Meteorology Flashcards
1
Q
Describe the Tropopause
A
The Tropopause is the boundary between the Troposphere and the Stratosphere where temperature stops decreasing with altitude. It starts at approximately 8 km above the poles at -40C to 18km and -70C over the equator
2
Q
Why does the stratosphere heat up with altitude above its isothermal layer?
A
Because the Ozone layer sits above the stratosphere. The ozone layer absorbs UV Ray’s which cause it to heat up.
3
Q
What is radiation?
A
Radiation - anything with a temperature above absolute zero (-273 C) radiates heat energy in the form of electromagnetic waves.
As heat increases, wave length decreases. The Suns is short wave radiation, known as insolation, and the earths is long wave radiation, known as terrestrial radiation.
Heat transfer through radiation can occur in a vacuum.
4
Q
What is convection?
A
The transfer of heat through movement of a body. The earths surface heats air which in turn decreases in pressure and rises. As it cools it travels horizontally, known as advection. It then sinks. Convection and advection together create circulation.
5
Q
What is conduction?
A
Conduction is the transfer of heat through contact. The earths surface is heated by insolation from the sun which then heats the air on the surface through conduction. Air is a poor conductor so only the air close to the surface is heated. This is why temperature decreases with height close to the earth.
6
Q
Less than half of the insolation from the sun is absorbed by the earths surface. Where is the rest lost?
A
Scattering by airborne particles and reflection from the ground and cloud tops.
7
Q
What is the greenhouse effect?
A
Terrestrial radiation from the earth is absorbed by water and carbon dioxide molecules in the atmosphere and radiated back to the surface.
8
Q
What is an isotherm?
A
A line on a weather map joining two places of the same temperature
9
Q
What is diurnal variation?
A
The change of temperature over 24hours. Over sea is usually 1C due to the high specific heat of the ocean. Desert can be around 20C.
Wind will mix up air and reduce diurnal variation compared to calm conditions.
Cloud cover prevents heat transfer to and from the surface, thus reducing diurnal variation.
10
Q
What is an isobar?
A
A line on a weather map joining places of equal pressure
11
Q
What is QNE?
A
Question nil elevation. With 1013.2 set on a subscale the altimeter will read pressure height or QNE.
12
Q
In what layer of the atmosphere is most of the water vapour, clouds and weather found?
A
The Troposphere
13
Q
What three factors affect density?
A
Pressure and density are directly related.
Temperature and density are inversely related.
Humidity and density are inversely related.
14
Q
How does an increase in density affect aircraft performance?
A
It improves aircraft performance.
15
Q
What three effects cause air pressure to change?
A
The movement of pressure systems.
The change in intensity of pressure systems.
The expansion and contraction of the atmosphere as it heats and cools.(semi diurnal variation)
16
Q
What is QNH?
A
QNH is the true sea level pressure. When set on a subscale it will read actual elevation AMSL.
Area QNH is the average for a given area and will not differ from an adjoining area or local QNH by more than 5 HPA
17
Q
What is the transition layer?
A
The layer in which aircraft may not cruise. It’s base is 10,000’ and its ceiling is determined by QNH. FL110-FL125. A decrease of approximately 17HPA from 1013 will lead to an extra 500’.
FL110 not available below 1013
FL115 below 997
FL120 below 980
FL125 below 963
18
Q
What will an altimeter read when flying level towards low pressure?
A
It will over read as if it were in a climb
19
Q
What does a adiabatic mean?
A
A temperature change due to a change in pressure
20
Q
What is a turbulence inversion?
A
Winds at low level may cause adiabatic expansion and cooling due to mixing. If the lower layer becomes cooler then an inversion forms.
21
Q
What is a subsidence inversion?
A
As a column of air subsides, the top of the column undergoes greater heating and compression than the bottom. This creates an inversion.
22
Q
What is a surface inversion?
A
Air close to the earths surface is cooled at night by conduction. Greatest at dawn
23
Q
What is a frontal inversion?
A
The boundary of two masses can become an inversion as Warm air is lifted by cool air
24
Q
Define stability, instability and conditional stability in the atmosphere.
A
Stability-when a parcel of air is lifted and its temperature becomes less than the surrounding environment and sinks once the lifting force is removed. ELR ELR > SALR
Instability - when a parcel of air is lifted and its temperature becomes more than the surrounding environment and continues to rise once the lifting force is removed. ELR > DALR
25
```
What are the lapse rates for;
Dew point?
Dry air?
Saturated air?
ISA?
Actual Environment?
```
0.6*C per 1000'
3*C per 1000'
1.5*C per 1000'
2*C per 1000'
Must be measured in actual atmospheres
26
What is relative humidity?
The ratio of water in a sample of air compared to its water volume at saturation.
Relative humidity = actual mass of water vapour \ mass of water vapour at saturation
27
What is latent heat?
The heat which is required to change a substance to a higher state and released to change to a lower state.
There is no change in temperature, just state.
Lower - Solid, liquid, gas - higher
28
How can you can determine the base of a cloud base?
Surface Temperature - Surface Dew Point Temp
Divide by the DALR-DPLR (2.4)
Answer is in thousands of feet
29
What is a Fohn Wind?
Fohn wind is the warm dry air on the lee slide of a mountain.
Humid air is forced to rise over a mountain forming cloud as it cools adiabatically. Latent heat is
released by the process of condensation and carried with the air down the lee side of the mountain as warm dry air.
30
What do the terms veer and back mean?
Veer - wind changes in a clockwise direction
| Back - wind changes in an anticlockwise direction
31
What is the difference between a squall and a gust?
Gusts are a sudden increase in wind lasting a short period of time.
Squalls are a sudden increase of wind of at least 16 knots and last for at least one minute reaching up to 22 knots
32
Describe pressure gradient force.
The tendency for air to flow from heigh to low pressure across isobars
33
Describe coriolis force
In the Southern Hemisphere the earths rotation causes air to deflect to the the left. The deflection is greatest at the poles and nil at the equator. The stronger the wind speed the greater the deflection
34
What is Buy Ballots Law?
With your back to the wind in the southern hemisphere, the wind will be on your right.
35
From the equator to the South Pole, list the main systems of the global circulation pattern.
Equatorial trough
Subtropical ridge
Sub polar low
Polar high
36
Why does Australia have a "wet season"?
In summer the equatorial trough moves poleward of the equator due to the tilt of the earths axis exposing one hemisphere to greater heating.
Air flowing towards the equatorial trough crosses the equator and is deflected by the Coriolis effect (left in SH) this gives rise to north westerly winds. As these winds pass over the sea they bring warm moist air.
As there is more land heating the equatorial trough in the northern hemisphere during summer it travels further poleward.
37
What winds form from the subtropical ridge in Australia during winter?
During winter in Australia air flows from the subtropical ridge to the equatorial trough. This brings moist air to the east coast but mostly dry air to northern Australia.
Strong north westerlies are generated below 40*S as air flows from the STR to the Sub Polar Low.
38
What are geostrophic and gradient winds?
Geostrophic winds flow parallel to straight isobars
Gradient winds flow parallel to curved isobars
39
What is the friction layer?
The lower 3000' of the atmosphere. Wind speeds reduce by ~2/3 over land and ~1/3 over sea.
Reduction in speed reduces the coriolis effect and winds tend to veer compared to winds aloft. 30 degrees over land and 10 degrees over water due reduced friction.
Winds aloft tend to flow parallel to isobars. Surface winds flow across isobars from high to low pressure.
40
How are land and sea braces created?
The breeze flows towards the hotter of the two to replace the rising air. As land heats and cools quicker than the sea (sea has a height specific heat) it is common to have a sea breeze during the day and land breeze night.
41
What causes katabatic and a anabatic winds?
Anabatic - Air on a mountainside is heated by conduction, density reduces and rises up the hill
Katabatic - air on a mountainside is cooled by conduction density increases and sinks down the hill
42
What clouds form in the lower etage?
Cumulus: mound or tower like clouds formed from convection or other lifting. Made up from supercooled/water droplets & ice crystals. Showers of rain or snow with occasional virga. Little icing risk.
Stratus: flat, grey cloud formed from orographic lifting of stable air, made of water droplets & drizzle is typical. Usually no icing.
Stratocumulus: greyish layer formed from surface level turbulence or cumulus spreading out in an inversion. Consists of water droplets, light rain or drizzle & occasional rime ice if above freezing level.
Nimbostratus: dark, low ragged clouds at the base, heavy, continuous rain or snow. Common in slow moving fronts, usually formed by convergence. Moderate rime ice, clear ice at lower levels especially in turbulence.
Cumulonimbus: Heavy, dense clouds with great vertical extent. Developed from any lifting in a conditionally stable atmosphere. Composed of supercooled/water droplets, hail, snow & ice crystals. Clear icing.
43
What clouds form in the middle etage?
Altocumulus: clumpy white or grey clouds made of supercooled/water droplets, formed in turbulent middle levels usually below an inversion, convection, orographic lifting in low humidity. Light rain or snow may fall in the form of virga, light rime ice.
Altostratus: thin, grey cloud covering the sky, formed from frontal lifting or convergence. Contains supercooled/water droplets & ice crystals. When thick, can produce intermittent rain or snow sometimes as virga. Moderate rime ice; clear ice possible in lower levels.
44
What clouds form in the higher etage?
Cirrocumulus: thin layer of cloud made of ice crystals & liquid droplets, associated with frontal lifting or convergence. Occasional light to moderate turbulence. Too high for significant icing.
Cirrostratus: thin, transparent cloud often covering the entire sky, associated with frontal lifting or convergence, made of ice crystals. Occasional light to moderate turbulence. Too high for significant icing.
Cirrus: associated with frontal lifting or convergence in a low pressure system. Made of ice crystals. Occasional light turbulence, no icing
45
What are castellatus clouds?
Clouds of turret or cumuliform shape from a common base. Generally associated with altocumulus and indicates instability.
46
The type of clouds formed depends on if the atmosphere is stable or unstable and what type of lifting mechanism is driving it. There are five different types of lifting mechanisms. What are they?
Convection - warm air rises + unstable atmosphere = Cu. If vertical development continues then = Cb.
Turbulence - wind blowing over an uneven surface creates an undulating cloud base as rising air condenses and sinking air evaporates.
Orographic uplift -
Mountain waves
Frontal uplift
47
What is Buy Ballots Law?
With your back to the wind in the southern hemisphere, the wind will be on your right.
48
From the equator to the South Pole, list the main systems of the global circulation pattern.
Equatorial trough
Subtropical ridge
Sub polar low
Polar high
49
Why does Australia have a "wet season"?
In summer the equatorial trough moves poleward of the equator due to the tilt of the earths axis exposing one hemisphere to greater heating.
Air flowing towards the equatorial trough crosses the equator and is deflected by the Coriolis effect (left in SH) this gives rise to north westerly winds. As these winds pass over the sea they bring warm moist air.
As there is more land heating the equatorial trough in the northern hemisphere during summer it travels further poleward.
50
What winds form from the subtropical ridge in Australia during winter?
During winter in Australia air flows from the subtropical ridge to the equatorial trough. This brings moist air to the east coast but mostly dry air to northern Australia.
Strong north westerlies are generated below 40*S as air flows from the STR to the Sub Polar Low.
51
What are geostrophic and gradient winds?
Geostrophic winds flow parallel to straight isobars
Gradient winds flow parallel to curved isobars
52
What is the friction layer?
The lower 3000' of the atmosphere. Wind speeds reduce by ~2/3 over land and ~1/3 over sea.
Reduction in speed reduces the coriolis effect and winds tend to veer compared to winds aloft. 30 degrees over land and 10 degrees over water due reduced friction.
Winds aloft tend to flow parallel to isobars. Surface winds flow across isobars from high to low pressure.
53
How are land and sea braces created?
The breeze flows towards the hotter of the two to replace the rising air. As land heats and cools quicker than the sea (sea has a height specific heat) it is common to have a sea breeze during the day and land breeze night.
54
What causes katabatic and a anabatic winds?
Anabatic - Air on a mountainside is heated by conduction, density reduces and rises up the hill
Katabatic - air on a mountainside is cooled by conduction density increases and sinks down the hill
55
What clouds form in the lower etage?
Cumulus: mound or tower like clouds formed from convection or other lifting. Made up from supercooled/water droplets & ice crystals. Showers of rain or snow with occasional virga. Little icing risk.
Stratus: flat, grey cloud formed from orographic lifting of stable air, made of water droplets & drizzle is typical. Usually no icing.
Stratocumulus: greyish layer formed from surface level turbulence or cumulus spreading out in an inversion. Consists of water droplets, light rain or drizzle & occasional rime ice if above freezing level.
Nimbostratus: dark, low ragged clouds at the base, heavy, continuous rain or snow. Common in slow moving fronts, usually formed by convergence. Moderate rime ice, clear ice at lower levels especially in turbulence.
Cumulonimbus: Heavy, dense clouds with great vertical extent. Developed from any lifting in a conditionally stable atmosphere. Composed of supercooled/water droplets, hail, snow & ice crystals. Clear icing.
56
What clouds form in the middle etage?
Altocumulus: clumpy white or grey clouds made of supercooled/water droplets, formed in turbulent middle levels usually below an inversion, convection, orographic lifting in low humidity. Light rain or snow may fall in the form of virga, light rime ice.
Altostratus: thin, grey cloud covering the sky, formed from frontal lifting or convergence. Contains supercooled/water droplets & ice crystals. When thick, can produce intermittent rain or snow sometimes as virga. Moderate rime ice; clear ice possible in lower levels.
57
What clouds form in the higher etage?
Cirrocumulus: thin layer of cloud made of ice crystals & liquid droplets, associated with frontal lifting or convergence. Occasional light to moderate turbulence. Too high for significant icing.
Cirrostratus: thin, transparent cloud often covering the entire sky, associated with frontal lifting or convergence, made of ice crystals. Occasional light to moderate turbulence. Too high for significant icing.
Cirrus: associated with frontal lifting or convergence in a low pressure system. Made of ice crystals. Occasional light turbulence, no icing
58
What are castellatus clouds?
Clouds of turret or cumuliform shape from a common base. Generally associated with altocumulus and indicates instability.
59
The type of clouds formed depends on if the atmosphere is stable or unstable and what type of lifting mechanism is driving it. There are five different types of lifting mechanisms. What are they?
Convection - warm air rises + unstable atmosphere = Cu. If vertical development continues then = Cb.
Turbulence - wind blowing over an uneven surface creates an undulating cloud base as rising air condenses and sinking air evaporates.
Orographic uplift -
Mountain waves
Frontal uplift
60
What does CAVOK mean?
* Vis ≥10km
* NSC
* NSW
61
Define NSW and NSC?
No Significant Cloud
no cloud below 5000’ AAL or highest 25nm MSA; whichever is greater and no cumulonimbus or towering cumulus at any height;
```
No Significant Weather means:
Mist
Dust
Dust storm
Drizzle
Funnel Clouds
Fog
Smoke
Hail
Small Hail Pellets
Haze
Ice Pellets
Dust Devils
Rain
Sand
Snow Grains
Snow
Squalls
Sand storm
Unknown Precipitation Type (from weather sensor)
Volcanic Ash
```
62
When a forcast has GOOD or Unrestricted visibility what is its range?
Greater than 10nm
63
When a forecast visibility is 9000 0600N, what is
A prevailing visibility of 9000m and a
| minimum visibility of 600m to the north.
64
```
What are the weather codes for:
Mist
Dust
Dust storm
Drizzle
Funnel Clouds
Fog
Smoke
Hail
Small Hail Pellets
Haze
Ice Pellets
Dust Devils
Rain
Sand
Snow Grains
Snow
Squalls
Sand storm
Unknown Precipitation Type (from weather sensor)
Volcanic Ash
```
```
BR Mist
DU Dust
DS Dust storm
DZ Drizzle
FC Funnel Clouds
FG Fog
FU Smoke
GR Hail
GS Small Hail Pellets
HZ Haze
PL Ice Pellets
PO Dust Devils
RA Rain
SA Sand
SG Snow Grains
SN Snow
SQ Squalls
SS Sand storm
UP Unknown Precipitation Type (from weather sensor)
VA Volcanic Ash
```
65
When is a SPECI issued?
a. when there is BKN or OVC cloud covering the celestial dome below an aerodrome's highest alternate minimum cloud base or 1500 ft, whichever is higher;
b. when the visibility is below an aerodrome's highest alternate minimum visibility or 5000m, whichever is greater.
a. Wind —
1. when mean direction changes by 30 degrees or more, the mean speed before or after the change being 20 kt or more; or
2. when the mean speed changes by 10 kt or more, the mean speed before or after the change being 30 kt or more; or
3. when gusts vary by 10 kt or more from a mean speed of 15 kt or more; or
4. when a gust exceeds the last reported gust by 10 kt or more.
b. Other conditions —
1. when any of the following begins, ends or changes in intensity — thunderstorm, hail-
storm, mixed snow and rain, freezing precipitation, drifting snow, duststorm, sandstorm, squall, fog;
2. at the incidence of any other phenomena likely to be significant to the operation of an aircraft;
3. when the QNH altimeter setting changes by 2 hPa or more;
4. when the temperature changes by 5 degrees C or more.
66
What is a SPECI?
Special Reports (SPECI) are aerodrome weather reports issued whenever weather conditions fluctuate about or are below specified criteria.
67
What is an AIRMET?
An AIREP from an aircraft published by the Met Office
68
How is the passage of a cold front evident?
A fall in temperature
Backing winds
Rise in temperature
69
How is the passage of a warm front evident?
A rise in temperature
A backing of wind
The pressure fall decreasing
70
What is a warm front?
Warm air rises up over a cold layer forming a sloping layer in the direction of movement. Warm air is generally stable resulting in Stratiform clouds and rain from nimbostratus. The approach of a warm front is evident by high level cloud then mid level cloud and eventually nimbostatus
71
What is and what weather is associated with an occluded front?
A cold front catches up to a warm front. Characterised by both warm and cold front weather it brings extensive cloud and widespread rain.
72
What is a quasi stationary front?
When a warm and cold front meet from opposite directions
73
What are air streams?
Air flowing from one region to another
74
What are warm air streams?
Generally flowing with a northerly component from warm source regions, latitudinal cooling of surface layers may form an inversion and generate stable conditions
75
What are cold air streams?
Air flowing from cold regions with a southerly component are warmed latitudinally at the surface. This gives rise to a large change in environmental lapse rate and generates unstable conditions.
76
What is the inter tropic convergence zone?
The zone over the equator between air flowing to the Northern and Southern Hemisphere.
77
What is a low pressure system?
A low pressure system is associated with convection and generates instability, often bringing cloud and rain. Visibility is good.
Wind circulates clockwise. Aloft winds are gradient. On the surface they flow slightly across isobars and inwards.
78
What is a high pressure system?
Highs are associated with subsidence, and subsidence inversions. This limits vertical cloud development and generates stable conditions. Visibility may be poor due to pollutants trapped beneath the inversion.
Cloud is usually stratiform in the lower layers. Winds are light and variable near the centre and blow anticlockwise around the high. In the friction layer winds tend to blowout across the isobars slightly, as friction slows the wind speed and reduces coriolis force. Aloft winds tend to be gradient (blowing parallel to the isobars).
79
What is a Col?
A col is a neutral area between two highs and two lows. Winds are usually light and variable, with poor visibility
80
What are streamlines?
Lines on a synoptic chart indicating wind direction in place of isobars
81
How strong is a gale force wind?
At least 34kt
82
How are tropical cyclones formed?
A tropical cyclone is an intense low pressure area, with wind speed greater than gale force (34 kt). A cyclone derives its energy from the heat stored in tropical waters, and from the release of latent heat during condensation and cloud formation. Cyclonesgenerally form between 5 and 15 degrees north or south of the equator. They rarely form within 5 degreesof the equator due to the lack of coriolis force to sustainthe cyclonicflow. The life cycleof a cyclonemaybe divided into four stages.
Formative stage. An existing depresslon deepens, Wind speed reaches gale force (34 kt). The eye forms (a calmareain the middle of the cyclone).
Immature stage. Pressure falls below 1000 HPA. Winds reach hurricane force (64 kt), Spiral bands form. Stormareastill relatively small (30 - 50 km radius).
Mature stage. Pressure steady. Area of the storm expands. Hurricaneforce winds may extend 300 km from the eye. In the southern hemisphere the most destructive area is the front left quadrant of the cyclone.
Decaying stage. Pressure begins to rise. Storm area contracts. If land is encountered the cyclone weakensinto a rain depression. As a cyclone moves inland wind and rain generally decrease.
The cyclone season in Australia runs from November to April (when the equatorial trough is in the region)
83
What is fog and mist and how does it formed?
Fog is cloud at ground level through which visibility is less than 1000 mt with relative humidity near 100%.
(If visibility is greater than 1000 mt it is mist with relative humidity lower than 100%). Fog forms when air is cooled below its dew point temperature, causing saturation and condensation
Air close to the ground may be cooled by radiation. This effect is strongest on clear nights with moist air and an inversion. A light wind (up to 6 kt) will deepen the fog as the cool air is mixed through a greater depth. Fog intensity is usually greatest around dawn when surface temperatures are lowest.
Warm moist air cools as it flows over a cool surface (eg: wind from the sea blowing over a cold land surface or cooler ocean current). Many fogs form as a result of both radiation and advection.
Fog disperses when either the air temperature is raised above dew point, or wind mixes drier air with the fog causing evaporation
84
What is the life cycle of a thunderstorm?
For a thunderstorm (cumulonimbus) to develop the following conditions must exist:
Unstable conditions (ie: a steep environmental lapse rate).
An abundant supply of water vapour (high humidity).
A lifting mechanism to trigger convection (eg: heat, a front, terrain, convergence).
The life cycle of a thunderstorm may be divided into three stages.
Cumulus Stage
A thunderstorm starts life as a cumulus cloud. Updraughtsonly are present asthe cloud builds. The cloud is warmer than the environment so vertical development continues.
Mature stage
The cloud has grown to a great height (over 40000ft in some cases). It is now a cumulonimbus cloud with the characteristic anvil shaped top composed of cirrus cloud and ice crystals. The whispy cirrus anvil is blown flat ontop bythe wind, indicating the direction of storm movement.
Precipitation (rain and/or hail) falls generating strong downdraughts. Strong updraughts (3000 ft/min) are still present resulting in wind shearandsevereturbulence.
Downdraughts may cause gusts and a drop in temperature ahead of the storm. Downdraughts undercutting the warm air at the front of the cloud base may form a roll of cloud which may detach and moveaheadof the storm itself.
Dissipating stage
Updraughts die out leaving only downdraughts. Rain gradually decreases and the cloud breaks up. The whole storm cycle typically lasts about 60 minutes, but may be longer
85
What are the different types of thunderstorms?
Thunderstorms may be named according to the mechanism which triggered their development. Two groups may be identified.
FRONTAL OR SQUAll LINE
These thunderstorms are formed as a result of instability at the boundary between two air masses.They are the most hazardous to aviation because they are often closely spaced, fast moving with a low base. Roll clouds may be associated with frontal activity.
AIR MASS THUNDERSTORMS
These Tend to be more isolated than frontal thunderstorms and form asa result of some other trigger.
Orographic Thunderstorms
Formed as a result of orographic uplift of a moist unstable air stream.
Cold Stream Thunderstorms
A Cold air stream flowing over a warm surface may be warmed from below. This generates instability in the lower layers.Thunderstorms can develop if the instability extends through a deep enough layer.
Night Equatorial Thunderstorms
At night in tropical regions cloud tops may cool due to radiation. This causes unstable conditions (a steep environmental lapserate) and a thunderstorm may develop.
Shear Thunderstorm
These thunderstorms are not named according to the trigger mechanism but after a wind shear effect which can increase the storm severity. If the cloud penetrates a wind shear layer the cloud may become completely separated by the shear in which case the storm will dissipate. Alternatively if the vertical development is strong enough the shear may simply spread the storm out more horizontally, reducing the friction between updraughts and downdraughts. This allows higher velocities inside the storm. Greater Turbulence can result, and for this reason shear thunderstorms are considered particularly hazardous.
86
What hazards are associated with thunderstorms?
A number of hazards may be associated with thunderstorm activity. These include heavy rain, hail, icing, electrical interference, lightning, and most importantly severe turbulence. The turbulence is caused by vertical wind shear between updraughts and downdraughts within the storm. Turbulence is always associated with thunderstorm activity. Severe icing will generally be concentrated around the middle of the storm, particularly around the freezing level. Severe turbulence may be found anywhere inside the storm but also to a considerable altitude over the cloud top. (As a rule of thumb, divide wind speed at the cloud top by 10 then multiply by 1000 to find the minimum safe altitude above a thunderstorm to avoid turbulence (eg: if the wind speed at the cloud top is 50 kt, allow 5000 ft clearance as a minimum). Hail is most likely to be encountered inside a thunderstorm between 10000 ft and 30000 ft.
87
Dust storms require three conditions to occur before they can form. What are they?
A source of dust
A mechanism of uplift
An unstable environment to keep the dust aloft
88
What are jet streams?
Jet Streams are fast moving narrow currents of air found near the tropopause. They are characterised by strong horizontal wind shear. Wind speeds may reach 150 kt. Knowledge of prevailing jet streams greatly benefits high flying jet aircraft. Jet streams are generated by the advection of upper air in the global circulation pattern and the thermal wind gradients produced by the temperature differential over different parts of the earths surface. They are aligned roughly east/west and are westerly winds. There are two major jet streams in the southern hemisphere, the sub tropical jet located at about 30 degrees South Latitude and the polar front jet further south at the location of the polar front. Both jet streams are located at an altitude of around the Tropopause (which varies with latitude).
89
What is wind shear?
Change in wind velocity
90
What is a low level jet stream?
A low level jet is a fast moving stream of air at low level. They are formed when air circulating around a high hits a mountain range and is accelerated into a narrow stream along the range. A surface inversion must be present to shield the flow from surface friction and allow the speed to increase.Low level jets may cover a large area (eg: Western side of the Great Dividing Range In winter, or may be produced on a much smaller scale by physical barriers). Maximum wind speeds are usually encountered in the early morning (pre-dawn) when the surface inversion is strongest. The main hazard to aircraft is strong horizontal windshear.
91
What is turbulence?
Turbulence may be defined as vertical wind-shear. Convective turbulence is caused by rising air currents such as found in and below cumulus and cumulonimbus cloud. Turbulence is particularly severe in cumulonimbus cloud where the boundary between up and down draughts produces strong vertical wind-shear. Strong Turbulence is a hazard to all aircraft.
Mechanical turbulence may be generated by wind blowing over and around surface obstructions. Severity depends on wind speed, the height of obstructions, surface roughness and the stability of the atmosphere. Stable conditions will result in considerable lee-side turbulence. Lee turbulence may be less in unstable conditions as rising air lifts the turbulent air. Mountain waves (lee waves, standing waves) arean example of lee turbulence with worst turbulence in the rotor zone beneath the wave crests.
Frontal turbulence is wind shear at the boundary of two air masses of different temperature (density).
92
What is hoar frost?
A fine light crystalline deposit of ice that forms by deposition when the aircraft skin temperature falls below o deg C in high humidity conditions. Hoar frost may form on an aircraft parked overnight and should always be removed before flight. While hoar frost does not add significant weight to the aircraft it may disrupt airflow over the aerofoil surfaces, reducing aerodynamic efficiency. Hoar frost may also form in flight if an aircraft with a skin temperature below 0 deg C flys through a warm moist layer. Water vapour, again through deposition, forms a coating of ice on the aircraft surface. Ice formed this way usually melts quickly asthe ski warms but loss of visibility from ice on the windshield can be a hazard.
93
What is rime ice?
A white opaque rough deposit of ice formed by small supercooled water droplets freezing on impact with an aircraft surface. Pockets of air may be trapped making rime ice light, opaque and brittle. The most common temperature range for rime ice is between -10 deg C and -20 deg C. These low temperatures allow rapid freezing of the supercooled droplets. The cloud types generally associated with rime ice are stratiform. This is because the droplets must be small to freeze rapidly. Nimbostratus is an exception, because of the large size of the droplets clear ice is most likely in nimbostratus.
94
What is clear ice?
A transparent sheet of ice formed by the slow freezing of large super cooled water droplets. The most common temperature range is from 0 deg Cto -15 deg C. Clear Ice is the most dangerous form of icing. It is difficult to remove, adds considerable weight and seriously reduces aerodynamic efficiency. Clear ice may be encountered in cumuliform clouds above the freezing level (ie. when the temperature is below 0 deg C). Cumulonimbus clouds produce severe clear icing conditions. Clear ice is also possible in thick altostratus and nimbostratus. Orographic cloud is also likely to produce clear ice due to the large droplet size.
95
What is carburettor icing?
Ice formed in the venturi of a carburettor as intake air is cooled by fuel evaporation and lossof pressure. Carburettor ice can cause engine failure if not detected early enough and removed by use of carburettor heat. Carburettor ice is possible over a wide range of outside air temperatures (up to 40 deg C), but humidity must be relatively high.
96
What type of icing is associated with each type of cloud?
Cirrus
No icing as the cloud consists of ice crystals.
Cirrostratus
No icing as the cloud consists of ice crystals.
Cirrocumulus
Rare as the cloud consists mainly of ice crystals.
Altostratus
Light rime ice may occur in this type of cloud as it consists mainly of supercooled water droplets. Prolonged flight in altostratus may produce a large build up of rime ice,which could prove hazardous to an aeroplane not equipped to remove the ice. Clear ice isa possibility in thick altostratus.
Altocumulus
If the cloud is thin, light rime ice may occur in the same way as it does in altostratus. It is important to remember however, that there is nearly always some turbulence associated with altocumulus and this enables the cloud to support larger supercooled water drops. The build up of rime ice may therefore be more rapid, and there is a greater possibility of ice forming.
Stratus
In nearly all cases in Australia, the temperature inside the cloud will be above 0 degrees and no ice will form. If the temperature is below freezing, light rime ice may form. Lack Of turbulence and hence lack of largewater drops, will prevent the possibility of clear ice.
Nimbostratus (orographic)
As supercooled rain is often present in this cloud type, clear ice is a very real risk in flight through this cloud at levels up to about 5000ft above the freezing level. Above This level light rime icing will occur.
Stratocumulus
Rime ice will occur providing the temperature is less than a degrees C, but this is rarely the case in Australia except over Tasmania and southern Victoria during winter.
Cumulus
Small cumulus cloud will nearly always be at a temperature above freezing, so no ice will form in them. However in large cumulus clouds clear ice may occur up to about 5000 ft above the freezing level. Above this rime icewill occur.
Cumulonimbus Large supercooled water drops will certainly occur at, and above the freezing level. From the freezing level up to where the temperature reduces to approximately -15 degreesC,clear ice will almost certainly occur. Above this level rime ice will occur up to the level where ice crystals predominate.
97
What are microbursts?
Microbursts are strong concentrated downbursts of air from the base of a convective cloud. They may pose a hazard to aircraft operating at low level. Microbursts are often associated with squall lines and thunderstorm activity and may also be associated with virga.
Virga is hail or rain which melts/evaporates before reaching the ground. Hail or rain which actually reachesthe ground is known as precipitation.
The typical life cycle of a microburst from ground contact to dissipation is around 15 minutes. An aircraft encountering the outflow from a microburst at low level will initially experience increasing headwind and improved performance followed by decreasing headwind then downdraught and sink followed by increasing tailwind. These effects can cause a large airspeed loss and severely degraded climb performance. In some cases the velocity of the downburst can exceed the climb capability of the aircraft resulting in ground contact.
98
What are the atmospheric pressures for the standard forecast levels?
850 HPA at 5000 ft
700 HPA at 10000 ft
500 HPA at FL185
400 HPA at FL235
300 HPA at FL300
250 HPAat FL340
200 HPA at FL385
150 HPA at FL445
99
What is an important hing to remember when calculating true altitude from local QNH?
An important polnt to note regarding true altitude corrections is that where an aircraft is operating on a local QNHfor an airport, then the altimeter error due to ISA variation is only applicableto the Height of the aircraft above the airport (AGl error only). The localQNHcancelsout any error upto the level of the airport and the altimeter will read the elevation of the airport when the aircraft lands.It is only the aircraft height above the airport which isusedinthe true altitude correction on the flight computer or using the rule of thumb
100
What is a quasi stationary front?
When a warm and cold front meet from opposite directions
101
What are air streams?
Air flowing from one region to another
102
What are warm air streams?
Generally flowing with a northerly component from warm source regions, latitudinal cooling of surface layers may form an inversion and generate stable conditions
103
What are cold air streams?
Air flowing from cold regions with a southerly component are warmed latitudinally at the surface. This gives rise to a large change in environmental lapse rate and generates unstable conditions.
104
What is the inter tropic convergence zone?
The zone over the equator between air flowing to the Northern and Southern Hemisphere.
105
What is a low pressure system?
A low pressure system is associated with convection and generates instability, often bringing cloud and rain. Visibility is good.
Wind circulates clockwise. Aloft winds are gradient. On the surface they flow slightly across isobars and inwards.
106
What is a high pressure system?
Highs are associated with subsidence, and subsidence inversions. This limits vertical cloud development and generates stable conditions. Visibility may be poor due to pollutants trapped beneath the inversion.
Cloud is usually stratiform in the lower layers. Winds are light and variable near the centre and blow anticlockwise around the high. In the friction layer winds tend to blowout across the isobars slightly, as friction slows the wind speed and reduces coriolis force. Aloft winds tend to be gradient (blowing parallel to the isobars).
107
What is a Col?
A col is a neutral area between two highs and two lows. Winds are usually light and variable, with poor visibility
108
What are streamlines?
Lines on a synoptic chart indicating wind direction in place of isobars
109
Is the tropopause a continuous layer around the globe?
The tropopause is not a continuous surface from the equator to the poles but is broken and uneven and there may even be more than one tropopause above a particular location.
110
What are jet streams, where and how do they form?
At the breaks in the tropopause, jet streams are often found. A jet stream is a strong narrow current of air concentrated along a horizontal axis in the upper troposphere or in the stratosphere. Jet streams are characterised by strong horizontal and vertical wind shears and wind speeds in excess of 60 kt. Jet streams may extend horizontally for thousands of miles but are usually less than 100 nm wide only several thousand feet deep. Jet streams form as a result of upper level pressure distribution in much the same way as surface winds. Pressure distribution in the upper levels is influenced by temperature distribution. If we consider two columns of air having the same weight therefore the same surface pressure, one at the equator and one at the pole, but different temperatures, the warmer (equatorial) column will be less dense and so be taller with a more gradual decrease in pressure with height than the cold (polar) column. At the surface the pressures are equal in both columns but as altitude increases the pressure at any given height in the warm column is greater than the pressure at the same height in the cold column. A pressure gradient has been produced by the temperature differential between the two columns. The pressure gradient causes a wind to blow towards the cold column (South in the Southern Hemisphere, North in the Northern Hemisphere). This wind is known as "thermal wind" since it is the result of a temperature differential between the two columns. This Southerly wind is deflected by Coriolis to the left (in the Southern Hemisphere, right in the Northern Hemisphere) resulting in a Westerly wind (in both hemispheres). The gradient intensity increases with height up to the tropopause causing the wind speed to increase with height up to the level of the tropopause. Isotachs joining points of equal wind speed show the maximum wind speed in the jet core.
The lower tropopause in the polar regions means that the temperature stops lapsing earlier and the temperature is relatively warmer abovethe polar tropopause than abovethe equatorial tropopause. Above the tropopause the warm column is over the pole and the cold column is over the equator. This causesa reversal of the thermal wind and creates a tendency for wind to blow North. This Northerly wind is deflected by Coriolis to the left (in the Southern Hemisphere) resulting in an Easterly wind above the tropopause which opposesthe Westerly Flow. High Above the tropopause, Easterly Winds prevail.
The two main jet streams are the Sub Tropical Jet found at an altitude of around 200 HPA(FL385) at about 30° latitude and the Polar front Jet found just below the level of the tropopause above the 500 HPA position of the Polar Front. The jet streams migrate with the overall movement of the general circulation during the seasons.In the Southern hemisphere summer, the patterns move South and so the Sub Tropical Jet stream moves a little further South in summer and North in winter, with the average location year round being 30° latitude. The Polar Front Jet is more mobile, following the position of the Polar Front and at times can merge with the Sub Tropical Jet so the two are asone. The STJ is generally the stronger of the two jet streams.
111
What is clear air turbulence?
Jet streams are often associated with areas of Clear Air Turbulence (CAT) due to the strong wind shears encountered around them. The CAT is considered to be most severe on the polar side of the jet stream below the level of the core. CAT is also considered to be more severe in winter when the jet streams are stronger, and when the jet stream is over land and/or curved. If CAT is encountered in association with a jet stream, it is likely to occur in shallow patches of only two or three thousand feet thick. In order to avoid the worst areas of CAT, change of level from one IFR level to the next (4000 ft) is usually sufficient to move out of the CAT. CAT can be categorised as either moderate or severe and forecast CAT areas are shown on a Significant Weather Prognosis (SIGWX PROG)chart.
112
How is it possible to predict a jet stream location based on OAT?
When flying North or South It is possible to predict the jet stream location relative to your aircraft based on OAT. If the OAT drops when flying South in the Southern Hemisphere, you are still below the tropopause and are therefore likely to fly under the jet stream. If the OAT rises when flying South, you are above the level of the tropopause and are therefore likely to fly above the jet stream. The opposite applies when flying North or in the Northern Hemisphere. Constant OAT when flying in either direction indicates a possibility of flight through the jet stream core.
