The Atmosphere

Question 1

Define Meteorology (pilot viewpoint)

Answer 1

A consideration of all available weather information to be able to plan and execute safe and economical flight operations

Question 2

Define Meteorology (scientific definition)

Answer 2

The study of the earth’s atmosphere and the interaction between the atmosphere and the surface of the earth.

Question 3

Define Atmosphere

Answer 3

The layer of air which surrounds the earth and extends upwards from the surface to about 500 miles and can be considered as four concentric gaseous layers.

Question 4

Where does the majority of flight take place in relation to the Earth’s Atmosphere’s?

Answer 4

Most flying occurs in the troposphere but high-flying jets cruise in the stratosphere.

Question 5

What is the Thermosphere? (2)

Answer 5

Upper limit is not defined. Temperature increases above the mesopause.

Question 6

What is the Mesosphere? (2)

Answer 6

Extends to between 260,000 and 295,000 ft. Temp falls with height to the mesopause.

Question 7

What is the Stratosphere? (4)

Answer 7

Extends to about 164,000 ft. Negligible water content. Temp overall increases. Contains the ozone layer.

Question 8

What are the major characteristics of the Troposphere? (4)

Answer 8

The troposphere contains almost all atmospheric water and therefore most of the weather, clouds, storms and temperature variances.
It is always moving. Moving horizontally and/or vertically, thus over-turning.
The troposphere contains 75% of the mass of the atmosphere in mid-latitudes. 50% of the mass of the atmosphere is found below 5 km.
Temp falls with height. Averaging close to 2 °C / 1000 ft. This change is known as the environmental lapse rate (ELR).
The tropopause varies in height from about 28,000 ft at the poles, 35,000 ft over UK latitudes, to about 56,000 ft over the equator; thus being elliptical in shape.
The height varies from day to day and is higher in summer than in winter (again, due to thermal expansion and/or contraction).

Question 9

Recite the order of the gaseous layers of the Atmosphere. (8)

Answer 9

Exosphere
Thermosphere
Mesopause
Mesosphere
Stratopause
Stratosphere
Tropopause
Troposphere

Question 10

What is the Exosphere?

Answer 10

Where the atmosphere merges into space.

Question 11

What is the Environmental Lapse Rate (ELR)/Temperature Lapse Rate (TLR)?

Answer 11

The fall in temperature in the tropopause.

Question 12

Why is the Tropopause significant in aviation? (4)

Answer 12

Clouds are rare above it.
Max. wind speeds are often found just below it.
Condensation trails occur just below but not above it.
Severe turbulence may be encountered close to it.

Question 13

Can the Tropopause overlap?

Answer 13

Yes - there may be multiple overlapping mid-latitude tropopauses separated by jetstreams.

Question 14

Describe the link between air masses and the Tropopause.

Answer 14

Air masses tend to ‘keep’ their tropopause, so in the northern hemisphere, with a moving air mass from the south, the tropopause will rise with the air mass. The tropopause is higher in each region in the summer than in winter.

Question 15

What is a significant trait of ozone?

Answer 15

It is an efficient absorber of ultraviolet radiation.

Question 16

Describe the temperature trend inside the Stratosphere.

Answer 16

The lower portion of the stratosphere is an isothermal layer where the temp remains constant (about -57 °C) with increasing height, and then in its upper layers the temp increases to around 0 °C at the stratopause.

Question 17

Compare and contrast temperature, air movement and water content between the Troposphere and the Stratosphere.

Answer 17

Troposphere: Temp - decreases with altitude with an abrupt change in the ELR at the tropopause.
Air movement - marked vertical movement with warm air rising and cool air descending.
Water content - contains almost all atmospheric water vapour.
Stratosphere: Temp - steady in the lower region but increases in the higher layers (ozone absorbs heat/ultraviolet radiation).
Air movement - little vertical movement.
Water content - little water therefore clouds are rare due to low humidity. Only exception to this is polar stratospheric clouds (PSC).

Question 18

Describe the most significant property of air.

Answer 18

Air is a compressible fluid. As such it is able to flow or change its shape when subjected even to minute pressures.

Question 19

Describe the nature of pressure in a fluid (2).

Answer 19

In fluids, the degree of cohesion of its molecules is so small that very small forces suffice to move them in relation to each other.
At any point in a fluid, the pressure is the same in all directions, and if a body is immersed in a stationary fluid, the pressure on any point of the body acts at right angles to the surface at the point irrespective of the shape or position of the body.

Question 20

List the composition of air by weight and volume:

Answer 20

Nitrogen: Volume = 78.08%. Weight = 75.5%
Oxygen: Volume = 20.94%. Weight = 23.1%
Argon: Volume = 0.93%. Weight = 1.3%
Carbon dioxide: Volume = 0.03%. Weight = 0.05%
Plus traces of other gases.

Question 21

What are the two factors that determine the amount of water vapour in the atmosphere?

Answer 21

The amount of water vapour in a given mass of air depends on the temp and whether the air is, or has recently been over large areas of water.

Question 22

What percentage of water vapour is found from the surface to between 26,000ft and 30,000ft?

Answer 22

The water vapour content fluctuates from between almost 0 to 5%.

Question 23

What is the relationship between water vapour and temperature?

Answer 23

The higher the temp, the greater the amount of water vapour the air can hold.

Question 24

What is the relationship between water vapour and air density?

Answer 24

A water molecule is relatively light and the presence of large numbers in an air mass decreases the overall density of that air mass (and therefore also pressure).

Question 25

Why is aircraft performance worse in humid conditions as compared to dry conditions?

Answer 25

Humidity affects the way an airplane flies because of the change in pressure that accompanies changes in humidity. As the humidity goes up, the air pressure for a given volume of air goes down. This means the wings have fewer air molecules to affect as they are pushed through the airmass. Fewer molecules = less lift.

Question 26

Describe atmospheric circulation. Is this an accurate depiction of how air masses move in reality?

Answer 26

The combination of the earth’s tilt and its curved surface means that the equatorial regions get more direct sunlight, and hence more surface heating from the sun. This heating causes convection within the atmosphere, resulting in circular motion of the air with warm, less dense air rising and being replaced by cooler, denser air. The warm air flows towards the poles where it cools, becoming denser, and sinks back towards the surface.

No - this is a simplified theory as these movements are modified by several forces, the most significant of which is the rotation of the earth.

Question 27

Define coriolis force and describe its effect over the movement of air across the globe.

Answer 27

This is the force created by the rotation of the earth. Coriolis deflects air to the right in the northern hemisphere and left in the southern hemisphere causing it to follow a curved path instead of a straight line. The degree of deflection depends on latitude. It is greatest at the poles and diminishes to zero at the equator.
The speed of the earth’s rotation causes three distinct cells to form in each hemisphere which gives us prevailing winds over certain regions in the globe e.g. north easterly and south easterly trade winds.

Question 28

Does coriolis have a greater affect on objects that travel over short or long distances?

Answer 28

Long distances. To a person standing on the earth, the effect of coriolis is imperceptible because humans move slowly and travel relatively short distances compared to the size of the earth and its speed of rotation. However, the coriolis force significantly affects bodies that move over great distances, such as air masses or bodies of water.

Question 29

List the factors which affect the circulation patterns of air within the atmosphere. (5)

Answer 29

Atmospheric circulation.
Coriolis force.
Seasonal changes.
Differences between the surfaces of continents and oceans.
Frictional forces caused by the earth’s topography.

Question 30

Using the north easterly trade winds as an example, describe the affect of coriolis in the northern hemisphere.

Answer 30

In the northern hemisphere, the warm air at the equator rises upward from the surface, travels northward, and is deflected eastward by the rotation of the earth. By the time it has travelled one-third of the distance from the equator to the north pole, it is no longer moving northward, but eastward. This air cools and sinks in a belt-like areas at about 30° N, creating an area of high pressure as it sinks towards the surface. Then, it flows southward along the surface back towards the equator. Coriolis force bends the flow to the right, thus creating the north-easterly trade winds that prevail from 30° N to the equator.

Question 31

At what height does friction alter the path of moving air?

Answer 31

At about 2000 ft.

Question 32

What is the purpose of international standard atmosphere?

Answer 32

ISA is used to create a standard reference. These standard conditions are a model which allow for the calibration of certain flight instruments and most aircraft performance data. ISA seldom applies to actual conditions.

Question 33

What are the assumed characteristics of ISA? (4)

Answer 33

The air is dry and its chemical composition is the same at all altitudes.

The value of G is constant at 980.665cm/sec2

At mean sea level: the temperature is 15 °C, the pressure is 1013 hPa and the density is 1.225 kg/m3.

The temp lapse rate is 1.98 °C per 1000 ft up to a height of the theoretical tropopause at 36,090 ft. Above this height the temp is assumed to remain constant at -56.5 °C until 65,617 ft where it then rises at a rate of 0.3 °C per 1000 ft.

Question 34

What is the temperature (ISA) deviation at 8,500 ft if the actual temperature is 5 °C?

Answer 34

ISA temp at 8,500 ft = ISA temp at sea level - (ELR x height (in 1000) ft)
= 15 - (2 x 8.5)
= 15 - 17
= -2 °C
Therefore deviation = +7. (Written as positive as the actual temp is warmer than ISA).

Question 35

What are the four basic properties of the atmosphere?

Answer 35

Pressure, density, temp and humidity.

Question 36

Define atmospheric air pressure.

Answer 36

The pressure of the atmosphere is the total weight of the column of air up to the upper limits of the atmosphere above a reference point. It amounts to almost 1 kg for every square cm of ground it covers. Measured in the hectoPascal (hPa).

Question 37

What is the difference between an analysis and prognosis chart?

Answer 37

A weather map showing current conditions i.e. the present position of pressure systems is called an analysis chart, whereas a map showing future positions, often 24 or 36 hrs ahead, is a prognosis chart.

Question 38

What is Buys Ballot’s law?

Answer 38

If you stand with your back to the wind in the southern hemisphere, the low pressure is on your right.

Question 39

What does Buys Ballot’s law imply?

Answer 39

It implies that, in the southern hemisphere, wind blows clockwise and cyclones and anticlockwise around anticyclones.

Question 40

Describe what is meant by a positive lapse rate.

Answer 40

Within the troposphere temperature usually decreases within increasing height. By convention, this is referred to as a positive lapse rate (even though the temp is decreasing).

Question 41

Describe what is meant by an inversion.

Answer 41

Occasionally temp will increase with increasing height in the troposphere, which creates a negative lapse rate.

Question 42

Describe what is meant by an isothermal layer.

Answer 42

An isothermal layer is one in which the temperature remains constant with increasing height.

Question 43

Describe the presence and importance of water vapour in the atmosphere.

Answer 43

Water vapour is the gaseous state of water in the atmosphere. It enters the atmosphere through the evapotranspiration process.

When water vapour changes state in the atmosphere - to its liquid or solid state it releases latent heat. This heat release destabilises the atmosphere. Conversely, the evaporation of liquid cloud droplets and the sublimation of ice clouds crystals cool the atmosphere and therefore increases atmospheric stability.

Question 44

Describe the presence and importance of aerosols in the atmosphere.

Answer 44

Aerosols are minute particles of sea-salt, dust, volcanic ash, smoke etc. floating freely in the atmosphere. Effectively, they are tiny solid particles with a crystalline structure at the molecular level. Water vapour and aerosols are the building blocks for all cloud and hence precipitation as well.
Inside every single cloud droplet, there is an aerosol.

Question 45

Why is there little or no cloud in the stratosphere or above?

Answer 45

Just like water vapour, all aerosols originate from the surface of the earth, so the higher we go, the less of these elements we encounter. Above the tropopause, there is virtually no water vapour and almost no aerosols; hence there is little or no cloud in the stratosphere or above.

Question 46

Describe the presence and importance of ozone in the atmosphere.

Answer 46

Ozone is present in small amounts which are concentrated within the stratosphere at about 30km above the earth’s surface. Ozone is vitally important as it absorbs incoming UVA and UVB light which, if not removed within the stratosphere, would greatly increase the incidence of concern in both humans and animals.

Ozone is also responsible for the increase in temp throughout the stratosphere.

Question 47

Describe the presence and importance of carbon dioxide in the atmosphere.

Answer 47

CO2 is also present in small (though slowly increasing) amounts. CO2 is the main gas contributing the global greenhouse warming. It does this by absorbing outgoing terrestrial radiation from the earth, which results in heating of the atmosphere.