used rain gauge

The tipping bucket type

355

How does the tipping bucket rain gauge operate

The unit consists of a collecting funds from which rainwater flows to the tipping bucket - the measuring unit. The bucket is divided into two equal compartments and pivots about a central point, one side resting against the stop. The bucket mechanism is adjusted so that as soon as 0.25mm of rainfall has entered a compartment, it over-balances and will come to rest against the other stop. The water collected is emptied out and the sequence repeats itself as the other compartment fills

356

What is the deciding factor in visibility

The transparency of air, which is directly related to the absence of solid or liquid particles within the air

357

What does meteorological visibility relate to

The horizontal plane, in other words, how far can an observer on the ground see an object of a given size against the horizon sky

358

How is visibility defined

As the greatest distance a black object can be seen and recognised against the horizon sky

359

Does visibility change during day or night

Whether it's day or night, if transparency doesn't change visibility doesn't change

360

What are the predominant factors of visibility

Precipitation, fog and mist, haze, smoke, sea spray

361

Mid latitudes visibility from precipitation

500m or less

362

Tropical latitudes visibility from precipitation

100m or less

363

Moderate rain visibility

4-10km. Light rain has a limited effect; visibility may well exceed 20km

364

Snowfall visibility

Moderate snowfall will reduce visibility to below 1,000m while heavy snow will reduce this to less than 100m

365

Visibility in drizzle

Depends on the associated weather. If drizzle is accompanied by mist or fog the visibility can be far less than if it had originated from low cloud such as stratus or stratocumulus. In the former, visibility can be less than 500m while the latter visibility may be reduced to 3km or less

366

Visibility in fog and mist

Fog is cloud on the surface when the visibility has reduced to less than 1,000m. Mist is reported when the visibility is 1,000m or more but does not exceed 5,000m

367

Visibility in haze

Haze relates to atmospheric conditions when visibility is reduced to 5,000m or less through the presence of dust particles. If dust or sand grains are blown off the surface and reduce visibility to less than 1,000m, they are referred to as sand or dust storms

368

How high do sand storms go

Sand storms, due to the size of sand particles, rarely go higher than some 100-150ft above the ground. Comparable smaller dust particles however, can be blown to much greater heights, in the region of 8,000-10,000ft

369

Smoke on visibility

Industrial and residential fires add innumerable particles of soot and carbon to the environment. These will reduce visibility through haze but additional problems are caused when these pollutants provide vast amounts of condensation nuclei. Thus if the airs moisture content is even remotely close to saturation, it does not take a great deal of cooling for the water vapour to condense prematurely

370

What is BLSN

Blowing snow above 2m

371

What is DRSN

Drifting snow within 2m of the surface

372

What is slant visibility

Slant visibility is a special problem where the surface is covered by a layer of mist or haze

373

What is the problem of slant range

Flying directly overhead the crew can easily see individual ground features such as runways etc. because the distance they have to see through the haze layer is relatively small when looking through it at right angles. However, having turned onto the final approach, the distance through the haze layer is now slant range, which is much greater than when overhear. Thus the visibility rapidly deteriorates

374

What is the aspect of height above the ground when considering slant range

Depending on the density of the haze or mist layer, there is a maximum eyesight penetration range. Thus if an aircraft flies just above the haze layer the number of miles one can see ahead on the ground is not great whereas increased height will still involve the same penetration range but the distance one can see ahead is now greater

375

What is runway visual range (RVR)

At some airports, the visibility prevailing along with runway, runway visual range, can be measured accurately through sensors positioned at the threshold, mid point and stop end of a runway. RVR is passed to pilots via METAR, ATIS or a controller whenever the prevailing visibility reduces below 1,500m. RVR applies to horizontal visibility along the runway as seen from 5m above the touchdown point

376

How does the bandar visibility sensor work

It measures the scattering of light by particles in the atmosphere and expresses the result as a voltage or frequency, which is proportional to the degree of scattering. The sensors microprocessor converts the info into a visibility range in miles and km's. The sensor projects an infrared beam while an infrared receiver detects radiation, which is scattered in the vicinity of the beam. If the air is perfectly clear no radiation is scattered to the receiver but as the air becomes more hazy, increasing amounts of scattering are received

377

How must visibility does the handcar visibility sensor measure

The sensor measure visibility only within the airspace within the vicinity of the infrared beam. Thus if visibility is different in another quarter it is not detected by the sensor

378

What is the definition of fog

Fog is cloud on the surface, whatever that surface ,at be. Since the formation of most clouds requires the initial presence of water vapour, a cooling mechanism and hygroscopic particles, it follows that when these factors are found near the surface fog is likely to form

379

What is the basis of radiation fog forming

When terrestrial radiation escapes and is not replenished by incoming solar radiation, a net loss of heat occurs at the surface. Thus, as the surface cools, conduction will cause the air molecules touching the surface to cool as well. Provided the RH (and thus dew point) is high, it takes only little cooling of air to reach saturation, and soon after, condensation. Air is a poor conductor of heat and therefore if no mixing of the surface air takes place, a very shallow layer of condensed water will lie on the surface, dew will have formed

380

What creates the radiation fog

If a light wind were to eventuate, sufficient to mix the surface air so that the cold surface temperature is distributed more extensively, the layer of air within which water drops are present thickens and fog will form. But if this wind is too strong the result is more likely to be a layer of cloud at a few hundred feet above the surface rather than fog on the surface

381

What are the conditions required to form radiation fog

A high RH so that little cooling is needed to saturate the surface air, a clear sky is that terrestrial radiation can readily escape, a light wind so that mixing within the surface air layer is assured and a generally stable atmosphere so that mixing and cooling are confined to a shallow layer

382

When are the requirements for radiation fog generally met

In anticyclonic conditions and when moist air is readily available. The weak pressure gradient usually associated with an anticyclone will help to produce the light wind, which is normally required for radiation fog

383

What prevents the radiation fog from lifting

The subsidence that invariably accompanies an anticyclone is likely to produce the stability of air, which prevents the fog bank from lifting off the surface while the clear skies associated with anticyclones encourage the uni pended escape of terrestrial radiation

384

What is the behaviour of the ELR during radiation fog

While radiation cooling of the surface progresses, the surface air temp on the ELR graph moves to the left indicating a narrowing gap between it and the airs dew point. Since air is a poor conductor of heat it follows that the ELR at some height above the surface will not be affected by the colder surface temperatures and the ELR starts to show a surface radiation inversion. As soon as the surface air temp reaches dew point values saturation will occur and further cooling will cause condensation

385

What happens when mixing takes place after condensation has occurred from radiation fog

Provided mixing takes place, a layer of fog will then be produced which, itself, influences the ELR. Cloud is a good absorber of terrestrial radiation, providing it with the capacity to become a source of radiation itself. This means that as soon as a fog layer has formed, the effects of escaping terrestrial radiation is offset by the warming within the layer of moisture and gentle mixing within it will cause the ELR to become isothermal rather than remain negative

386

What happens when ascending and descending within the isothermal radiation fog bank

If one ascends within this fog bank, the temp will remain more or less steady with height but near the top of the bank there will be a small increase in temperature, indicating the remnant of the inversion. Above that, the temp will decrease again consistent with the ELR prevailing in the upper air

387

What happens with more mixing when radiation fog is forming

The more through the mixing, the higher the position of the inversion, the deeper the layer of fog. If the wind causing the mixing is too strong however, the fog will likely be lifted bodily off the surface and a layer of low stratus is formed

388

What time of day is radiation fog most likely to occur

Cooling of the surface is most common during night time, which means that the longer the night, the more intense the cooling process. Additionally, the cooler the surface before night fall, the greater the chance of the surface air cooling to and below its dew point

389

What time of year is radiation fog most common to occur

During wintertime, when nights are long and the surface already fairly old even during the day, radiation fog is more likely to occur than during summer. However, autumn air is still relatively warm which enhances the presence of water vapour and therefore fog may occur when early dips in night time temp or the occasional frost is experienced

390

When will radiation fog persist until

Once radiation fog has formed in the early stages of the night or even towards dawn, it will likely persist until well after sunrise. Radiation fog cannot form over the sea because the water temperature does not cool sufficiently quickly

391

What happens to radiation fog after sunrise

After the sun has risen, the particles within the friction layer become excited and mixing intensifies. The top of the fog bank will experience some of this turbulence with the result that the height of the fog bank increases. Additionally, the initial solar input into the fog encourages the droplets at and near the surface to evaporate. This process requires latent heat, which is drawn from wherever it is available, including from the air above the bank

392

What causes the thickening of radiation fog after sunrise

The combined effect of extra mixing and demand for latent heat will produce a thickening of the fog bank after sunrise

393

What happens to the ELR when radiation fog is dispersed

Solar radiation does have a limited ability to penetrate moisture; it also has some warming effect on the water content within the fog bank. These factors combined will cause a steepening of the ELR; a positive difference develops between the surface air temp and its dew point. The fog will lift off the surface in places, a few holes will appear and direct solar input onto the surface will further steepen the ELR. As a result a layer of low cloud will form which initially takes the form of low stratus, but sometimes it turns directly into stratocumulus

394

What are the orographic influences on radiation fog

If radiation fog forms in hilly or mountainous terrain, the effect of katabatic flow will intensify its development. Thus katabatic flow will enhance the formation of valley fog. Once formed in hilly terrain, fog will invariably settle into low lying areas and often flow through valleys and gorges to the adjacent planes

395

What is the timing of radiation fog

Although radiation fog is very often a night occurrence - it is not unusual for it to form after sunrise when the RH has been high but a lack of mixing during the night prevented its formation. The mixing produced by solar input after sunrise and the demand for latent heat as some surface moisture evaporates may cause radiation fog to form at that time

396

When is dispersal of radiation fog normally assured

Dispersal of radiation fog is normally assured once solar radiation is introduced, i.e after sunrise

397

What surface does advection fog require

Advection fog requires a surface that is already cold so that when moist air is transported over it (advection) the air can cool to temperatures lower than dew point

398

How is the requirement for mixing met when forming advection fog

The requirement for mixing is generally met by the action of the wind, which transports air, and a high RH assures that the airs water content is sufficient to produce condensation

399

Can advection form over water

Advection fog can form over water and land

400

How long can advection fog persist for

Advection fog can last for days and even weeks under some circumstances

401

What conditions does advection fog need

Radiation fog normally requires anticyclonic or col conditions whereas advection fog does not necessarily require this. Provided the wind is not too strong and moist air is blown across a sufficiently cold surface, advection fog may form

402

How is advection fog dispersed

Two very common causes of advection fog dispersal are, firstly, a change in the direction and/or strength of the wind, and secondly, advection of less moist air

403

When will valley fog form

This fog tends to form when an abundance of moisture from rivers and streams is assured and a rapid radiation cooling process is available from rock walls, which will have been in shadow for some time during the afternoon. Valley fog can often be very persistent especially in winter when the low sun cannot penetrate down to the valley floor

404

When does up slope fog form

When a light wind forces moist air up a cold slope such as a snow covered mountain, fog may form. The end of this fog bank is found where the up slope wind stops

405

What is cold stream fog

When a cold ocean stream is surrounded by warmer oceans, the cross flow over the cold surface can produce fog. It is then expected that this fog will be restricted to the cold stream boundaries

406

What type are warm and cold stream fog

The advection type

407

What is warm stream fog

The warm air above a warm stream is likely to be very moist and if this is transported over adjacent cooler ocean surfaces very extensive fog can be produced

408

What are the two main causes for steaming fog

Addition of moisture from a warm wet surface to colder air above to the extent that it becomes saturated or transport of very cold land air over a relatively warm sea surface

409

When can the first mentioned cause of steaming fog be experienced

Can occasionally be experienced in geothermal areas where geyser activity expels such large amounts of water that the air saturates. On rate occasions fog can develop when the sun comes out following a heavy shower or thunderstorm as moisture evaporates from the warm wet ground. Usually such fog dissipates rapidly because the warm surface tends to make air unstable but if this fog has formed towards the end of the day, it may persist for some time

410

What front is more likely to form frontal fog

Because fog is associated with light winds, the chances of frontal fog occurring with warm fronts are better than with cold fronts because the first mentioned are invariably more gentle in style

411

What does the world meteorological organisation define a thunderstorm as

One or more electrical discharges manifested by lightening and thunder

412

What are the requirements for a thunderstorm

The development of CB. Flight conditions will involve moderate to severe turbulence, icing, possibly lightening strikes and severe windshear at low levels

413

Where is most of the energy come from to form a thunderstorm

These dramatic and violent occurrences require vast amounts of energy most of which are thermal in origin. Development of the necessary CB cloud requires strong instability, which is often a con eques even of high surface temperatures

414

What is a strong provider for instability to create thunderstorms

Provided the moisture content of air is sufficiently high, the convectively rising air will soon experience condensation so that cumulus cloud is formed. This process in itself is a strong provider for instability because the latent heat released during condensation adds heat to the air in the immediate vicinity. Thus the clouds temperature is warmer than the surrounding air and added impetus is provided to the rising process

415

What is the process of entrainment during thunderstorms

With increasing height, surrounding colder air outside the cloud is sucked in, a process known as entrainment, which causes the warming process to diminish. Furthermore, as less moisture becomes available with height, the release of latent heat reduces so that the convection process slows down the combined result is a levelling off in instability at altitude which invariably produces high level layer type cloud in the form of the typical anvil shape often found when CB's have often passed their maximum intensity

416

What are the requirements for the presence of thunderstorms

An atmosphere that is unstable through a deep layer, an adequate supply of moisture from below, a trigger action that causes the initial upsurge of the unstable air and a mechanism that will produce sufficient electrical charge differences

417

What is lightening in thunderstorms

There must be something that sets the ordinary CB apart from those that are associated with thunderstorms. The difference is the presence of lightening with its associated thunder. Lightening is the result or difference in electrical potential. A build up in this difference causes a spark to jump the distance between differently charged particles or between these particles and say, the earth surface

418

What is the induction theory of lightening

The ionosphere is positively charged while the earths surface is often negatively charged. Thus cloud droplets would acquire a negative charge on top and a positive charge at the bottom. Differences so caused, involving millions of drops, may be sufficient to produce the required potential

419

What is the rate at which air is moved up during the growing stage

All draughts are upwards. The rate at which air is moved up can exceed 2,000ft/min while the horizontal extent of development can involve an area of some 10km across

420

What heights can the CB grow to in the growing stage

Large quantities of latent heat are released which enhance the rising process while at the same time they strongly influence (and raise) the height of the freezing level within the cb cloud. The top of the cloud continues to rise, in some instances in the tropics this may well involve rising into the stratosphere. In mid latitudes, tops of cb's rarely go beyond 25,000-35,000ft. The cloud tops have a boiling up appearance; there is no high level stratiform development accompanying the cb at this stage

421

How long does the growing stage take

The growing stage takes some 15-30mins but this timing can be quite different either way under certain conditions and in different latitudes

422

What's the affect of the drag during the mature stage

The drag between the precipitating mass and the updraughts has a strong diminishing effect on the velocity of the updraughts so that after a brief period of time, some downdraughts are produced. Normally these downdraughts are restricted to the front portion of the cb while updraughts continue to function at the rear

423

What does the air at the downdraughts warm at during the mature stage

The air in the down draughts warms at the SALR because the descending air is saturated. Due to this low rate of warming, the down draughts are increasingly colder compared to the surrounding unsaturated air. This has the effect of accelerating the velocity of the down draught so that the greatest vertical speed is generally found towards the bottom of the cb

424

Where is an often violent turbulent zone formed during the mature stage

Between the up and down draughts

425

What is the gust front during the mature stage

At the down draughts hit the surface, they have no option but to split and spread out horizontally. The speed at which this flow travels horizontally is greatest close to the cb cloud - speeds in excess of 45kts have been recorded. This high velocity squall is known as the first gust also known as the gust front. In some cases, the horizontal flow can rise off the surface some distance in front of the cloud and return to the cloud in the shape of a roll cloud

426

Where is the maximum strength of the gust front during the mature stage

Although the surface deflected down draughts produce a horizontal flow all around the base of the cloud, the maximum strength is usually to the front of the cb i.e in the direction of travel

427

How does the freezing level oscillate during the mature stage

The freezing level oscillates, lower in the forward half of the cloud where the downdraughts prevail and higher where the up draughts are found (usually the rear). The onset of down draughts causes precipitation in the form of heavy showers of water and more often, hail. At about the same time, lightening flashes are seen

428

What forms the anvil at the end of the mature stage

Towards the end of the attire stage and certainly the decaying stage, equilibrium with the surrounding air is reached near the top of the cb cell. This gives rise to horizontal outflow of ice crystals aloft which forms into stratiform cloud often in the shape of an anvil. Usually the anvil points in the direction of travel of the cb

429

What is the average time of the mature stage

Approx 15-20 mins

430

What happens during the decaying stage

Up draughts within the particular cell cease so that a generally descending motion takes place. Turbulence and gusts decrease and in the absence of other cells at different stages, the high level stratiform cloud settles and the cb slowly disappears

431

Is precipitation common during the decaying stage

Precipitation is common during this stage and will occur underneath the entire base of the could. The freezing level moe bulges down relative to the level outside the cloud

432

How can the regeneration of thunderstorms occur

Not all cells are at the same stage of development. For instance, while one cell within the cb is in the growing stage, another one may be in the decaying stage and so on. Research has also indicated that the cycle of one cell can be interrupted or affected by the behaviour of the neighbouring cell. As well, the lifting of air at the gust front may be the trigger action necessary to set off the development of another cell

433

What is another feature that may have a strong influence on not only the intensity but also the regenerative capacity of a thunderstorm cloud

Is the presence of a windshear layer aloft. It is generally found that air above the shear level is drier than that below and due to the altitude where this type of shear normally occurs, the strong airflow is also cold

434

When are orographic thunderstorm found

This type is found when unstable moist air is forced to rise over mountainous terrain

435

Do days or seasons have a major influence on orographic thunderstorms

Time of day or season have no major influence although the added effects of hear during the afternoon in summer and autumn may aggravate thunderstorm conditions

436

How long do orographic thunderstorms last

Provided the unstable moist air is continuously transported onto the mountains, the orographic type can preserve sometimes for days on end

437

What precipitation is associated with orographic thunderstorms

With moisture content often high in orographic cloud, the associated precipitation can be very heavy. Under these circumstance, it is not unusual for mountains and even the adjacent plains, to have a heavy coating of hail, even in the summer

438

What are the requirements for heat type (thermal) thunderstorms

Warm, moist air to be heated from below so that strong instability is encouraged

439

What must the thermal thunderstorm be close to

The heat type thunderstorm, under most circumstances (but not always), is diurnal and land related and must be sufficiently close to a water source for cb developments to progress

440

What are the thermal thunderstorms in temperate latitude

This favours coastal regions during the summer time and generally restricted to the early to mid afternoon

441

What are the thermal thunderstorms in tropical regions

The heat thunderstorm can well be a daily occurrence throughout the year, starting in the early afternoon and sometimes persisting until well after sunset

442

What cb will develop provided surface heating is strong enough for a thermal thunderstorm

Provided surface heating is strong enough, thunderstorms can develop from cb clouds, which have relatively high bases and little or no precipitation. Lack of rainfall onto the surface is often caused by evaporation within very warm air usually found between cloud base and the hot surface

443

When are convergence thunderstorms formed

This type can be found in a variety of circumstances when convergence takes place. Thus the heat type cb may well also fall within the convergence type found, for example, during a warm summers day when sufficiently strong sea breeze activity and unstable conditions force moist air aloft. Alternatively, the convergence of moist unstable air associated with low pressure systems may be sufficiently strong to produce cb's. A final example of this type is the frequent thunderstorm activity associated with the intertropical convergence zone when it is active

444

Where is the nocturnal equatorial thunderstorms most common to form

Is a common occurrence over oceanic areas approx 10 degrees latitude either side of the equator

445

What happens to the ELR during the nocturnal equatorial thunderstorm

The sea temp in those latitudes remain very warm throughout the year and records show that temps around 30°C are not unusual. Thus the air above the sea surface remains similarly warm but due to radiation losses of air at high levels, a steepening of the ELR follows as the air and clouds cool aloft

446

When are nocturnal equatorial thunderstorms most active

This type is at its most active towards dawn when the ELR has reached its steepest value. After day break, the thunderstorm persists for some time but under normal conditions, they cease to exist around mid morning

447

What is the cold stream (cold advection) thunderstorm

When relatively cold air moves over warm sea areas, it absorbs large amounts of water from the sea surfaces while at the same time its lower levels warm rather quickly. As the ELR steepens and vertical mixing intensifies, the stage is set for the development of cb cloud

448

Where can the cold stream (cold advection) thunderstorms be found

This type is not unusual in the Australian region when cold polar air moves towards the continent in both winter and spring time. It can also be experienced in NZ region under similar conditions during spring and early summer. In either case, the intensity of thunderstorm activity through this cause is not very great in relative terms

449

Are the up or down draughts stronger in a thunderstorm

The ups are often stronger and of greater vertical extent than the downs

450

Do the up and down draughts always cause severe turbulence

Up and down draughts do not necessarily cause severe turbulence if the side of the draughts is large. However, maintaining altitude is a problem where the aircraft may be bodily forced to rise or descend through many thousands of feet in some instances

451

Where are the up and down draughts most forceful

Up draughts are considered to be more forceful from the middle of the cb cloud to its top. Down draughts tend to be stronger from the middle of the cloud to the surface

452

Where is the turbulence from up and down draughts

Up and down draughts are associated with turbulence where draughts meet. Thus flight in mature stage cb clouds will more than likely produce moderate to severe turbulence on the boundaries between draughts

453

What are the principle problems with draughts in thunderstorms

Altitude control and the secondary problem is turbulence especially when opposing draughts are closely spaced

454

What is the major cause of turbulence in thunderstorms

The major cause of severe turbulence is the presence of gusts which are short term and localised fluctuations in both vertical and horizontal airflow, superimposed on the up and down draught pattern

455

What are the main structural dangers of gusts to aircraft in thunderstorms

Severe load factors imposed on the airframe; possibly large changes in airspeed which may cause a pilot to exceed the speed limits for turbulent flight and abrupt changes to the aircraft attitude which may cause structural failure if the pilot attempts to correct these changes too quickly or harshly

456

Where are the gusts most likely to be found in a cb

Although gusts can be found in many parts of the cb, the most likely places are in the dividing sectors of strong up and down draughts found in the mature stage

457

How is a shear zone formed from a thunderstorm

The cold dense outflow, which can frequently reach up to 1,500-2,000ft at speeds up to some 40kts or thereabouts, propels itself along the surface beneath the warmer environment air resulting in: a gust front forms some considerable distance ahead and a shear zone is formed between the cold outflow and the warmer air above which often travels in the opposite direction back into the lower part of the cb

458

Where are icing conditions worst in thunderstorms

Above the freezing level, icing conditions are worst within the first 6,000-8,000ft because within that band of height, supercooled water and droplet size combine to produce clear ice, which readily builds up and adheres strongly to the aircraft; it is difficult to get rid of

459

What ice is found at the tops of thunderstorms

Towards the tops of thunderstorm clouds, and supercooled water that may have survived will likely freeze in such a way as to produce the more brittle rime ice, which is somewhat easier to remove. But then the risk of ice ingestion in turbine engines becomes a real risk

460

What is icing like below the thunderstorm cloud

Flight below the cloud base will cause less icing problems but this may well be offset by the dangers of turbulence and downdraughts normally to be expected there. Aircraft performance permitting, flight through the tops or above the tops would clearly be preferable

461

What are the problems caused by lightening

Problems caused by lightening strike involve possible loss of aerials because when stuck, these will cut off neatly and instantly. Magnetic compasses will be strongly affected and large deviation errors result. It will be necessary to swing the compass before it's reliability is restored. Electronic equipment may suffer severe damage and reliance on instruments and aids that rely on such equipment may be impaired. Finally, temporary blinding may occur when crew and passengers are in dark or dimly lit environments. For this reading it is recommended that cockpit and cabin lights are turned to full bright

462

What is the use of discharge wicks and static strips for lightening

While dealing with electrical charges and charge differentials, an aircraft flying through cb clouds, heavy precipitation or even sand storms, may build up a potential charge so great that discharges can occur between the aircraft and the cloud, or aircraft and precipitation and even the aircraft and the ground. Controls attached to the airframe through hinges, can similarly be charged differently from the airframe and for this reason, amongst others, bonding is invariably provided between controls and airframe. Similarly, the use of static strips or wicks will do much to dissipate charge potential differences between aircraft and surrounding air

463

What is St. Elmo's fire

Under all the charged conditions, it is possible that highly charged air, when rapidly accelerated through a propelled disc, past a nose cone or through a heli disc, may become a visible discharge known as St. Elmo's Fire. Although the phenomenon looks very frightening, it is totally harmless

464

What size hail stones pose a risk to aircraft

Statistics show that stones larger than 1/2 inch pose a serious threat especially to high speed aircraft

465

Where is the worst part of the thunderstorm for hail

The worst parts of thunderstorm clouds, from the point of view of hail, are between some 8,000 and 25,000ft in mid latitudes. In tropical regions, these values are much higher

466

Is hail at sea level generally a problem in tropical and sub tropical regions

Hail at sea level is not generally a problem with CB clouds in tropical and sub tropical regions because the freezing level is fairly high and stones will have melted to a large degree by the time they reach lower heights

467

Is hail a problem in mid latitudes

In mid latitudes, hail can persist to sea level and statistical evidence shows that even in summer time, the odd thunderstorm can produce a substantial coating of ice on the ground; admittedly, it is not likely to remain there for long

468

What is the potential loss of instruments and impairment of accuracy during thunderstorms

Flight below the base of CB, especially in the growing and the early mature stages, will involve substantial local convergence. The associated reduction in pressure may be very serious if ground clearance is not great and visibility poor because the altimeter will tend to over read i.e the crew will be lead to believe that aircraft height is more than shown on the altimeter

469

What happens if the effects of a thunderstorms block your pitot source

A blocked pitot source through ice or affected by heavy water ingestion, will seriously impair the workings of the ASI

470

What should your choice of height be approaching a thunderstorm

This depends mainly on latitude, aircraft performance capability and terrain a. If possible, flight should be conducted over the tops of cloud and if that is not possible, flight beneath the base should only be attempted if visibility there is adequate and turbulence acceptable. In tropical regions, tops of cb clouds can penetrate the stratosphere but in mid latitudes, tops should be found at some 25,000-30,000ft. Thus away from tropics, flight above cloud tops should be feasible for modern jet aircraft

471

How should deicing equipment be used during flight through a thunderstorm

It is prudent to switch on all anti or de-icing equipment at an early stage and not wait until the symptoms of blocked intakes present themselves

472

What airspeed should you have during flight through a thunderstorm

This should be reduced to below maximum turbulence penetration speed or to a speed indicated in the aircraft flight manual

473

How should your lights be operated during flight through a thunderstorm

Switch lights to bright

474

How should you have your flight controls when flying through a thunderstorm

Unless the flight manual dictates otherwise it is common practice to disconnect the autopilot. Abrupt and violent control inputs should be avoided and the main attempt should be to hold the attitude as steady as possible under the circumstances. If necessary, request for greater aircraft separation should be made

475

How should you track when flying through a thunderstorm

A line of thunderstorm clouds should be crossed as close to right angles as possible so that minimum time is spent within them

476

What use is radar when flying through thunderstorms

If airborne radar is available, this should be use to maximum advantage in order to fly between CB's rather than through them

477

What is the ground base radar used for

Ground based radar designed for the purpose of thunderstorms is used to map areas of precipitation and to track their movement, providing information on the size and intensity of individual cells. MetService has a chain of such stations providing coverage for most of the country. Aircraft tracking radar and wind finding radar are not particularly effective for locating storms

478

How is the airborne radar of use when flying through thunderstorms

Airborne radar especially colour display, is most useful in avoiding cells and intense precipitation areas. Range graticule and angle off displays are usually provided but it must be remembered that whereas at some distance, a gap between CB's my show a likely avenue for safe flight, by the time the aircraft gets there, growth of cells may have closed the gap entirely

479

How are the worst areas of turbulence shown on an airborne radar

The worst areas of turbulence on an airborne radar display are shown as red while areas of moderate turbulence are painted yellow. Green areas indicate where turbulence is light

480

What are tornadoes

Tornadoes are rotating funnels of air sucked up from below the base of CB cloud. They are caused by extremely strong upward motion within the cloud

481

How is the high rotational velocity of tornadoes produced

The high rotational velocity of a tornado is produced when there is an existing rotation of air beneath the CB and this rotating air is drawn into the base of the cloud through convergence. As this convergence develops it increases the rotational velocity of air (conservation of angular momentum)

482

What axis is the rotation of air below a CB in a tornado

The rotation of air beneath a CB is very often around a vertical axis and the increase in rotational velocity through convergence as it is drawn in is therefore readily understood. But rotation due to wind shear can also produce tornadoes

483

How does wind shear exist below tornadoes

When low level wind shear takes place beneath or in the vicinity of a CB, air is made to rotate around a horizontal axis like a roll of dough when it is rolled along a table top by hand. Should part of the rolling air be drawn into the base of the CB, the axis of rotation becomes vertical and convergence will then cause rotational velocity to increase

484

How high do rotational velocities of tornadoes get to

Rotational velocity in many tornadoes are often so high (values in excess of 300kph have been recorded) that atmospheric pressure in the core can drop hundreds of hectopascal's

485

What is the diameter of the tornado core

The diameter of the core is only a matter of meters but some cases have been recorded where the diameter reached in excess of 200 meters. This core diameter must not be confused with the diameter of the total tornado, which can be 2 or even 3km

486

Can tornadoes form over water

Tornadoes can form over water areas Ao that the typical water spout which sucks up water develops

487

What are small scale tornadoes

Small scale tornadoes can develop in hot arid regions such as the Australian interior. The well known willy-willies or dust devils are a very common occurrence during the dry season. These small tornadoes can develop in a very short space of time and can travel for quite some distances before decaying

488

What is turbulence defined as

Turbulence can be defined as small scale, short term, random and frequent changes to velocity of air. In other words, when there are rapid changes in either the airs speed or its direction of movement or both, conditions are said to be turbulent. These changes are brought about by the imposition of vertical (or non horizontal) velocity components on the horizontal movement of air

489

What is thermal turbulence

When the surface is sufficiently warm, vertical currents involving irregular eddies produce turbulence, which can reach great heights in conditions of strong instability. The degree of surface heating is uneven due to different surfaces. Therefore, the intensity of turbulence can change quickly as these differing terrains are overflown

490

How can you avoid eddies from thermal turbulence

When thermal eddies have formed above a warm surface the effect can invariably be avoided by flying above the friction layer or above convective cloud tops

491

What happens during thermal turbulence when absolute instability has been established in the friction layer

When absolute instability in the friction layer had been established, turbulence will occur. In many situations this instability extends beyond the friction layer and, when the moisture content of air is high enough, cumulus clouds will form which, through release of latent heat, add to the instability. Under those circumstances turbulence will be experienced from the surface up to cumulus top levels (where the inevitable inversion is found). In the absence of cloud, the top of the turbulence layer can often be seen at the level where improvement in visibility begins

492

How can the steepening of the ELR be done for thermal turbulence

Although steepening of the ELR is very often caused by warming of the surface through insolation, the same result will show up when a cold air stream moves into warmer latitudes i.e cold advection. Thus any factor that has the capacity to steepen the ELR may be instrumental in producing thermal turbulence

493

When is thermal turbulence strongest

Turbulence caused as a consequence of insolation will be subject to diurnal fluctuations. Thus conditions will be most turbulent in the early afternoon and smoothest during the night and especially so just before dawn

494

What is mechanical turbulence caused by

Interference of surface features on the horizontal flow of air

495

What influences mechanical turbulence

Speed of the wind, size of the obstruction, shape of the obstruction and the prevailing ELR

496

What causes the worst mechanical turbulence

The stronger the wind, the larger the obstacle, the rougher the obstacle and the steeper the ELR, the worse the turbulence and vice versa

497

What are conditions from small scale interference when wind is less than 15kts

When the speed of the wind is less than some 15kts, low hills, buildings, tree lines etc. will produce light turbulence in the lee of these obstructions. In general, under these wind conditions, small scale turbulence does not extend much above approx 1/3 the height of the obstruction above the obstruction

498

What are the conditions from small scale interference when the wind is more than 15kts

In winds slightly stronger than some 15kts it is often found that standing eddies form at both the windward and lee side of solid obstructions. The eddies in the lee tend to travel downwind so they their influence can be felt some distance away from the obstruction. However, when wind speeds become strong and gusty, the flow becomes highly irregular, standing eddies no longer occur and moderate turbulence can be experienced

499

What cloud will form from small scale interference

When the moisture content of air is sufficiently high, cumulus cloud may develop from the windward side of the hills and spread over subsequent ridges

500

Where will the wind move around large scale interference

The wind approaching mountains will move laterally around them, over them or through valleys and saddles. In almost all cases the speed of the wind and the probability of turbulence increase where the flow bends.

501

What is a major factor determining the flow pattern of air from large scale interference

A major factor determining the flow pattern is again stability of air. In conditions of string stability aloft the low level airflow is prevented from rising much above mountain height. In most cases this situation is linked with reduced wind shear (slower at altitude) so that rotor streaming and associated moderate to severe turbulence is likely to be encountered at low levels in the lee of the mountain

502

What happens with large scale interference in less stable conditions

In less stable conditions, the interference of a mountain range on the flow of air can be felt at great altitudes as evidence in wave action and rotor turbulence. The most extensive turbulence due to large scale interference is generally to be expected from high mountain ranges when air is unstable

503

Large scale interference - valley winds

Valley winds, even when strong, tend to follow the confines of valleys up to approx 2/3 the height of the surrounding mountains. Above that height the prevailing (geostrophic/gradient) wind is usually experienced. Thus turbulence may be remarkably light near valley floors. This factor must not be relied upon however, because valleys are invariably intersected by others so that conflicting airflows and turbulence can be experienced in the vicinity of valley junctions. As well, saddles and slanting ridges can have a strong influence in lowering the cross valley wind height

504

When moisture content is favourable in large scale interference, what cloud will form

When moisture content of air is favourable, the areas of worst turbulence can often be identified by cumulus cloud formations. Particularly between the ridges and the base of these clouds, turbulence can be moderate to severe

505

What is the low level shear of wind shear turbulence

Low level shear is invariably associated with a very stable friction layer. When the ELR near the surface is shallow or negative, mixing of air is at a minimum so that the wind flow at higher levels is not carried down. Thus in conditions of strong low level stability and strong higher level instability, the shear zone is characterised by moderate local turbulence

506

What happens when the ELR from the surface is steep with wind shear turbulence

When the ELR from the surface is steep, so that conditions are unstable, mixing of air will spread the stronger winds from aloft into the lower levels so that an averaging of winds occurs which prevents shear (and its turbulence) even though mechanical turbulence may occur

507

What effects does the friction layer have on wind shear turbulence

When the friction layer is at its most dense (just before dawn) turbulence in the shear zone is a maximum while at the warmest time of the day (early afternoon) it is a minimum or absent

508

What is wake turbulence

Aerodynamic lift is produced when a pressure differential exists between the air above and below the lifting surface. A circular flow pattern results when (relative) high pressure air below the lifting surface moves towards deducted pressure air above

509

Which was does each vorticy travel from wake turbulence

This pattern develops around the wing tips or blade tips and looking from behind, a clockwise rotating vortex is shed from the left tip and an anticlockwise vortex from the right tip

510

What is wake turbulence directly proportional to

The mass of the aircraft, aircraft configuration and indicated airspeed

511

When is the wake turbulence the worst

The higher the all up weight (AUW), the cleaner the aircraft configuration and the slower the airspeed, the bigger and more intense the vortices and the greater the risk to the following aircraft

512

How long can wake turbulence last for

It can persist for as long as 5-6 minutes in calm conditions

513

How much wake turbulence can follow a plane in straight and level

Wake turbulence generated by an aircraft in straight and level flight usually settles about 1,000ft beneath its flight path

514

What are the classification of turbulence

In reports and forecasts turbulence is classified only as moderate or severe

515

What is considered light turbulence

This involves occasional movement of the aircraft around its three axes so that mild rolling, pitching and yawning may be experienced

516

What is considered light turbulence

This turbulence involves more or less persistent and abrupt movement of the aircraft around its three exes, i.e there is rather severe rolling, pitching and yawing. There is general passenger discomfort

517

What is considered severe turbulence

This turbulence involves abrupt and large movements of the aircraft about its three axes during which pilot control is frequently (but briefly) lost. It's is quite difficult to maintain altitude and direction and poor piloting techniques may cause airframe stress damage. Passenger discomfort is marked

518

What is considered extreme turbulence

This turbulence involves frequent and prolonged periods during which control of the aircraft is lost so that altitude and direction control cannot constantly be maintained. It is likely that some airframe stress damage will be experienced. Passengers are severely discomforted

519

What is PMK

Polar maritime cold advection

520

PMW

Polar maritime warm advection

521

Does a depression have boundaries

A depression has no boundaries but interacts with adjacent pressure systems such as cold and anticyclones and is also continually changing with time. It's associated frontal system will have a different structure at different points along its length. Although depressions and anticyclones have life cycles, developing and decaying over time, it is not possible to define with any precision a start point or an end point

522

What is the polar front theory also known as

Norwegian theory

523

What is the polar front theory based on

The theory was based on observations of what was actually happening in the atmosphere (known as empirical observations) rather than on an explanation in terms of physics or flux dynamics (largely used in modern meteorology)

524

What is the polar theory identified as

The theory identified a more or less narrow zone of bad weather separating the very cold polar air from warm air of the tropics and sub tropics. This zone was called the polar front. In one place along the front cold air was seen to be advancing (the cold front) and in another palace warm air would be advancing (the warm front)

525

What is the surface pressure at the sharp end of the wave of a polar front

Often the surface pressure at the sharp end (the cusp) of the wave could be seen to fall and a depression would develop

526

What is the polar front theory

A warm front and a cold front start as a stationary front, then starts a circulations. The isobars then close around the centre of the wave so that an identifiable depression is formed. The advancing cold air gives rise to the cold front while the warm air gives rise to the warm front. The higher speed of the cold front causes it to overtake the warm front so that the warm sector is squeezed up and becomes smaller. The front is said to be occluded. Since the warm sector has been lifted to mid troposphere altitudes, the associated stratiform cloud has demonised. The remainder of the occluded front is covered by various amounts of cumulus and possibly some patches of layer type cloud. The frontal system has almost decayed, the depression travels as an independent system

527

What is the meridional front

One observed fact was that most of the fronts in the Southern Hemisphere lay north-south along the meridians with their northern parts sometimes penetrating into the tropics. The term meridional front was used to describe these common fronts to distinguish them from the polar fronts of the theory

528

What are air masses defined as

Air masses may be defined as large blocks of air that have uniformity in certain properties in the horizontal plane extending over many square miles. The characteristics involved in this definition relate to the airs temperature, moisture content and temperature lapse rate

529

What are source regions

A study of global air movement indicated that the major areas of air stagnation are found within the belts of anticyclonic subsidence, i.e the sub tropical and polar regions. These regions are therefore referred to as source regions

530

What must a parcel of air do to absorb the characteristics of a source region

For a mass of air to absorb the characteristics of a source region, it needs to reside in that region for some 5-7 days. Weather conditions must be as static as possible, which favours the presence of anticyclones because in those systems pressure gradients are fairly slack, the wind is usually light and variable and subsidence with its slow divergence provides the imprint from the region onto the air. In addition, the requirement for a relatively steady ELR is generally met in large anticyclonic situations

531

What is the main factor in air masses

The main deciding factor in air masses is temperature, which means that air masses are either of polar or tropical origin

532

What are polar regions effected by

Both polar regions, due to subsidence, are for a large part of the year affected by high inversions from the surface to about 5,000ft elevation. Beneath this inversion, small amounts of cloud may cause occasional light snow

533

What is migration to different latitudes referred to as

Migration to different latitudes is referred to as air stream advection meaning horizontal transfer of air

534

What are air streams that are judged to be cold on arrival have

Air streams that are judged to be cold on arrival in a new locality have the letter K appended while air streams that are judged to be warm on arrival in a new locality have the letter W appended

535

What is cold advection

This means cold air moving towards warmer regions. Thus it could involve PmK or PcK depending on the origin of the moving air. Regardless of surface type, the temperature of the lower layers of the horizontally moving air will become warmer. It then depends on the degree of mixing as to how extensive the temperature effect will be in the vertical

536

What will northbound flow of warm advection cause

Northbound flow towards the equator from Southern Hemisphere source regions is caught up in the generally easterly winds occurring on the equator side of the subtropical anticyclones. When confluence with the northern hemisphere easterlies takes place, often massive developments of both cumuliform and stratiform cloud occur in a zone called the intertropical convergence zone, the ITCZ

537

What will southbound flow of warm advection cause

Southbound warm air leaving sub tropical source regions in the Southern Hemisphere travel over cooler surfaces so that lower layers will be affected by these surfaces and cool. The ELR will therefore shallow and stability is enhanced. Since on arrival at colder locations, this air will be judged as warm, the symbols TcW or TmW will apply

538

What is dynamic modification

Modification of airstreams involves thermal processes when air is transported horizontally over colder or warmer surfaces. These processes are adjective and radiative in the main and, particularly in the case of warm advection, certainly not adiabatic. However, when air streams are forced to rise up in front of, or sink in the lee of mountain ranges, adiabatic factors are involved in the modification process; the associated vertical movement of air and mixing is then referred to as dynamic modification

539

What are the predominant causes of air stream modification

Whereas thermal process changes may well be the predominant factors in air stream modification in the lower layers of the troposphere, dynamic processes are the major causes of air stream modification in the middle and upper troposphere

540

How often do satellite meteorology give reports

Pictures are in both the viable spectrum and infrared so that night time imagery is available. Geostationary satellites cover large areas of the globe every 3 hours (in some cases hourly) and orbiters provide very high resolution imagery so that fine details of cloud and fog distribution can be determined

541

What can the satellite meteorology give information on

Sensors can give approximate vertical temperature profiles in areas where upper air surroundings cannot otherwise be made. They can also give a picture of the water vapour content at various levels, sea surface temperatures, cloud top temperatures, or flight level, upper winds estimated from the movement of identifiable cloud elements, and other info. They are not forecasts themselves

542

What do the colours used in satellite photos represent

Cloud tops against flight levels

543

Satellite met photos dark grey up to

FL050

544

Satellite met blue between

FL100/FL200

545

Satellite met yellow between

FL300/FL400

546

Satellite met purple higher than

FL450

547

Satellite met light great between

FL050/FL100

548

Satellite met green between

FL200/FL300

549

Satellite met red between

FL400/FL450

550

How can an air mass or air stream be resolved

The actual characteristics of an air mass or air stream can be resolved to a reasonable degree of accuracy from a network of upper air observations, reports from aircraft, remote sensing by satellites and by computer analysis and prediction

551

What do modern meteorologists look at fronts and depressions as

Modern meteorologists look at fronts and depressions in terms of the principles of fluid dynamics rather than as phenomena resulting from some grand battle between different air masses

552

How are upper air flows shown on contour charts

Upper airflows are shown on contour charts, which use contour lines instead of isobars. These contour lines are read in the same way as isobars i.e closely spaced contours imply strong winds and vice versa. Furthermore, the direction of the wind flow can be assessed from the pattern of contours

553

What do the waves indicate on contour charts

Unlike closed isobar patterns at low altitudes, contour charts rarely show closed circulation. Instead, airflow aloft is characterised by wave flows cumulating in troughs and ridges. When a wave indicates airflow detecting to the right (clockwise), a trough is involved whereas a wave flow deflecting to the left (anti clockwise), implies a ridge. Both situations are applicable to the Southern Hemisphere

554

What are jet streams on contour charts

Jet streams are more or less horizontal ribbons of high velocity airflow near the tropopause characterised by strong values of wind shear both in the horizontal and the vertical. Thus in the centre of a jet stream the wind velocity will be very strong while above, below and to each side, the wind is less strong. These jet streams, which have a beginning and an exit, are found in preferred locations the Australasian region being one of them

555

Does the speed of airflow on contour charts increase in waves

The speed of an airflow does not necessarily increase in every wave or curve of a jet stream, speed maxima are sometimes identified along straight flows

556

What is vorticity

Vorticity is a measure of the rotational motion or turning in an airflow (or other fluid) which can be cyclonic or anticyclonic

557

What can the absolute vorticity of a parcel of air include

Vorticity due to curvature of the flow, vorticity due to windshear and vorticity due to earth rotation

558

What is vorticity due to curvature

Air flowing around a curve will have vorticity that will by cyclonic when airflow deflects to its right and anticyclonic when it turns to its left in the Southern Hemisphere

559

What is vorticity due to wind shear

This vorticity is the result of horizontal wind shear. Where a belt of strong winds lies alongside a belt of lighter winds, as in a jet stream for example, there will be rotation of the faster flow around the slower flow. Depending on the orientation of the faster flow to the slower flow, shear vorticity can be cyclonic or anticyclonic

560

What is vorticity due to earths rotation

Since the earth is turning, air has vorticity due to earths rotation even when the air is calm relative to the earths surface, i.e when relative vorticity is zero. Vorticity due to earths rotation is cyclonic regardless of Hemisphere

561

What is relative vorticity

Vorticity at any one point is the sum of curvature and shear components. Shear and curvature vorticity apply only to a flow of air relative to the earths surface and for that reason it is referred to as relative vorticity

562

What is absolute vorticity

When relative vorticity due to earths rotation at a given location and level are added together, the absolute vorticity is obtained

563

What is the predominant vorticity

Since vorticity due to earths rotation is predominant it follows that absolute vorticity is always cyclonic. It is the change in this absolute vorticity that plays such a large part in the formation and development of major weather systems such as anticyclones, depressions and fronts because vorticity changes will produce convergence or divergence in association with conservation of angular momentum

564

What is the energy of the airflow expressed as

Although air is not dense compared to solid structures, it has mass; many thousands of tons are involved as air flows around the curve of a trough or ridge line. The energy of the airflow is expressed by its momentum

565

For motion in a straight line, how is momentum measured

For motion in a straight line, momentum is measured as M x V (MV). It is a fundamental law of nature that momentum, like other forms of energy, is conserved - it remains unchanged unless energy is lost to or gained from outside sources

566

What happens when the flow of motion involves curvature

When the flow involves curvature, the momentum is measured as angular momentum and is given by the product of the mass, the angular velocity (rpm, indicated by w) and the square of the radius of the centre of gravity (CG) of the mass from the axis of rotation: M w r 2

567

What is the principle of conservation of angular momentum

The principle of conservation of angular momentum means that if one of the factors changes, then one or both of the others must change in the opposite sense to keep the total value constant. The importance of this understanding in terms of its relationship to vorticity is that a change in angular velocity (rpm) must affect vorticity (turning motion)

568

What is a good example of the conservation of angular momentum

The classic example of conservation of angular momentum is the ice skater spinning on a spot. When the arms are brought closer to the body (I.e radius of total mass decrease), angular velocity (rpm) increases to keep the total momentum constant

569

How can changes in rotational velocity as centre of mass is re-positioned can be put in the context of convergence and divergence

Bringing the mass closer towards the centre of rotation is mass convergence, which increases rotational velocity and therefore increases vorticity. Alternatively, moving the mass further away from the axis of rotation is mass divergence, which decreases rotational velocity and therefore decreases vorticity

570

What happens to angular velocity and vorticity when air is diverging and converging

When there is convergence in a rotating mass of air there will be an increased angular velocity (rpm) and therefore an increase in vorticity. And the reverse hold true also - when there is an increase in vorticity there must be increased angular velocity and therefore convergence

571

What is vorticity advection

Going back to the ice skater, rotational velocity (rpm) only changes when the arms are in the process of extension or contraction, in other words, when the arms are steady there is no change in rotational velocity. The same principle must apply to vorticity. Only when the radius of rotation of a parcel of air is changing - only when convergence or divergence are taking place - will vorticity change. This change in vorticity is known as vorticity advection

572

How does cyclonic flow create convergence aloft

When relative vorticity (curvature plus shear) happens to be cyclonic as well, absolute vorticity increases and there must then be mass convergence. Considering the principle of convergence, when convergence takes palace aloft, air will subside (in an anticyclonic system)

573

How does anticyclonic flow cause surface convergence

When relative vorticity is anticyclonic (or less cyclonic), absolute vorticity decreases and this decrease must be associated with divergence. When this divergence occurs at high altitude it sucks up air from low levels where surface friction often slows down the rate of surface convergence with the result that pressures near sea level reduce

574

What is the formation of the development of pressure systems and fronts

The formation and development of pressure systems and fronts are basically caused by speeding up or slowing down of air in upper level air streams and not by events taking place at sea level

575

What happens when high level divergence has resulted in the formation. And development of a depression and/or a front

Large quantities of low and mid tropospheric air are transported. When this involves movement into different latitudes, conservation of angular momentum will play an important role in causing this low or mid tropospheric air to either speed up or slow down (and increase or decrease its vorticity) depending on whether the migration is towards higher or lower latitudes. E.g low level air will experience increased vorticity if it is transported to higher latitudes because the radius on which it moves becomes smaller (towards the pole). The consequence of this is that low level air will converge so that it will ultimately rise

576

What are the two types of depressions found in the Southern Hemisphere

One is the mid latitude depression, which brings warm moist air from low latitudes. The other is the polar depression, which forms entirely in cold air

577

What is the mid latitude depression

This is the most common type of depression, which involves advection of warm moist air from the tropics and subtropics. When air moves towards the poles from tropical latitudes, the radius on which is rotates becomes smaller (and the poles the radius would have become zero)

578

How does the mid latitude depression form

Consistent with the principle of conservation of angular momentum, the angular velocity of the flow increases and convergence of low and mid troposphere air will cause this air to rise. When this tendency is reinforced by the shear and curvature components of vorticity, very strong upward vertical motion may result. The resulting cloud sheet will be enhanced by the release of latent heat from the condensing moisture so that an extensive cloud band is formed. Near the rear edge of this sheet there will be a temp contrast between the warm air from the north and the colder air to the rear of the trough. This is where the cold front will be drawn

579

How is the polar depression formed

When a southerly or southwesterly jet stream (the polar jet stream) moves over an upper trough, the curvature term (from the trough) and the shear term (from the jet stream) reinforce each other giving high values of cyclonic vorticity and hence high values of cyclonic vorticity advection (i.e change in vorticity). As the flow slows down rapidly downstream of the trough line there will be enhanced divergence aloft, accompanied by strong upward motion through the troposphere and convergence at low levels. This may result in the formation of an area of mainly cumulonimbus cloud typically in the shape of a comma. The rapidity of the process may mean that the upper divergence is not matched by the low level convergence and a small but intense depression may form

580

How is the polar depression shown on satellite imagery

The comma shaped cloud structure will occasionally be seen on satellite imagery. More often, the imagery will show speckled cloud patterns of cumulus and cumulonimbus in the cold air where the decrease of absolute cyclonic vorticity at high levels is not sufficiently strong to trigger off the complete process. The cloud pattern on satellite imagery is termed a CVA-max (cyclonic vorticity advection maximum)

581

What is the longitudinal limit to the position of mid latitude depressions

There is none

582

What does the extent of fronts depend on

The amount of moisture involved
The stability/instability of the rising warm air
The slope of the front
The speed of the front, and;
The contrast in temperatures across the front

583

What cloud tends to form in fronts

The cloud will be of the cu,ulus variety if rising air is unstable or if the rate at which it is forced to rise is great. Thus in steeply sloping fronts, such as in many cold fronts, it is most often found that cumulus cloud, including CB, is involved. In stable conditions or when the slope of the front is relatively shallow (as in many warm fronts), the presence of layer type cloud is the norm

584

What is the rough relationship between the geostrophic wind in the vicinity of fronts and fronts speed of travel

Cloud fronts travel at about 0.8 x speed of the wind and warm fronts at about 0.6 x the speed of the wind

585

What is the usual slope of the cold front

The slope of the interface between the cold and warm air (the front) is relatively steep, something in the order of 1-in-80 to 1-in-120. This means that for every 100m forward, the front rises about 1m

586

What causes the cold front to be a curved slope

The drag caused between the moving air and the surface slows down the lower part of the front while near the top of the front, the slope also becomes less steep so that the overall effect is a curved slope

587

What are greatly responsible for the division of cloud fronts into two types

Firstly the rate at which the rising warm air is being displaced i.e its vertical velocity (which is generally associated with its stability and degree of high level divergence) and secondly, the steepness of the frontal slope

588

What is a type 1 cold front (kata type)

This type normally has a mean slope in the order of 1-in-120, which may be considered shallow for a cold front. There is a general sinking of warm air at high levels (hence the term kata for katabatic) which causes the vertical cloud development to be somewhat restricted. While most cloud fronts are associated with cumulus and especially CB clouds, type 1 cold front has both stratiform and cumuliform clouds

589

What is the mean slope of the frontal interface of the type 1 cold front (kata type)

The mean slope of the frontal interface is somewhat shallower than the lowest part between the surface and some 5,000-6,000ft. This implies that the vertical velocity of the rising warm air from the surface is initially much more rapid that at greater altitudes. Thus stratocumulus and possibly cumulonimbus will form initially but as the frontal slope shallows and vertical velocity decreases, altostratus and nimbostratus will also be found

590

What precipitation is expected from the type 1 cold front (kata)

Since the slope is shallower aloft, the precipitation to be expected from the type 1 cold front is rather widespread. Some precipitation in the form of showers (possibly associated with hail and thunderstorms) takes place in advance of, and immediately behind the surface front while further behind the surface front, precipitation may involve rain from the overlying nimbostratus and altostratus

591

What happens if the cold air behind the precipitation band is unstable in the type 1 cold front (kata)

Cumulus cloud with associated showers will develop. However, if the cold air is not unstable or marginally so, skies should generally be clear with good visibility

592

What is the speed of the type 1 cold front

Around 20kts, which is slow for a cold front

593

What slope does the type 2 cold front have (ana)

It's slope is quite steep, somewhere in the region of 1-in-150 to 1-in-180, there is instability and rising of warm air at high levels (hence the term ana for anabatic), and the speed at which the front travels can be in excess of 30kts

594

What cloud is associated with the type 2 cold front (ana)

The associated cloud band is rather narrow and is often topped by an inversion near the tropopause, which causes cirrus in the shape of an anvil to develop. The predominant cloud is cumulus and cumulonimbus. It is therefore to be expected that thunderstorms and associated hail showers will be part and parcel of this front but being only some 100-150km wide, duration of precipitation will be relatively short lived

595

Do skies clear fast from the type 2 cold front

Often rapid clearing skies behind this type of cold front. The rapid change from heavy showers and thunderstorms to brilliantly blue skies can be dramatic

596

What do conditions behind the type 2 cold front depend on

Conditions behind the cold front depend to a large extent on the stability of the cold air. If the flow of air has cyclonic vorticity, the air behind the front will be unstable, with cumulus and cumulonimbus cloud and showers

597

What's the usual slope of the warm front

The slope of most warm fronts is in the order of some 1-in-150 to 1-in-180, quite shallow. In addition to this, the speed at which warm fronts travel is generally around 10-15kts, slower than cold fronts

598

What happens if the advancing air is unstable in a warm front

If the advancing warm air is unstable and consistently rises relative to the frontal interface (ana type lifting), it is likely that a combination of layer and cumuliform clouds are present. However, if the warm air is stable and the air sinks at high levels, cloud is predominantly stratiform of reduced vertical extent. It is therefore convenient to subdivide warm fronts into stable kata type and unstable ana types

599

What clouds will the approach of a kata type warm front have

With the stable kata type warm front, the approach of the front is heralded by cirrus and cirrostratus, which may appear as anything up to 1,000km in advance of the surface front (which is that part of the front actually in contact with the surface). As the front nears the observer, the cloud layer thickens into a combination of cirrostratus and altostratus. Finally, the sector of the front within some 4-500km from the surface front combines stratiform cloud from the surface up i.e nimbostratus

600

What precipitation can be found from a warm front

Precipitation may fall from each type of cloud but any falling from high level layers will evaporate before reaching the ground. However, as the cloud base lowers, light rain will initially be experienced at the observers station, gradually turning into more heavy and persistent rain. Depending on the altitude of the freezing level, snow and/or sleep may be involved

601

What types of conditions can be expected with an ana-type warm front

If the advancing and ascending warm air is unstable in whole or in part, the ana-type warm front results. Cloud development is generally similar to that found in the kata type but cumulus cold, including cumulonimbus, is embedded. Thus precipitation, especially within the first 3-400 or so kilometres in advance of the surface front, will include both rain and heavy showers. In some very unstable situations, thunderstorms and hail may be experienced

602

What is the cold sector like below the frontal interface of a warm front

The cold sector air below the frontal interface will become very moist as the result of large amounts of precipitation penetrating from above. The fact that the air is already cold and that evaporation of some of the precipitation demands latent heat, adds up to a situation where almost 100% relative humidity is readily reached. Thus the air in advance of the surface front will have very poor visibility and this will be made worse by development of low cloud, which will form when saturation takes place. Indeed, in the low level triangular sector between the surface and the frontal slope, frontal fog may well be experienced

603

Why will the air behind the warm front have substantial amounts of residual cloud

The surface will have received large amounts of precipitation over some 24-36 hours of persistent rain and showers. Thus evaporation from these water areas will add to the moisture content of the air and development of cloud is very likely, most often low level and surprisingly extensive. Secondly, the warm air behind the front will invariably have come from a region of relatively high temp and high moisture content. Thus substantial low cloud with drizzle and even some fog can be expected. In the case of stable kata type warm front, conditions improve some heat quicker once the surface front has passed compared to an unstable ana type warm front

604

What are the characteristics of a stationary front

Stationary fronts are weak and often represents the remains of a decaying frontal system. Thus not a great deal of cloud, wind or weather is associated with them

605

What is a type 1 cold front (kata type)

This type normally has a mean slope in the order of 1-in-120, which may be considered shallow for a cold front. There is a general sinking of warm air at high levels (hence the term kata for katabatic) which causes the vertical cloud development to be somewhat restricted. While most cloud fronts are associated with cumulus and especially CB clouds, type 1 cold front has both stratiform and cumuliform clouds

606

What is the mean slope of the frontal interface of the type 1 cold front (kata type)

The mean slope of the frontal interface is somewhat shallower than the lowest part between the surface and some 5,000-6,000ft. This implies that the vertical velocity of the rising warm air from the surface is initially much more rapid that at greater altitudes. Thus stratocumulus and possibly cumulonimbus will form initially but as the frontal slope shallows and vertical velocity decreases, altostratus and nimbostratus will also be found

607

What precipitation is expected from the type 1 cold front (kata)

Since the slope is shallower aloft, the precipitation to be expected from the type 1 cold front is rather widespread. Some precipitation in the form of showers (possibly associated with hail and thunderstorms) takes place in advance of, and immediately behind the surface front while further behind the surface front, precipitation may involve rain from the overlying nimbostratus and altostratus

608

What happens if the cold air behind the precipitation band is unstable in the type 1 cold front (kata)

Cumulus cloud with associated showers will develop. However, if the cold air is not unstable or marginally so, skies should generally be clear with good visibility

609

What is the speed of the type 1 cold front

Around 20kts, which is slow for a cold front

610

What slope does the type 2 cold front have (ana)

It's slope is quite steep, somewhere in the region of 1-in-150 to 1-in-180, there is instability and rising of warm air at high levels (hence the term ana for anabatic), and the speed at which the front travels can be in excess of 30kts

611

What cloud is associated with the type 2 cold front (ana)

The associated cloud band is rather narrow and is often topped by an inversion near the tropopause, which causes cirrus in the shape of an anvil to develop. The predominant cloud is cumulus and cumulonimbus. It is therefore to be expected that thunderstorms and associated hail showers will be part and parcel of this front but being only some 100-150km wide, duration of precipitation will be relatively short lived

612

Do skies clear fast from the type 2 cold front

Often rapid clearing skies behind this type of cold front. The rapid change from heavy showers and thunderstorms to brilliantly blue skies can be dramatic

613

What do conditions behind the type 2 cold front depend on

Conditions behind the cold front depend to a large extent on the stability of the cold air. If the flow of air has cyclonic vorticity, the air behind the front will be unstable, with cumulus and cumulonimbus cloud and showers

614

What's the usual slope of the warm front

The slope of most warm fronts is in the order of some 1-in-150 to 1-in-180, quite shallow. In addition to this, the speed at which warm fronts travel is generally around 10-15kts, slower than cold fronts

615

What happens if the advancing air is unstable in a warm front

If the advancing warm air is unstable and consistently rises relative to the frontal interface (ana type lifting), it is likely that a combination of layer and cumuliform clouds are present. However, if the warm air is stable and the air sinks at high levels, cloud is predominantly stratiform of reduced vertical extent. It is therefore convenient to subdivide warm fronts into stable kata type and unstable ana types

616

What clouds will the approach of a kata type warm front have

With the stable kata type warm front, the approach of the front is heralded by cirrus and cirrostratus, which may appear as anything up to 1,000km in advance of the surface front (which is that part of the front actually in contact with the surface). As the front nears the observer, the cloud layer thickens into a combination of cirrostratus and altostratus. Finally, the sector of the front within some 4-500km from the surface front combines stratiform cloud from the surface up i.e nimbostratus

617

What precipitation can be found from a warm front

Precipitation may fall from each type of cloud but any falling from high level layers will evaporate before reaching the ground. However, as the cloud base lowers, light rain will initially be experienced at the observers station, gradually turning into more heavy and persistent rain. Depending on the altitude of the freezing level, snow and/or sleep may be involved

618

What types of conditions can be expected with an ana-type warm front

If the advancing and ascending warm air is unstable in whole or in part, the ana-type warm front results. Cloud development is generally similar to that found in the kata type but cumulus cold, including cumulonimbus, is embedded. Thus precipitation, especially within the first 3-400 or so kilometres in advance of the surface front, will include both rain and heavy showers. In some very unstable situations, thunderstorms and hail may be experienced

619

What is the cold sector like below the frontal interface of a warm front

The cold sector air below the frontal interface will become very moist as the result of large amounts of precipitation penetrating from above. The fact that the air is already cold and that evaporation of some of the precipitation demands latent heat, adds up to a situation where almost 100% relative humidity is readily reached. Thus the air in advance of the surface front will have very poor visibility and this will be made worse by development of low cloud, which will form when saturation takes place. Indeed, in the low level triangular sector between the surface and the frontal slope, frontal fog may well be experienced

620

Why will the air behind the warm front have substantial amounts of residual cloud

The surface will have received large amounts of precipitation over some 24-36 hours of persistent rain and showers. Thus evaporation from these water areas will add to the moisture content of the air and development of cloud is very likely, most often low level and surprisingly extensive. Secondly, the warm air behind the front will invariably have come from a region of relatively high temp and high moisture content. Thus substantial low cloud with drizzle and even some fog can be expected. In the case of stable kata type warm front, conditions improve some heat quicker once the surface front has passed compared to an unstable ana type warm front

621

What are the characteristics of a stationary front

Stationary fronts are weak and often represents the remains of a decaying frontal system. Thus not a great deal of cloud, wind or weather is associated with them

622

What are the main requirements for the formation of an orographic depression

A mountain range of substantial size and a wind more or less at right angles to it. Some of the air pushed onto the range will deflect around the outside edges, i.e the airflow splits up at the windward side of the barrier. Some air however, will flow over the top of the range and through the inevitable valleys

623

Where are high and low pressures found in an orographic depression

The air on the windward side is somewhat compressed causing a localised high pressure area. On the lee side, especially at sea level, a suction effect (and therefore convergence) takes place which causes atmospheric pressure to reduce. Thus a low pressure area forms on the lee side with winds flowing clockwise around it. It must be noted that this depression rarely extends to great heights and never higher than mountain top height

624

What weather is typically associated with an orographic depression

Generally clear of cloud, a gusting wind and possibly a fog effect producing warm temperatures. If cloud does appear, it is likely to be of minor extent and generally associated with orographic cloud formed over the mountains and drifting downwind. Similarly, some cloud may drift through valleys and penetrate into the lee side

625

What happens during an orographic depression if a frontal band approaches more or less parallel to the mountain range

The lower part of this front will lag and the formation of a lee depression will be much encouraged. If the low level air on the lee side is warm, it follows that the higher frontal sector, unobstructed by the mountain range, will be much colder than the low level air. Thus a steepening of the ELR on the lee side will produce substantial, and sometime severe instability

626

What is a secondary effect of a mountain range on the passage of a front during an orographic depression

Is to cause the front at the range to stop while the parts beyond either end travel on. A good example of this is the effect of the Southern alps on a cold front travelling from west to east

627

Which side of the mountains intensifies the weather during an orographic depression

Mountain ranges generally tend to intensify the weather (and fronts) on the windward side and diminish the effects on the lee side

628

What is a thermal (heat type) depression

When land masses warm in summer, the associated rising of air will produce a circulation similar to a huge sea breeze with low level pressures reducing sharply. This low pressure causes surface convergence and due to coriolis influence, clockwise air movement will result

629

How high can the thermal (heat type) depression get to

Depending on the degree of surface heating, the top of the depression can be a mere few thousand feet in temperate latitudes on a fine summers day but many thousands of feet in some other situations e.g parts of the Australian interior in summer time

630

What happens in mild situations of the thermal (heat type) depression

In a mild situation (e.g a summer sea breeze over large coastal plains such as Canterbury), the wind which should cross the coast at right angles, will actually cross at an angle much less than 90 degrees to the circulation associated with the thermal low. It is not unusual to have a northeasterly wind in areas near the coast while well inland the wind can be northwest. The weather in these situations involves various amounts of CU but will little or no precipitation

631

What is referred to as a complex trough

Often it is found that frontal waves develop secondary depressions involving three or four smaller disturbances. Each depression advanced in more or less the same direction as the master depression

632

What are freezing nuclei

Freezing nuclei are relatively rare. It is generally considered that freezing nuclei come from fine soil particles and volcanic dust

633

What is latent heat of fusion

Latent heat of fusion is heat energy released when water changes its stable state

634

What is the unit of heat energy

The unit of heat energy is the joule (previously the calorie). By definition, 4.18 joules (1 calorie) of heat energy are required when the temperature of one gram of water is to be raised by one degree Celsius. Again, the reverse applies in that cooling of one gram of water by one degree Celsius will release 4.18 joules (1 calorie) of heat energy

635

What is the difference between changing temperature and changing state

To change temperature of water, each gram involves 4.18 joules (1 calorie) of heat energy per degree of change. However, if the state of water is to be altered, the number or joules involved is vastly more. For example, water liquid changing into ice through the freezing process releases 334.4 joules (80 calories) of heat energy

636

What is the release of latent heat at a temp -1°C to -80°C

At a temperature between -1°C (where into a tiny part freezes initially) and -80°C (where all the water freezes initially), the released latent heat of fusion is instrument, partly, in the freezing process of a portion of the supercooled droplet and the remainder warms the partly frozen drop

637

How does the freezing process between -1°C to -80°C work

1/80th (4.18/334.4) of the drop freezes instantaneously for every degree Celsius that the supercooled drop is below 0°C and the remainder of the heat energy is dissipated in raising the temperature of the partly frozen drop resulting in heat losses through evaporation and conduction

638

What is the process of conduction with ice

The less cold the supercooled droplets, I.e the closer they are to 0°C, the smaller the part that freezes on impact and the larger the portion that flows back

639

What is clear ice

Supercooled water drops are large, if drops are small, the amount of liquid water flowing back is less.
Temperature between 0°C and -15°C

640

What are the dangers of clear ice

Extra weight involved, icing development may not becomes apparent quickly. Clear ice is hard. May freeze controls if allowed to flow back. If flight enters areas where the temp is above 0°C, clear ice will likely break off the aircraft in large segments and damage may be caused as a result

641

What cloud is clear ice most likely to form in

Flight in cumulus cloud within the first 6,000-8,000ft above freezing level will be very prone to clear ice accretion. The reason for this is that the up and down draughts as well as turbulence within this type of cloud are very likely to cause growth of drops while an ELR close to the SALR involves about 10,000ft between the freezing level and the -15°C level

642

What's the most dangerous clouds for clear ice

Cumulus and especially CB development in frontal situations will be dangerous particularly if the aircrafts track is not at right angles to the front

643

What is rime ice

When small supercooled water droplets are disturbed, the amount of water flowing back (freezing as it flows back) will be relatively limited. If, in addition to that the temperature of the droplets is colder than some -15°C, then freezing of the droplets is practically instantaneous. As a result, numerous pockets of air will be trapped within the accumulating ice build up, which gives the ice a milky, opaque appearance

644

What are the characteristics of rime ice

Rime ice is rough and uneven in shape. As it builds up on frontal areas such as leading edges and so on, the aerodynamic qualities will be affected. If rime ice develops as far back as hinge lines, the risk of control locking is less than with clear ice because a few rapid movements of the controls concerned will break the ice

645

What cloud is most associated with rime ice

Cloud most likely associated with rime ice is stratiform cloud because the usual lack of turbulence within it will minimise collision and coalescence so that droplets remain small. Stratiform cloud associated with a warm front will often have some cumulus clouds embedded within it so that one must always be vigilant in anticipating conditions and likely type of icing

646

When does hoar frost form

When the temperature of the skin of an aircraft is colder than 0°C and the environment air is moist but not necessarily saturated, deposition may occur. This involves the process of water vapour directly changing into the ice state without going through the liquid state

647

What are the dangers of hoar frost

The boundary layer around lifting surfaces is obviously affected while the all up weight of the aircraft can be substantially increased

648

When can hoar frost form

Hoar frost can form on an aircraft in flight whenever the skin of the aircraft is very cold and the flight path takes the aircraft rapidly into warmer and moist air. Aircraft don't need to be in cloud for this type of ice to form. It may instantly clover the windscreen so that vision is lost at a critical time

649

What is freezing rain

When a warm layer of cloud lies above air which has a temperature of less than 0°C and rain falls from this warm cloud into the cold zone, an aircraft can be covered in ice from freezing rain in a matter of minutes

650

What are typical conditions for freezing rain to occur

Typical conditions for this happening in flight the cold sector beneath the slope of a warm front or in the cold sector just behind a front. Alternatively, the presence of a strong inversion intersecting the freezing level can sometime produce the same results

651

What's the best remedy to freezing rain

If freezing rain conditions are encountered, the best remedy is to vacate the area forthwith by either descending, climbing or altering heading

652

What happens when dry snow is in countered during flight

It will not generally adhere to aircraft parts. However, when supercooled water is mixed with snow, the combination will increase the rate at which ice accumulates

653

How is snow a problem for aircraft on the ground

Snow has proved to be a serious problem for aircraft on the ground waiting for take off. Even light snow accumulation can be sufficiently severe for aircraft to be placed at risk during the take of and initial climb out phase

654

What is hail defined as

Hail is defined as precipitation of circular hard pellets of ice that fall from cumulonimbus cloud

655

What do hail stones consist of

Close examination of a hail stone indicates that it consists of a number of layers of clear and rime ice which supports the theory that hail is formed as the result of strong up and down draughts in which a formative stone is repeatedly exposed to temperatures ranging from positive to grossly negative

656

What part of the cb is hail restricted to

Hail formation is generally restricted to only part of a cloud cell so that aircraft in flight only experience relatively brief encounters

657

What does soft hail mainly consist of

Soft hail consists mainly of rime ice, which has formed in clouds that have limited water content

658

What factors influence rate of ice accretion

Cloud water content, aircraft characteristics, kinetic energy and adiabatic heating

659

What are the effects of cloud water content on ice accretion

In mid latitudes, icing will be more severe in summer than in winter. Flight above the freezing level in polar regions is less likely to produce high accretion rates. The very low temperatures encountered in polar regions often produce rime ice or dry ice crystals, which rarely adhere in sufficient quantities to cause serious problems

660

Where are areas on an aircraft that increase ice accretion

Areas where pressures decrease due to higher flow velocity, such as over the wing or around a pointed nose cone, will experience a drop in temperature and a better opportunity for ice to form

661

What speed increase ice accretion

The higher the aircraft speed the greater the rate of accretion but this is generally applicable only to aircraft that have a speed range below 200-250kt mark

662

How does kinetic heating effect ice accretion

Friction between the aircraft skin and passing air molecules causes the skin to heat. Clearly, the higher the aircraft speed, the greater the temperature increase. At about 500kt, skin temperature can increase by some 25°C

663

How does adiabatic heating effect ice accretion

In accordance with bernoullis theorem, a fluid or airflow slowed down or brought it rest will increase in pressure and an associated increase in temperature is then involved. Thus, any parts or components of aircraft, which tend to slow the adjacent airflow down, will experience this warming and through that reduce or avoid icing

664

Which ice increases drag more

Rime ice may be worse that clear ice due to its ragged nature

665

How does ice distort aerodynamic shape

Aerofoil shapes can be badly distorted by a cover of ice (especially rime ice) so that production of lift can be severely reduced (and drag increases)

666

How does ice increase stall speed

Aerodynamic distortion and increase in all up weight have a dramatic influence in increasing an aircrafts stall speed

667

How does ice cause a loss of thrust

It is highly unlikely for ice accretion to be the same on all blades, thus an unbalancing factor is introduced

668

What will happen if ice blocks pitot/static blockage

Flight in icing conditions will almost certainly block the aerodynamic pitot intake (if not heated) so that airspeed readings are in error. Under certain conditions the static intake may also block which will then deprive the crew from useful readings from all pressure instruments

669

How does ice cause a loss of aerials

Especially in older type aircrafts, which do not have the modern smaller and sturdier aerials, ice accretion may be sufficient to break them off and deprive the crew of radio or navigation aid reception/transmission

670

How does icing effect fuel tank vents

Often these bends are built into the fuel cap and when blocked, ambient pressure will be prevented from entering the fuel tank and fuel starvation will soon follow

671

What are mechanical de icing methods

Normally referred to as de icing boots and involves rubber or plastic tubing manufactured within rubber or plastic skins installed on leading edges of wings, tailplanes or fins

672

What is the fluid de icing method

Fluids such as alcohol and glycol which have a freezing temperature well below 0°C may be used to combat icing. Strategically placed nozzles eject spray over parts of the aircraft most vulnerable to icing

673

What are the disadvantages of the fluid de icing method

Weight of the fluid and when fluid has been exhausted the aircrafts capacity to combat ice has been eliminated. Some of the fluids are night corrosive

674

What is the thermal de icing method

Perhaps the most common method, involves either electrical heating through heat coils and pads or through ducting of hot air over affected aircraft parts. Electrical elements are used for items such as pitot tubes, windscreens and often, propeller blades. If the required equipment is not fitted, has malfunctioned or exhausted, flight below the freezing level will remove it

675

Is there icing problems at high altitude

Flight at high altitude will involve either dry ice crystals or greatly reduced water contents which means that icing problems there are virtually nil

676

What causes carburettor icing

This ice problem is caused by two fundamental processes: the reduction of temperature when air is de pressurised and the reduction of temperature due to demand for latent heat when fuel vapourises

677

What is throttle icing

Any moisture present will turn to ice and cling to the various components found in side the Venturi

678

What is refrigeration icing

When fuel is injected into the air stream flowing through the Venturi, it vapourises and a demand for latent heat occurs. The associated reduction in mixture temperature can cause ice to form, which is referred to as refrigeration icing

679

What temperatures is carburettor icing found

OAT +25°C and -15°C

680

What's a common method for clearing carburettor icing

The ice of carburettor heat, supplied by hot air ducted from around the exhaust manifold

681

What are the main icing problems for turbine engines

The main problem with turbine engines is the build up of ice on and near intakes, which can lead to compressor stall and flame out

682

Which clouds do not contain supercooled water droplets

Cirrus, cirrostratus and cirrocumulus

683

Which clouds contain super cooled water

Middle and low level clouds

684

Which clouds have the presence of liquid water and ice

Middle and low level clouds

685

Which clouds have no presence of liquid water and ice

High level clouds

686

Which clouds have nil icing

Cirrus, cirrostratus and cirrocumulus

687

Which clouds have light/moderate icing

Altostratus, altocumulus, stratocumulus and cumulus

688

Which cloud has light icing

Stratus

689

Which clouds have light/moderate/severe icing

Cumulonimbus and nimbostratus

690

What icing is altostratus associated with

Altostratus is usually associated with rime ice although within the first 5,000ft above the freezing level (where the mean temperature is between freezing and -10°C) some clear ice may encountered

691

What icing does altocumulus cause

Altocumulus can similarly produce clear ice but due to the generally larger drop size, the most likely zone is the layer from the freezing level to the -15°C level

692

When do CB' have moderate to severe icing

CB in the growing and mature cells can have moderate to severe icing up to the -15°C level but records show occasional severe icing conditions in vigorously growing cells at levels of -20°C. Summertime and tropical CBs are the worst in these respects

693

What icing is associated with nimbostratus

NS can produce moderate to severe clear ice within the first few thousand feet above the freezing level and this likelihood is enhanced when the water content of the cloud is high such as in tropical or summer nimbostratus development or when orographic causes are involved. The fact that NS is generally widespread points to a high amount of ice accretion because the aircraft is in the cloud for a longer period of time

694

How does cloud type and ice accretion depend largely on latitudes and time of year

For instance, in mid-high latitudes in winter, when the freezing level is below ground or sea level, even stratus cloud can produce a light coating of ice. Ice and snow can be found in large quantities in AS, AC and CU cloud developed in and around mountainous terrain in winter and sometimes even in summer

695

What is classified light icing

Minor coverage of aircraft parts by either rime or clear ice. Airspeed loss is very small or nil and, in the absence of a deterioration in ice accretion, no change or altitude or track is needed

696

What is classified moderate icing

Substantial build up of clear or rime ice on wings and other major aircraft parts. Airspeed loss may become sufficient to cause concern. The safety of the aircraft may be jeopardised if rate of accretion increases or is anti icing equipment is not coping adequately. It may be desirable to alter altitude or heading

697

What is classified severe icing

Accretion is sufficiently great to endanger the aircrafts performance. Change of altitude and/or heading are deemed essential if anti icing equipment is to adequately coping with the problem

698

What is this Hadley cell

The tropics generally are regions where the air is tending to ascend due to the intense heating under a sun, which at some time of the year will be vertically overhead. The rising air spreads pole wards at high levels but because of the coriolis effect it does not travel all the way to the poles and as a consequence it sinks towards the surface in the belt of sub tropical highs in each Hemisphere. The surface air then moves towards the equator as the trade winds, blowing from the southeast and the northeast in the southern and northern hemispheres respectively

699

What are the doldrums

A zone of ascending air within the regions at or near the meteorological equator caused by convective action from warm surfaces. These regions contain the equatorial trough and are referred to as the doldrums

700

What are the horse latitudes

Subsiding air within the regions covered by the semi stationary anticyclones centred at average latitudes 30°N and 30°S. these regions are referred to as the horse latitudes

701

What convergence and divergence is found in the Hadley cell

High level divergence over equatorial regions and high level convergence over the anticyclonic regions

702

What are the trade winds

Low level air flows (winds) from the anti cyclonic regions towards the equatorial trough. These winds are known as the trade winds

703

What are the thermal and meteorological equator

The slanted orientation of the earths spin axis causes maximum insolation from a vertically overhead sun to oscillate annually either side of the geographical equator, giving rise to the opposing seasons of each Hemisphere. Thus the terms thermal equator or meteorological equator were introduced to seperate the geographical equator from those latitudes where solar input is temporarily a maximum

704

What establishes the equatorial trough

The heated surfaces at or near the thermal equator obviously produce a high degree of instability and convection so that the main thrust of air movement within the affected regions is upwards. This results in the establishment of the equatorial trough, a system of comparative low pressure and surface convergence

705

Where is maximum convergence, convection and cloud development normally located

A few degrees of latitude away from the equatorial trough on the geographical equator

706

Is the equatorial trough and ITCZ permanent features

The equatorial trough is a more or less permanent feature of convergence and convection whereas the ITCZ is an erratic feature, the presence and intensity of which depends on the variable degree of trade wind convergence. Put differently, the equatorial trough is a general term for the belt of comparatively low pressure around the globe in the tropics. The ITCZ is a synoptic weather feature within this trough

707

What are winds like in the doldrums

The latitudes covered by the equatorial trough are known as the doldrums. Within these areas, winds are generally light and variable mainly due to the fact that the predominant movement of air is upward rather than horizontal. However, the convergence associated with doldrums is not of a constant nature

708

What is the seasonal location of the equatorial trough and ITCZ in the northern summer (may to October)

The entire trough is located in the northern hemisphere but, due to the strongly differing nature of the surface, the latitudinal change in position and intensity of the trough varies greatly

709

What happens to the equatorial trough over oceanic regions

Over the major oceanic regions, the equatorial trough is fairly constant in terms of latitude but the influence the continental land masses, their differing mountainous natures and local pressure systems cause the trough to shift considerably and even fracture in a few localities

710

What is the seasonal location of the equatorial trough and ITCZ in the Southern summer (November to April)

There is a totally different picture. A large deflection takes the trough south through South Africa while a smaller wave brushes over the northern parts of Australia for similar reasons as stated for the northern hemisphere i.e influence of variations in surface characteristics

711

What is the major reason for the equatorial trough staying in the northern hemisphere over the central pacific and Atlantic oceans

The major difference is the position of the trough in the central pacific and Atlantic oceans where the equatorial trough remains in the northern hemisphere. The main reasons for this are firstly, stronger trade wind strength in the northern compared to the Southern Hemisphere and secondly, the influence of oceanic surface temperatures

712

What weather is associated with an active ITCZ

Considering the high moisture content of air especially when the ITCZ is positioned over oceanic regions, any lifting will almost immediately involve the SALR when air saturates at low levels. Thus the zone becomes associated with massive cloud developments, which can involve a horizontal width of some 5-600k. And a depth of some 50,000-60,000ft

713

What clouds are found in an active ITCZ

Cloud consists mainly of the cumulus variety such as CU, CB, and AC but also some stratiform cloud at high levels (CS) where tops of CB clouds reach a balance with the surrounding cold air. It is not uncommon for tops of CBs to exceed the height of the equatorial tropopause through simple momentum

714

What are the main dangers with a active ITCZ

Turbulence, icing and other factors associated with CB activity

715

What are the dangers of turbulence with an active ITCZ

Can be moderate to severe. It will be difficult to avoid this phenomenon unless the flight path can be maintained above the tops of CBs. Heavy precipitation associated with the cloud presence will strongly reduce visibility and especially when flying over ocean areas, the horizon will appear merged with the cloud base. Depending on the stage of cell development, strong up or downdraughts can be expected below the cloud base.
Strong convergence below growing cells will reduce ambient pressure with the altimeter over reading

716

What's the risk of icing with an active ITCZ

The high water content experienced in ITCZ cloud developments poses a risk of moderate to severe icing above the freezing level and especially within the first 10,000-15,000ft above it

717

What are other CB factors with an active ITCZ

Exposure to dangers associated with thunderstorms e.g wind squalls, lightening strikes etc. when the ITCZ extends over continental areas, its charcoal will be influenced by the nature of the land, the temperature differences of the air masses either side of the zone and the presence of major airflows

718

What weather is associated with an inactive ITCZ

The zine is associated with various oktas of cumulus and some shower activity from the more developed clouds. Winds are light and variable

719

What is the South Pacific convergence zone (SPCZ)

A zone of intense cloudiness due to convergence of two major airflows is frequently identified from the eastern tip of Papua New Guinea to slightly southeast of French Polynesia. This is known as the South Pacific convergence zone, which like the ITCZ, varies in intensity

720

What converging airflows from two distinct systems caused the SPCZ

The semi stationary anticyclonic system in the eastern Pacific. The easterlies on the equator side of this system turn north easterly on approaching the western Pacific; and,
The anticyclones originating and travelling eastward from the Australia/New Zealand regions. The south easterlies preceding these systems extend to the region where SPCZ forms

721

When is the SPCZ more active

When the north easterlies and the south easterlies meet they form a convergence zone with weather characteristics very similar to the ITCZ. The zone is mor active in summer than winter

722

What front tends to strengthen the SPCZ

The SPCZ appears to be associated with the northeast passage of mid latitude cold fronts when the cloud band of the latter merges with that of the convergence zone. Thus cloud fronts appear to strengthen or re-activate the SPCZ

723

Where is the northern limit of the SPCZ

The northern limit of the SPCZ is often taken to lie over Papua New Guinea and fluctuations in its intensity may bring cloudiness and rainfall down into Australians Northern Territory and northern Queensland from time to time. It is thus occurrence that may seem to indicate incorrectly that the ITCZ extends into Australia

724

What is the strength of trade winds

Trade winds are quite steady and persistent in strength. In the pacific region they vary throughout the year from 10-20kts with the occasional increase to 30kts. In global terms, trade winds are generally slightly stronger in winter than summer. The winds above the trade winds are usually from the west, often light

725

Are there variations in trade winds

Although their direction in the Southern Hemisphere is mainly from the south-east there are minor variations in direction due to underlying terrain (islands versus ocean for instance) and pressure gradient fluctuations

726

What is the weather in the trade wind zone influenced by

The weather, especially at the source of the trade wind zone, is for a large part influenced by subsidence inversions, which vary in height from 6,000-8,000ft above sea level

727

What's conditions below the inversion in the trade wind zone like

Below the inversion, visibility is usually fair to good with varying amounts of cumulus with bases between 2,000-3,000ft and tops at some 8,000ft. These amounts and vertical limits are prone to increase when trade winds cross the windward side of islands so that increased wind speeds and shower activity are likely in those localities

728

What are trade winds converging at the equatorial trough subject to

Directional change depending on season. For instance, in mid Pacific, the trough never enters the Southern Hemisphere, which means that the south-east trade winds (of the Southern Hemisphere) cross the equator to flow to the trough (in the northern hemisphere)

729

Where is the equatorial trough during the Southern winter

In the northern hemisphere north of Australia so that trade winds in that region also cross the equator. However, in summer time in Australia, the trough is south of the equator so that the northern hemisphere trade winds will have to do the crossing

730

What happens when the south-east trade winds cross the equator en-rout to the equatorial trough in the northern hemisphere

They will be subjected to the coriolis force applicable to the northern hemisphere i.e veers to the right. Admittedly, the strength of the force is weak but present nonetheless. Therefore, after crossing the equator, the trade winds blow initially towards the trough from the south and ultimately from the count west

731

What happens to the north-east trade winds from the northern hemisphere crossing the equator towards the trough near northern Australia in summer

Will initially become northerly and ultimately north-west. Arriving over land after travel over very warm waters, these northwest winds (or from other northerly quarters depending on local topography) can produce some intense cloud developments in coastal regions. This, coupled with the activity of the equatorial trough, a heat low and occasionally the SPCZ, produces the wet season over the northern parts of the continent between December and march

732

What is the influence of islands during the summer

In summertime the land of major islands warms to a greater degree than the surrounding seas so that sea breeze effects develop over the land. Consistent with this, air pressure will be slightly lower over the land than over the sea and instability increases. Thus coastal regions exposed to the southeast experience an increase in cloud development and possibly heavy precipitation depending on the nature of terrain. In some situations this effect will be felt even at the lee side of islands

733

What are the influences of islands in winter time

The land is colder than the surrounding seas so that pressures are somewhat higher inland and stability is enhanced. Although seem cloudiness may still occur at the southeast coasts from exposure to the trade winds, generally conditions are much drier and more stable

734

What are the main disturbances in the tropical circulation

Individual cumulus, mesoscale convective areas (MCAs) wave disturbances, monsoons; and, tropical cyclones

735

How do individual cumulus disturb tropical circulation

These are the smallest disturbances, measured in a life span of only a few hours and a dimension some 5km across, resulting from turbulence experienced in the trade wind friction layer. These individual clouds often align themselves in cloud streets more or less parallel to the wind direction, or they are arranged in honeycomb patterns

736

How does the mesoscale convective areas (MCAs) disturb the tropical circulation

These are systems comprising of grouped clusters of cumulus and CB developments within areas from 100km-1,000km across. MCAs are characterised by instability through deep layers - they can often be part of the equatorial trough. Although many MCAs last no longer than one or two days, others can persist for extended periods of time

737

How do wave disturbances effect tropical circulation

Disturbances demonstrate themselves in wave form in the equatorial east elise where waves travel from east to the west but not necessarily at the same speed as the trade winds

738

Where do wave disturbances originate from

The formation of a wave generally originates some two to four degrees of latitude on the pole ward side of the equatorial trough. Of all the wave disturbances that form in the tropics throughout a year, approximately 25% develop to the tropical depression stage while some 10% become named tropical cyclones. Easterly waves are more frequent in the northern hemisphere than in the South Pacific

739

How long do easterly waves associated with wave disturbances last

Easterly waves normally have a life span of about 10 days, have an average wavelength of some 3,000km, travel approximately six degrees in longitude per day and slope eastward with height

740

What is the wet monsoon

When large land masses of the world heat during their summer season, intense instability and convection cause air to rise so that replacement air from the oceans travel towards the land and island. The great water content of this air produces large cumulus developments so that thunderstorms, heavy precipitation and squalls result. Mountains barriers accentuate the development. This reflects the large scale wind regime of the wet monsoon, which may well involve winds contradicting the prevailing wind

741

What is the dry monsoon

When landmasses cool in winter the reverse development takes place. Subsidence on a grand scale over the land causes air to flow outward towards the oceans, which have retained a great deal of their stored heat. Thus the weather over the land is generally clear due to adiabatic warming. This reflects the dry monsoon

742

Where are the major monsoon regions found

With the exception perhaps of the north Australian continent, the major monsoon regions are found in the northern hemisphere. The most developed monsoon winds are concentrated along the south Asian continent when in summer time the transport of moist air from the Indian Ocean causes widespread precipitation and cloudiness in India, Pakistan, Myanmar etc. the wet season begins around June and ends towards late September. In the case of India for instance, about 75% of its annual rainfall is concentrated within this monsoon period

743

Where a monsoon depressions a frequent occurrence

In the Bay of Bengal region. These are systems extending up to some 25,000ft and lasting about three days on average. These depressions have a strong influence on the distribution of monsoonal rain

744

What's the predominant wind direction of the south Asian monsoon

From the south-southwest but local topography has strong influences

745

What are the characteristics of the Australian monsoon

In the Australian situation, mention was mad before of the influence of the equatorial trough, the South Pacific convergence zone and the northwest winds on the northern parts of the continent during summer. The wet monsoon takes effect in December and peaks during February. During this period, heavy precipitation and thundery conditions may prevail for many days. In the dry winter season, the weather in northern Australia is governed by the southeast trade winds, which are generally dry

746

What are tropical cyclones

Tropical cyclones are low pressure systems that from over warm tropical waters and have gale force winds (sustained winds of 34kts or greater and gusts in excess of 49kts) near the centre. Technically they are defined as a warm core, non frontal low pressure system of synoptic scale developing over warm waters having organised deep convection and a maximum mean wind speed of 34kts of greater extending more than halfway around near the centre and persisting for at least 6 hours

747

What is considered a severe tropical cyclone

The gale force winds can extend hundreds of kilometres from the cyclone centre. If the sustained winds around the centre reach 64kts (gusts in excess 89kts), then the system is called a severe tropical cyclone

748

What is the diameter of the tropical cyclone eye

The circular eye or centre of a tropical cyclone is an area characterised by light winds and often by clear skies. Eye diameters are typically 40km but can range from under 10km to over 100km. The eye is surrounded by a dense ring of cloud about 16km high known as the eye wall which marks the belt of strongest winds and heaviest rainfall

749

What temperature does the sea need to be for a tropical cyclone

Tropical cyclones derive their energy from the warm tropical oceans and do not from unless the sea surface temperature is above 26.5°C, although once formed, they can persist over lower sea surface temperatures. Tropical cyclones can persist for many days and may follow quite erratic paths. They dissipate over land or colder oceans

750

Where do most cyclones originate

Many tropical cyclones originate as the result of a wave disturbance particularly in the northern hemisphere. In the pacific, tropical cyclones often appear to develop in situ with no detectable wave disturbance. Most cyclones originate in the equatorial trough

751

What are the requirements that must be met if a tropical wave disturbance is to develop into a tropical cyclone

Relationship with the equatorial trough, thermal energy, high level divergence, formation of the cyclone eye and the presence of a warm cyclone core

752

When is maximum cyclone activity normally attained

Maximum cyclone activity is normally attained when the trough is at its most northerly or southerly position which means that in the southwest pacific, the tropical cyclone season starts about October and peaks around February

753

What are contributing features to the lack of cyclones in the eastern South Pacific

Generally lower sea surface temperatures due to the cold humboldt current and the almost permanent anticyclone, giving rise to the regional dry zone

754

What areas do most cyclones form in

Most cyclones form within an area from northwest of Australia to approximately 160°W. due to the total absence of coriolis formed at the equator it is not feasible that cyclones form and develop within 5 degrees latitude of the equator. Due to other influences, such as ocean temperatures, it's not usual for cyclones to form beyond latitude 20°S

755

How is thermal energy a requirement for the development of tropical cyclones

The immense kinetic nerdy involved in the high velocity airflows around a tropical cyclone is fundamentally derived from thermal energy. Although part of this is obtained from the required high ocean surface water temperatures which must be at least 28°C for the formation of a tropical cyclone, the warm ocean condenses into liquid water

756

Why is high level divergence a requirement for the formation of tropical cyclones

This upper air outflow must be present to facilitate the fall of pressure at sea level so that the depression can develop into the cyclone stage. It appears that apart from the action of the upper air high pressure system, the eastern limbs of high level troughs in the westerlies can produce the necessary divergence as well. Thus upper air circulation has a great influence on the growth and also on the direction of travel of tropical cyclones

757

What is the formation of the cyclone eye a requirement for the development of a tropical cyclone

The narrow cylindrical column from the surface to the very top of the storm involves subsidence of air so that adiabatic warming dissipates much of the cloud. It is also clear that without the presence of the eye, pressure at sea level is unable to fall much below 1,000 hPa

758

Why is a warm core a requirement for a tropical cyclone

The warming action is provided by immense latent heat released from a few hundred cumulonimbus towers within a small radius from the centre. The warm core is instrumental in the stimulation of the storm because it encourages the development of an upper level high pressure zone, which, through the associated high level divergence, causes lower air to rise and surface air to converge

759

What are the stages of development for a tropical cyclone

The formative stage, the immature stage, the mature stage and the decaying stage

760

What is the formative stage of the tropical cyclone

The surface pressure within a wave disturbance drops below 1000hPa, the eye begins to form and the wind in at least one quadrant increases to gale force strength (34kts). The affected area encompassed by a few closed isobars has a radius of some 50km. The cyclone is designated a tropical depression

761

What is the immature stage of the tropical cyclone

Surface pressure continues to fall to under 1,000hPa. A closely spaced isobar pattern forms with a clearly defined eye in the centre (which will ultimately be of some 40km diameter) and winds reach hurricane force strength (64kts) within some 50km radius. Spiral bands are established and weather conditions commonly associated with CB developments are experienced within an expanding area. Now classified as a tropical cyclone and gets names

762

What is the mature stage of the tropical cyclone

Surface pressures settle at its lowest value, which can be as low as 950hPa. Winds of hurricane force and stronger are experienced within a radius of some 200km from the centre while gale force winds are experienced up to 500km radius. Torrential rain, thunderstorms and violent squalls. The quadrant of the cyclone where winds and weather are at their worst is at the left forward side in the Southern Hemisphere

763

What is the decaying stage of tropical cyclones

Surface pressure begins to rise and the tropical cyclone diminishes in intensity and is soon classified as a tropical depression

764

What are the main causes for the decaying stage to be reached

Movement over substantial land areas so that the fundamental source of energy, latent heat from an abundant water source, is cut off.
Movement into latitudes where ocean temperatures are too cold for the required evaporation to be provided.
Ingestion of cold air from whatever source.
Separation of the cyclone from the high altitude divergence pattern

765

What is the life span of a tropical cyclone

The average life span of tropical cyclones is 10 days nut substantial variations either side of this have been recorded

766

What is the movement of tropical cyclones

In the southwest pacific tropical cyclones tend to move southeast wards initially, particularly in the early part of the season (December to April). In the latter part of the season they may move west of southwest before re-curving to the southeast

767

How fast do tropical cyclones usually travel

The mean track of most tropical cyclones is from west to east and the average speed is 20kph but erratic changes in direction and speed are often noted. It is not possible for tropical cyclones to cross the equator

768

What are extra tropical depressions

When cyclones move into higher latitudes, there strength is still such that even ordinary depressions they cause can cause a lot of harm and damage. When moving away from tropical latitudes, the decaying cyclone is referred to as extra tropical depressions

769

What is the average pressure gradient of a tropical cyclone within a 300km radius from the centre

Approximately 1hPa per 6km

770

What are signs of an approaching cyclone

An increasing sea swell due to the outward flow effect of squalls and strong winds and a change in the semi diurnal variation of surface pressure which, in tropical regions, is readily evident due to the normally static nature of sea level pressures

771

What is the frequency of tropical cyclones

An average occurrence of 6 tropical cyclones per annum in the southwest pacific region. For the entire Southern Hemisphere the number is close on 25 per annum which is only half the recorded occurrences in the northern hemisphere where in excess of 50 hurricanes/typhoons are experienced each year

772

What is the Southern oscillation

The entire Pacific Ocean south of the equator is influenced by a circulating weather system known as the walker circulation. The system balances about two pivotal areas, high pressure off Peru and low pressure to the northwest of Australia

773

What is the picture in the east of southern oscillation

The Pacific Ocean along the west coast of South America is strongly influenced by a cold ocean current, originating from polar regions and flowing north, known as the Peruvian current (also known as the humboldt current). Off the coast of Peru this cold current forces itself to the surface with the result that the sea water temperature there is colder than it ought to be considering its position in the tropics

774

What is the consequence of the cold current off the coast of Peru in southern oscillation

A consequence of this is that the air above the region becomes more dense and develops into a large subsiding anticyclonic system rather than a rising air mass, which is more characteristic of tropical environments. Under the influence of the subsiding motion of the air off the Peruvian coast, surface divergence towards the west (into the centre of the pacific) strengthens the trade winds so that the general low level easterly flow across the ocean is enhanced

775

What is the picture in the west of southern oscillation

Under normal conditions the sea temperature in the western Pacific is warmer than in the east, this difference can amount to as much as 5°C. Thus the less dense air over these warmer sea surfaces produce a region of comparative low pressure within which air rises and at altitude some of this air flows eastwards towards the subsiding air mass off the Peruvian coast

776

What is developed from southern oscillation

A huge closed circuit has developed right across the Pacific Ocean with westbound air travelling at low lev Los and east bound air travelling at altitude. These are the elements of the walker circulation

777

When is the walker system said to be neutral or normal

When the pressures are somewhat lower in the west than in the east the walker circulation is said to be a neutral or normal state. This is reflected by a mildly neutral or very slightly positive ENSO index (El Nino - southern oscillation index), which can be measured by comparing the prevailing sea level pressure at Darwin in the west to that at Tahiti in the east

778

What is a positive ENSO index

When the pressure in the west (Darwin) is lower than normal and that in the east (Tahiti) higher than normal and stronger easterly surface winds in tropical latitudes of the pacific are the norm

779

What is a strongly negative ENSO index

When the pressure in Darwin is greater than Tahiti and easterly surface winds are reduced with strengthened westerly winds in mid latitudes, there is said to be a strong negative index

780

When does the el Nina events take place

When pressures decrease further than normal in the west and increase further than normal in the east, the walker circulation intensifies, the index becomes more positive and the La Niña event takes place, enhancing easterly winds

781

When do the El Nino events take place

When pressures increase in the west and decrease in the east the walker circulation weakens the index becomes more negative and the El Nino events take place, enhancing reduced easterlies in tropical latitudes and stronger westerlies in mid latitudes

782

What is the fundamental reason for the ENSO index oscillation

It is believed to be a change in the state of the oceans surface water. The ocean surface in the western Pacific can be more than 30cm higher than in the east

783

What is the summary of the walker circulation

The walker circulation is normal (or neutral) when the pressure index is very slightly positive/mildly neutral. The circulation intensifies, when an associated increase in the trade wind strength, when the index becomes positive and the La Nina event is established. When the circulation weakens and sometimes even reverses itself, the index becomes strongly negative mid latitude westerlies become stronger and the El t is established

784

How does El Nino affect NZ

When the index becomes strongly negative (low pressure in the east, high pressure in the west, El Niño) the overriding wind blows from the west-southwest and regions exposed to that direction such as both west coasts become wet while dry conditions are experienced on the lee of the main reaches

785

What does El Nina effect in NZ

When the index is positive (high pressure in the east, less pressure in the west, La Niña) the overriding wind is from the north/north easterly quarter. Regions exposed to the north and northeast such as bay of plenty, northland, Nelson and the sounds experience wetter conditions than normal

786

What is streamline analysis used for

Used for tropical latitudes because the coriolis force is so small that the pressure gradient is the overriding force which means that air will flow more or less directly from high to low pressure and across the isobars

787

What do streamline analysis show

In the tropics, the only method whereby the wind can be presented in useful form on a chart is by observing the actual wind over a wide area and drawing these observations on streamline charts. Directional streamlines show the direction of the wind for a given level

788

How is the info gathered for streamline analysis charts

The information is gathered by a network of observation stations and from aircraft. Streamline analysis is also provided for upper air charts such as the 700,500hPa levels etc

789

What does streamline analysis only provide information on

It only provides information on the direction of the wind and nothing else. There is not similar link between the spacing of streamlines and windspeed

790

How is wind speed and direction shown on streamline analysis charts

There is a need for wind speed as well as direction and the use of isotachs satisfies this requirement. An isotach is a dashed line that is generally curved or closed around regions where a given wind speed has been observed. They use wind vanes to indicate wind strength

791

What is the pressure at 5000ft

843hPa +5°C

792

What's the pressure at 10,000ft

697hPa -5°C

793

What's the pressure at 15,000ft

572hPa -15°C

794

What's the pressure at 20,000ft

456hPa -25°C

795

What's the pressure at 25,000ft

377hPa -35°C

796

What's the pressure at 30,000ft

301hPa -45°C

797

What's the pressure at 35,000ft

239hPa -55°C

798

What is the pressure at 36,090ft

226.3hPa -56.5°C

799

What's the pressure at 40,000ft

188hPa

800

What does the term general circulation apply to

The large scale pattern of pressure systems and winds, which endure worldwide throughout the year

801

What are low and high latitude regions

Low latitude (equatorial) regions and high latitude (polar) regions

802

What are the predominant factors that control the transfer of heat around the globe

Constant angular momentum of the atmosphere and vertical and horizontal circulations

803

What is the principle of angular momentum of the atmosphere in terms of general circulation

If large masses of warm air leave their tropical origins and move into higher latitudes, the smaller radius upon which they travel will cause their velocity (from west to east) to increase. This increase gives added stimulus to the poleward transfer of heat

804

What must there be a global balance of to stop the angular velocity of the earth altering

There must be a groans balance between easterlies and westerlies otherwise the angular velocity of earth would alter

805

How is the transfer of heat and the maintenance of an average global temperature achieved throughout global circulation

Vertical transfer in the two tropical Hadley cells (one in each hemisphere) and predominantly horizontal mixing of mid latitude and polar air rough the continued action of travelling extra tropical highs and lows, semi stationary highs and particularly their associated wave patterns at high altitude

806

What play a very important part in transferring heat pole ward and cold equator wards

The long Rossby waves and jet streams

807

Where does the entire circulation pattern shift with seasons

In the Southern Hemisphere summer, jet streams, semi-stationary highs and mid-latitude pressure systems tend to move towards the south pole whereas in winter, the systems move equator ward. Due to the more intense thermal gradient in winter, the winter hemisphere experiences a stronger general circulation than is the case in summer

808

What are rossby waves

These are long waves on contour charts that are break offs from jet streams

809

What can be interpreted by rossby waves (long waves)

By interpreting the direction of curvature of long waves it is possible to determine where the ridges and the troughs are. The importance of these waves in their function of transferring heat pole-ward and cold equator-ward

810

What is the principle cause of rossby waves (long waves)

The principle cause of long waves are firstly, the influence of major mountain features such as the Andes in South America and the high ground of the eastern Antarctic and secondly, the influence of major ocean currents and their cold upwelling in coastal areas

811

What has analysis shown of long waves over long periods

Analysis over long periods has shown that the wave pattern in the Southern Hemisphere is more or less the same summer and winter, which is due mainly to the predominance of oceans, about 80% of the total area

812

What's the difference between long waves in the northern and Southern Hemisphere

Ocean and land coverage of the northern hemisphere is almost evenly divided, it comes as no surprise that the long wave pattern involves a greater number of ridges and troughs due firstly, to strongly differing heat and cold sources such as warm ocean currents in winter and hot land masses in the summer and secondly, large mountain barriers

813

What does zonal index refer too

The term zonal index refers to the strength of the mid latitude upper level westerly winds, which are also referred to as zonal winds

814

What is a high zonal index

When the zonal winds are strong and the wave pattern does not display large fluctuations there is said to be a high zonal index. Under these conditions, surface and low tropospheric systems such as highs, lows and their fronts, travel at relatively high speed from west to east. The upper level westerlies and jet stream associated with high index situations are characteristically strong and homogeneous (uniform in structure), with the jet stream showing little change in speed along its axis and little curvature

815

What is a low zonal index

When the zonal winds are less strong and when the long waves have larger northward and southward fluctuations. In low index situations the upper level westerlies are weaker overall in mid latitudes and the jet stream shows considerable variation in speed along its axis and also shows strong curvature. Localised high speed maxima occur in the regions of strong cyclonic and anticyclonic curvature

816

Which way do pressure systems move in high zonal index situations

Pressure systems in the lower troposphere move towards the east at slower speeds than was the case in high index situations

817

What does the low zonal index situation (extreme) involve

Complete breakdown of zonal westerly winds with the result that closed contour patterns are formed at altitude. Deep lows close to the subtropics and deep anticyclones at high latitudes are commonly found under these conditions

818

What are the wave length numbers of zonal index's

Wave number 1 means one wave around the hemisphere i.e a wavelength of 360 degrees. Wave number 2 means two waves around the hemisphere with a wavelength of 180 degrees. Wave number 6 means 6 waves around the hemisphere with a wavelength of 60 degrees and so on

819

What are the wavelengths are considered long and short

Wavelengths 1-3 are considered long and wavelengths 4-7 are short. Long waves are very stable features of the hemispheric circulation and may persist for weeks with little Chaney or movement. Short waves are less stable, they have a life cycle of days and show greater mobility

820

What are wave lengths of 8 or shorter considered to be

In the synoptic scale and therefore last a short time

821

What are the potential dangers of wind shear

Sudden loss of airspeed, sudden reduction in lift, increase in the rate of descent, increase in the angle of descent, increase in take off and landing path when head wind/tailwind changes, shallower climb angle when headwind changes to tailwind, might to moderate turbulence

822

What are the most common causes that produce low level wind shear

Descending into a dense friction layer, most likely around dawn.
Descending into a cold zone i.e aerodrome and surroundings covered in snow.
Descending into the lee of sand dunes when a sea breeze blows.
Descending into low levels where the reported wind is lighter and/or blowing from a different direction than that above.
Ascending or descending through a sea breeze boundary

823

What does a microburst involve

A microburst often involves downward spiralling of air usually within a small circumference. The vertical velocity within microbursts can be as high as 6,000ft/min

824

What happens upon microbursts hitting the ground

On hitting the ground, microbursts form a vortex ring, with winds flowing outwards at speeds exceeding 50kts up to a radius of some 1-2km from the core. Not all microbursts involve spiralling down draughts - in some cases the draughts are straight down so that, on contacting the ground, they form (vertical) vorticies about a horizontal axis. The height to which these vorticies reach can be over 2,000ft AGL

825

How many microbursts can one weather system produce

More than one

826

How long do microbursts last for

From the time the microburst first contacts the ground, the speed of the wind continues to intensify for approximately five minutes. After that time period the intensity decreases and the event should have dissipated after some 20 minutes

827

What might microbursts be associated with

Microburst may be associated with convective cloud developments and/or heavy precipitation but it is not necessary that the base of the cloud is low

828

What cloud top base is said to from microbursts

Research into microbursts and microbursts is continually carried out and there are indications that very heavy precipitation from clouds with temperatures at their base of +22°C or warmer can produce very powerful downdraughts

829

What is the difference between anti icing and de icing

Anti icing is a preventive measure, and de icing is a remedial measure. Pre flight anti icing can normally be achieved by applying a solution of cold, rich freezing point depressant (FDF) fluid which under certain circumstances can prevent snow, from adhering to aircraft skins for up to 45 minutes in ambient temperatures as low as -14°C. Pre flight de icing can also be achieved by application of FDF fluids

830

What can cause hoar frost in flight

Cooling of surfaces can be aggravated by very cold fuel affecting the temperature of metal surfaces after long periods of flight at altitude. Rapid and sometimes instant development of hoar frost is then likely

831

What happens to the freezing level when stable and unstable saturated air is forced up orographically

When stable, saturated air is forced up orographically, the freezing level will lower over the mountains. When unstable attracted air is forced to rise orographically, the freezing level will rise over the mountains

832

What is the rate of ice accretion in lenticular clouds

High

833

Where does sleet build up

VFR flight in sleet can cause a build up of snow in areas where components join such as near wing roots, wing strut attachment points and around undercarriage components

834

How does volcanic ash affect aircrafts

It may cause engines to flame out due to blocking of intakes. It may cause extensive damage inside the engine due to the abrasive action of the ash particles and also because the particles may melt at the high temperatures to form a layer of glass which totally distorts the shape of all components. It may affect the operation of instruments due to blocking of pitot heads and it may cause total loss of visibility through the windscreen due to the sand blasting effect

835

Where is volcanic ash a bigger hazard

AHRS is a greater hazard at the high altitudes used by jet aircraft as compared to piston engine aircraft. At lower levels, the ash disperses quite quickly due to turbulence

836

What affects depth perception

The presence of moisture or dust in the air has an adverse effect on the persons ability to judge distances, both vertically and horizontally

837

What affects visual distortion of ground features

Precipitation and low cloud very often have the effect of changing the appearance of ground features such as valleys, saddles and ridges

838

What is grey out

It is possible to experience serious loss of depth perception through grey out in unsaturated air when flying in mountainous terrain close to sunset when the friction layer (which will extend above mountain tops) is dense as in stable atmospheric conditions. The shadow sides of ridges and particularly cliffs merge with the dimming light and it then becomes almost impossible to determine distances

839

What is the remedy to a situation when flying below cloud bases

A partial remedy to this situation is to select a height equal distance between the cloud base and the surface

840

What's the best way to get out of cloud when toy inadvertently entered it

The execution of an immediate 180 degree turn will navigate the aircraft quickly into the clearer airspace outside the cloud

841

What is jet engine flame out

When jet aircraft experience severe turbulence or heavy rain there is a risk of flame out. For that reason, many pilots turn engine igniters into continuous when experiencing these conditions

842

How can you avoid turbulence

Use altitude, fly above turbulence cloud, select a route on the windward side of mountains or ridges, slow down and read the clouds

843

How far can wake turbulence travel on parallel runways

In calm conditions wake turbulence will affect areas within approximately 750 meters from the centre line of the runway

844

What's the delay of wake turbulence

If wake turbulence cannot be out climbed after take off, a delay of 6 minutes in totally calm conditions will normally be sufficient. This time frame can be reduced as wind speed increases so that in winds of more than some 20kts a time delay of around one minute is normally more than adequate

845

What is dynamic aqua planning

When water has accumulated on a runway, a hydrodynamic pressure is developed between the footprint and the surface; thus pressure increases with increasing aircraft groundspeed. When the lift produced by hydrodynamic pressure equals the weight resting on the tyre, any further increase in aircraft ground speed will lift the wheel cleanly off the surface. Under certain circumstances this can cause the wheel to stop rotating or even start rotating in the opposite direction when the water build up in front of the foot print produced the required rotational force

846

What does dynamic aqua planning cause

The loss of contact between the tyre and runway surface reduces or eliminates braking effectiveness and directional control

847

What is needed for dynamic aqua planning to develop

It is necessary that substantial water accumulation exists on the runway e.g a flooded or heavily puddled runway

848

How much water is needed to cause dynamic aqua planning

The smoother the runway and the smoother the tyre, the less the water required to develop aqua planning. Studies have revealed that only 3mm of water is needed between a smooth tyre and smooth surface for the problem to develop whereas a rough runway surface and a threaded tyre needs some 8mm of water

849

What is the equation for dynamic aqua planning

Vp = 9 square root p
Vp is the dynamic aqua planning speed in knots, p is the tyre inflation

850

What is viscous aqua planning

Can occur on a wet runway that is very smooth either by design such as smooth asphalt, or due to the rubber accretion such as found in the touchdown area. The amount of water required to develop viscous aqua planning is dramatically reduced and the speed at which it can occur is at least 35% below dynamic aqua planning speed

851

What is steam (or reverted rubber) aqua planning

This type of often the consequence of dynamic or viscous aqua planning. It occurs when the wheels are locked and the aircraft aids along the wet runway. The heat produced by the friction between tyre and runway surface wears off the rubber and causes particles to accumulate at the rear of the tyre foot print. This forms a barrier that prevents water from escaping. As the trapped water is heated, its stream pressure lifts the tyre off the runway

852

When can steam aqua planning develop

When a pilot experiences dynamic or viscous aqua planning after touchdown and on sensing reduced or nil braking effectiveness, applies progressively greater brake pressure so that the wheels ultimately lock. The prevention of this type of aqua planning is not locking the brakes

853

How does tyre condition affect aqua planning

Worn or poorly inflated tyres lower the ground speed at which aqua planning can occur and it is therefore important that pilots check their tyres throughly for wear and pressure prior to flight when the destination airfield is likely to have a wet runway

854

What are the consequences of aqua planning

Increase landing roll and reduction or loss of directional control. The latter is particularly important to remember because nose wheel tyre pressure is normally less than main wheel tyre pressure

855

What are operational techniques for aqua planning

The aircraft should be landed at the minimum safe touchdown speed as close to the opening threshold of the runway as possible. Use of braking devices such as spoilers, reverse thrust etc should be used with care and initial contact with the runway should be firm

856

What can be used on runways to reduce aqua planning

Use of coarse chips in the runway sealing coat greatly reduces the risk of aqua planning and the best protection against aqua planning is lateral grooving of the runway cover

857

What are substances commonly used on aircraft that are affected by UV radiation

Plastics, nylons, paints, dyes and rubber. In contrast, metals are non organic which means that metal aircraft, with the exception of their organic components, are not subject to UV degradation

858

What is the degree of degradation to UV exposure determined by

The distance from the source of radiation, the exposure time and the presence or absence of screening

859

What latitudes are less affected by UV exposure

The higher the latitude the less the radiation per area and therefore the risk of exposure is somewhat less

860

What's used to reduce UV damage

The use of suitable screens between the sun and an organic substance can reduce ultraviolet damage considerably

861

What coloured materials are worse for UV radiation

Experience has shown that dark coloured materials, such as Dacron used in many microlight aircraft, has a better protective quality from UV radiation than light or white coloured cloth

862

What are visual signs of ultraviolet degradation

Differences in discolouration of surfaces facing up compared to those facing down, or those facing vertically

863

What are the three major influences that govern the climate in NZ

Latitude, oceanic surroundings and topography

864

What are the influences of tropical factors in the far north

It produces a climate there that is characterised by small annual fluctuations in temperature, a generally high relative humidity, reduced visibility and, apart from the occasional tropical disturbances passing by, light to moderate winds

865

What are the average temperatures of the north and South Island

The north island experiences greater fluctuations in annual temperatures where an average around 14°C. The South Island has similarly fluctuating annual temperatures with the average hovering around 10°C

866

What are the prevailing winds in NZ

The country lies at the northern edge of the mid latitude westerlies, which causes the prevailing winds to be from the west. Auckland/Northland experience westerlies about 50% of the time whereas Southland experiences these winds some 70% of the time

867

What is the average sea level pressure in NZ

Between 1,000hPa and 1,020hPa

868

What is the usual track of decaying tropical cyclones in NZ

The usual track of decaying tropical cyclones is southeastward, with the centre of the depression passing to the north of northland

869

What is the influence of the oceanic surroundings of NZ

The influence of the oceans around NZ is to temper the prevailing temperature and produce a relatively moist climate. This is reflected in the generally high average relative humidity values and cloudiness pertaining throughout the country

870

Where is the steadiest rate of rainfall

In terms of variation, the steadiest rate of rainfall is in the southwest of the South Island whereas the most variable pattern is found on the east coasts of both islands and the north

871

In terms of seasonal fluctuations, where is the most rainfall recorded

Most rainfall is recorded in the northern and central regions of New Zealand during the winter and least in the summer

872

What is the topography of NZ

The axis of the main mountain ranges lies SSW to NNE which is very close to right angle obstruction to the prevailing westerlies. When the high moisture content due to surrounding oceans is added to the orographic effect, it can be seen that regions exposed to the west experience a greater degree of cloudiness and precipitation than those to the east

873

Where is turbulence more in NZ

Turbulence will be more prevalent on the east of both islands

874

Where is a funnelling effect found in NZ

A funnelling (and Venturi) effect is readily established through cook trait whenever the wind has a west or northwest component. In similar vein, when the prevailing wind blows from the south, a southeast wind will funnel through the affected area. The effect on both winds is to increase their strengths, often to gale force

875

What is the weather like when the wind through cook strait becomes a strong gale force nor'wester

There is often little cloud at low levels other than stratocumulus on the hills to the north of Wellington. Also, turbulence is generally restricted to the friction layer. When the wind is forced through as a south easterlies, cloud bases are then lee while turbulence is nil or light over the water to the South of Wellington but often moderate over the land

876

What are the conditions formed from a North westerly

On the west: increasing stratiform cloud, rain developing in SW, flying conditions generally smooth
On the East: little or no clouds turbulent flying conditions, fohn winds likely in South Island and wairarapa/Hawkes bay, good visibility

877

What are the conditions found in a south westerlie

On the west: showers, generally smooth flying conditions and visibility good to excellent outside showers
On the East: mainly fine, turbulent flying conditions and visibility good

878

What are the conditions found in a south easterly

On the West: fine weather, turbulent flying conditions, good visibility
On the East: showers, generally smooth flying conditions and visibility good to excellent outside showers

879

What is meant by normal progression

Over NZ, the passage of an anticyclone followed by a cold front followed by the next anticyclone, the entire sequence involving some 6-8 days

880

What are the most common surface winds at Auckland International Airport

The most common winds are south west and north-north east with the first mentioned the more predominant south westerlies have their greatest percentage occurrence during spring and summer while the north easterlies are spread fairly evenly throughout the year

881

What is the main direction of a sea breeze in Auckland

The main direction of the sea breeze is from south-south west

882

Is turbulence very common at the Auckland airport

Surface friction induced turbulence is not very common but thermally induced turbulence is often experienced in and around the nearby hills

883

What fog is common at the Auckland airport

Radiation fog is common in winter (between April and late august) when synoptic situations indicate a high pressure system over the area

884

What is the most common cause of low cloud at the Auckland airport

The most common cause of low cloud formation is a moist north east air stream when a cloud base of some 500-600ft and sometimes as low as 300ft can be experienced. When formed, this cloud can persist for anything up to 24 hours

885

What is the prevailing wind at the Wellington International Airport

Due to the orientation of the mountain ranges, the prevailing wind in the region is north/north west or south/south east

886

Does fog form at the Wellington airport

Fog is very uncommon at Wellington airport

887

What is the prevailing wind at the Christchurch International Airport

The prevailing wind at the airport is from the north east (about 20% of the year). Southerly winds occur about 15% of the year and the north westerlies blow approximately 13% of the year

888

How can you anticipate the onset of a north westerly in Christchurch

The onset of hot and turbulent northwesters can be anticipated by monitoring the pressure difference in a northerly and westerly direction. When the QNH difference between Cape Campbell (high) and Dunedin (low) exceeds 4hPa and when the QNH difference between hokitika (high) and Christchurch (low) exceeds 5hPa and the air is unstable, the surface wind at Christchurch will be a north westerly

889

When is low cloud most common in Christchurch

In terms of season, the months of May to September have the highest incidence of low cloud and/or poor visibility

890

What are the current weather conditions reported in

METAR, SPECI and METAR AUTO

891

What provides information on forecasted weather

TAF

892

What is the world area forecast centres (WAFC)

Global en-route forecasts for international air navigation are provided by the world area forecast centres (WAFC) London and Washington, which distribute forecasts of upper wind, temperature, humidity, tropopause height, maximum wind, icing, turbulence, CB cloud and significant weather to meteorological offices throughout the world

893

What does an automatic weather station consist of (AWS)

The AWS consists of a central data collection, processing and communications system in which a range of sensors can be incorporated to monitor specific weather elements

894

What phenomenon do the AWS monitor

Surface wind speed and direction, visibility, present weather elements, cloud, air temp, dew point, atmospheric pressure (QNH)

895

What are the limitations of data obtained from AWS

Pilots should be aware that the information on cloud and visibility obtained from automatic weather stations may not be representative of the conditions over the entire aerodrome and immediate vicinity

896

How is measurement of upper air wind velocity, temperature, pressure and humidity carried out for the AWS

By radiosondes, which are small lightweight (200 grams) packages sent aloft by balloon

897

What principles does the radiosonde sensor work on

Temperature: measured by a thermistor consisting of a small element coated in a special chemical
Humidity: measured by a small capacitor (resistor), which contains a lithium chloride element
Atmospheric pressure: measured by a small aneroid capsule
Wind velocity: obtained by radar tracking of metal refractors attacked to radiosondes or by GPS radiosondes

898

What times are given on aerodrome reports

METAR, SPECI and METAR AUTO are reports of weather conditions at a given time and are not forecasts, therefore they will have an issue time but cannot have validity times. They describe conditions at a certain time within a radius of 8km from the aerodrome reference point

899

What are METAR and SPECI reports based on

METAR and SPECI reports, based on human observations are available in New Zealand only from Milford Sound and the RNZAF aerodromes

900

What are the METAR AUTO reports based on

METAR AUTO reports are based on AWS data available for the international and secondary aerodromes. SPECI are not reported from METAR AUTO sites

901

What are manual observations (METAR and SPECI)

METAR and SPECI are provided in New Zealand only at the RNZAF aerodromes (Whenuapai and Ohakea) and at Milford sound. METARs are issued routinely on the hour during the hours of ATC service. SPECI are issued between routine reports when changes in weather conditions meet certain defined criteria. When SPECI conditions exist at the time of a routine hourly report, the report is coded as a METAR and not as a SPECI

902

What are the phenomena that warrant the issue of a SPECI

Wind velocity, gustiness, visibility, cloud, the onset or cessation of moderate or heavy precipitation or obscuration and air temperature

903

When is a SPECI issued

A SPECI is issued immediately when one or more of the phenomena goes through predetermined values. When an improvement above predetermined values has persisted for 10 minutes a new SPECI is issued with the words SPECI CONDX CEASE or SPECI CEASES added as a remark

904

When is the word Nil used in a manual observation

If a report is missing

905

When is the word COR used in a manual observation

If a report is corrected for any reason

906

What is surface wind for METAR and SPECI in

Degrees true and knots

907

What is the letter G in METAR or SPECI

When gusts exceed the mean speed by 10kts or more they are reported using the letter G after the wind speed

908

When is the letter V used in METAR and SPECI

When the direction of varying by 60 degrees or more, a second group is used showing the extremes separated by the letter V

909

When is the term VRB used in a METAR and SPECI

The term VRB is used when the wind direction is variable and the wind speed is less than 3kts

910

What visibility is taken from information in METAR and SPECI

If the lowest visibility is observed in more than one direction, then the most operationally significant direction is reported. When the visibility is fluctuating rapidly and the prevailing visibility cannot be determined, only the lowest visibility will be reported with no direction indicated

911

What does the abbreviation VC in METAR and SPECI mean

The abbreviation VC (in the vicinity) is used to identify phenomena between 8km and 16km from the aerodrome reference point

912

How is cloud cover given in METAR and SPECI

Used using the abbreviations FEW, SCT, BKN, OVK. The type of cloud is not reported unless it is CB or TCU (towering cumulus without the anvil). The base of reported cloud is given as a 3 figure group in feet above aerodrome level

913

What does /// mean in METAR and SPECI

No dew point reported

914

What does RERA in METAR and SPECI mean

Moderate or heavy rain in the past hour (which was heavier than currently included in the METAR or SPECI)

915

How is wind shear reported in METAR and SPECI

Wind shear is reported using the format WS RWY followed by the runway designator