What the air is actually made of
Meteorology is the branch of science dealing with the earth's atmosphere and the physical processes occurring in it. You are not studying it to become a forecaster. You are studying it because weather is the one factor in modern aviation over which nobody has any control, and a pilot who understands it can at least anticipate what it is about to do.
The syllabus asks you to be able to do five things: understand the physical processes going on in the atmosphere, understand the hazards and how to reduce the risk from them, identify what weather information a given flight needs, interpret actual and forecast weather from the documents you are handed, and evaluate all of that before and during the flight.
The atmosphere, defined
Oxford gives you the definition to reproduce in the paper: the spheroidal gaseous envelope surrounding a heavenly body. Joshi describes the same thing more usefully: an envelope of a roughly uniform mixture of gases, called air, held to the earth by gravity, turning with the earth at the same speed and in the same direction.
It is about 480 km thick, but there is no line where it stops. It simply thins until it merges with space. What matters for flying is that it is bottom heavy: about 80 per cent of the whole atmosphere sits inside the lowest 16 km.
Composition by volume
| Gas | Per cent by volume | Worth knowing |
|---|---|---|
| Nitrogen | 78.09% | Inert, does nothing for weather |
| Oxygen | 20.95% | What keeps you conscious |
| Argon | 0.93% | Inert |
| Carbon dioxide | 0.03% | Joshi quotes 0.035%, a greenhouse gas |
Plus traces of neon, krypton, hydrogen, nitrous oxide, carbon monoxide, ammonia, helium, xenon, methane, nitrogen dioxide, sulphur dioxide, iodine and ozone. On top of the gases the air also carries solid particles and water vapour, and from a weather point of view water vapour is the single most important thing in the list.
Nitrogen and oxygen together are almost 99 per cent of the air. Joshi gives the ratio in a form DGCA has asked for directly, and these are the four numbers from this page that keep turning up in papers.
Homosphere and heterosphere
Up to roughly 80 km the mixing processes of the lower atmosphere keep everything stirred together, so the mixture stays the same. Joshi calls this the homosphere. Above it the mixing stops, the gases separate out by weight under gravity, and that region is the heterosphere. Oxford quotes the changeover as at least 60 km rather than 80. Either figure is accepted, and no DGCA option has turned on the difference.
Variable gases, and why they matter
Water vapour, carbon monoxide, sulphur dioxide, nitrogen dioxide and methane vary from place to place. They concentrate over industrial areas, cities and water bodies, and thin out over open country. They are tiny in quantity and enormous in effect.
Greenhouse gases
Water vapour, carbon dioxide, ozone and methane let short wave solar radiation straight through, but partly absorb the long wave radiation coming back off the earth and re-radiate it. That is the greenhouse effect, and it keeps the planet warmer than it would otherwise be.
Carbon dioxide
Carbon dioxide comes from burning fuel and wood. It is heavy over industrial areas and negligible in polar regions and at altitude. A great deal of it is dissolved in the oceans, and plants take it in and give oxygen back.
Ozone
Ozone forms in the upper atmosphere, where it absorbs ultraviolet radiation from the sun and warms the air doing so. The molecules then grow heavier, sink and collect lower down. Appreciable ozone is found between 10 and 50 km, with the maximum concentration at 20 to 25 km. Oxford adds a figure that sticks in the memory: if the entire ozone layer were brought down to sea level it would be only 3 mm thick.
The ozone hole is the recent thinning of that layer, mainly over the poles, caused by halocarbon refrigerants, solvents and chlorofluorocarbons. On reaching the stratosphere they release halogen atoms by photodissociation, and those break ozone down into ordinary oxygen.
Solid particles
Solid particles, salt from evaporating sea water, dust from arid regions and industrial soot, block some solar radiation and cut visibility. They also act as condensation nuclei, which means they let water vapour condense earlier than it otherwise would. That is why fog and smog are worse over cities.
Water in the air
Water exists in the atmosphere in all three states: solid as snow, ice and hail, liquid as drizzle, rain and showers, and gas as water vapour. The amount of vapour air can hold depends mostly on its temperature, and warmer air can hold more. Vapour content falls away quickly with height, and is almost nothing at the poles and above 30 000 ft.
In the tropics, between 23½°N and 23½°S, air can hold as much as 4 per cent water vapour by volume. At that point the air is saturated and has 100 per cent relative humidity. Below that it is unsaturated, which Joshi calls dry air.
Properties you can be asked to list
The earth's atmosphere varies both vertically and horizontally in pressure, temperature, density and humidity. It is fluid, it has weight and therefore exerts pressure, it is compressible and expandable, it occupies space and has no definite shape, it supports life only at the lower levels, and it is a poor conductor of heat and electricity.
Atmosphere of the earth is an envelope of homogeneous mixture of gases, called Air. It surrounds the earth and is attached to it due to gravitation.IC Joshi, Aviation Meteorology, chapter 1
The spheroidal gaseous envelope surrounding a heavenly body.Oxford ATPL Vol 9, 1.3
Oxford gives the definition to write down. Joshi gives the picture to think with. This course keeps both, because the paper rewards the first and flying rewards the second.
How the atmosphere is heated
Everything else in this subject rests on one sentence, so learn it before anything else:
Solar radiation passes through the air fairly freely and heats the surface of the earth. The surface then heats the air sitting on it, by conduction into the layer touching the ground, by convection carrying that warmth upward, by radiation, and by the release of latent heat when water vapour condenses.
Move away from the surface and there is less of that warming to go round, so temperature falls steadily with height. That is the whole reason for the lapse rate you are about to meet, and the reason the lowest layer of the atmosphere behaves the way it does.
Where the heat actually comes from
Joshi splits the flow of heat from the surface into the atmosphere into two parts, and the split is not what most students guess:
| Route | Mechanism | Share |
|---|---|---|
| Sensible heat | Conduction, convection, radiation | 23% |
| Latent heat | Evaporation, condensation, sublimation | 77% |
Three quarters of the heat that reaches the atmosphere gets there inside water molecules. Water evaporates at the surface, taking heat with it, rises, condenses higher up and lets that heat go. It is worth pausing on that, because it explains why water vapour drives so much of the weather you will study later.
Where the air actually is
The atmosphere is held down by gravity, so it is densest at the bottom and thins with height. Joshi gives the vertical distribution of mass in a form that is easy to remember and easy to be asked:
- Half of the atmosphere by mass lies below 6 km
- Three quarters lies below 10 km
- 99 per cent lies below 35 km
Oxford states the same idea by layer rather than by height: the troposphere, the lowest layer, holds about three quarters of the total atmosphere by weight and contains almost all of the weather. Both are saying that essentially all of aviation happens in the bottom sliver of the atmosphere.
Lapse rate
The rate at which temperature falls as you climb is called the lapse rate. In the troposphere the average is about 6.5°C per kilometre, which is 1.98°C per 1000 ft. In practice everybody uses 2°C per 1000 ft for mental arithmetic, and that is the number you will use for the rest of this subject.
That figure is an average through the whole layer, not a promise about any particular day. Chapter 7, adiabatics and stability, is entirely about what happens when the real lapse rate on the day differs from it.
The layers, from the ground to space
Satellite and high altitude flights confirmed that the atmosphere sorts itself into well defined horizontal layers, and the thing that defines them is what temperature does with height. Where the trend reverses, a new layer begins, and the boundary takes the name of the layer below it with "pause" on the end.
The figure below is Joshi's Figure 1.1 rebuilt so you can move through it. Drag the slider to fly up through the atmosphere and watch which way the temperature is going.
Troposphere
The lowest layer, and the one where temperature decreases with an increase in height. It reaches 16 to 18 km at the equator and 8 to 10 km at the poles, and the reason it is deeper at the equator is the rising hot convection currents there. It holds about three quarters of the atmosphere by weight, 99 per cent of the water vapour and the aerosols, and almost all of the weather. It is generally unstable, it is turbulent and well mixed, and it is where almost all flying is done.
Joshi subdivides it, and DGCA has asked for the boundaries:
| Sub layer | From | To |
|---|---|---|
| Lower troposphere | Surface | 2.1 km |
| Mid troposphere | 2.1 km | 7.6 km |
| Upper troposphere | 7.6 km | Tropopause |
Tropopause
The boundary between the troposphere and the stratosphere, and the level where temperature ceases to fall with an increase in height. Oxford gives the practical test: it is taken as the height where the fall in temperature drops below 0.65°C per 100 m, that is 2°C per 1000 ft. Joshi says the same thing from the other side, that the lapse rate at the tropopause reduces to 1 to 2°C per km.
The tropopause gets a page of its own next, because it carries more exam weight than any other boundary here.
Stratosphere
The layer above the tropopause. Temperature stays constant for the first 8 to 10 km, to an average height of about 20 km, then increases, slowly at first and sharply towards the top, reaching about −2.5°C at 47 km. Above about 51 km it starts falling again.
The cause of that increase is the one thing to remember about the layer: ozone absorbing ultraviolet radiation from the sun. The absorption releases heat, so this layer is warmed from above, the exact opposite of the troposphere. The ozone layer itself sits inside it at around 25 km.
For a pilot the stratosphere is attractive air. It is very dry, stable, has no weather, and its winds are steady, strong and horizontal. The only cloud you will see is nacreous cloud, also called mother of pearl cloud, occasionally in the upper stratosphere at high latitudes in winter.
Its upper boundary is the stratopause, at an average height of about 50 km in temperate latitudes.
Mesosphere
From the stratopause up to about 80 km. There is little solar absorption here and weak vertical mixing, so temperature falls again with height. On rare occasions noctilucent cloud is seen in the upper mesosphere over polar regions.
Mesopause
The top of the mesosphere, where temperature stops falling. It is the coldest part of the whole atmosphere, about −100°C, which is 173 K, at 80 km. Meteors burn up in this region.
Thermosphere, ionosphere and exosphere
From the mesopause upward, temperature increases with height again. The atmosphere above about 60 km is also called the ionosphere, which is what makes long range radio propagation possible, and which is why serious radio interference happens during solar disturbances. Above the thermosphere is the exosphere, extending up to 10 000 km, where molecules and atoms escape into space. The air there is so thin that individual particles can be extremely energetic while the layer as a whole holds almost no heat at all.
The Stratosphere is the layer above the troposphere where temperature initially remains constant to an average height of 20 km then increases to reach a temperature of −2.5°C at a height of 47 km, then above 51 km temperature starts to decrease again. The reason for the increase is the action of ultraviolet radiation in the formation of ozone.Oxford ATPL Vol 9, 1.6
The inversion is produced by the absorption of UV radiation from sun by ozone, which safeguards earth from harmful radiation. It is a very dry and stable region with low humidity and no weather.IC Joshi, Aviation Meteorology, chapter 1
The tropopause, and why it earns a page
Two facts control everything in this section, and they point in opposite directions, which is exactly why students get them backwards.
Higher tropopause, colder tropopause. The temperature at the tropopause is controlled by its height.
Put together: the equator has the warmest surface, so it has the highest tropopause, so it has the coldest tropopause. The poles have the coldest surface, the lowest tropopause, and therefore the warmest tropopause. Every tropopause question is some version of that chain.
The numbers to carry
| Where | Height | Temperature |
|---|---|---|
| Equator | 16 to 18 km | −75°C to −80°C |
| 50°N, the ISA case | 11 km, 36 090 ft | −56.5°C |
| Poles | 8 km, Joshi says 8 to 10 | −40°C to −50°C |
Breaks, folds and the three sections
There are two places where the tropopause abruptly changes height, called breaks or folds, at approximately 40° and 60° of latitude. The break at 40° is the more prominent one. Jet streams occur at these breaks, which is the whole reason the breaks are examinable. They divide the tropopause into three sections:
- Tropical tropopause. Usually at the 100 hPa level, from the equator out to about 35° to 40°. Over India it sits at about 16 to 16.5 km.
- Middle tropopause. Near the 200 hPa level, between the polar and tropical sections. Over India it is at about 11.5 km with a temperature near −45°C, and in winter it can reach down to 23°N. The subtropical jet stream sits at its southern end.
- Polar tropopause. Near the 300 hPa level, poleward of about 45° to 60°. It is occasionally noticed over Srinagar in winter when polar air pushes south. The polar front jet stream sits at its southern end.
Why the tropopause height matters to you
Its height usually marks:
- the maximum height of significant cloud
- the presence of jet streams
- the presence of clear air turbulence, now written as TURB in forecasts
- the maximum wind speed
- the upper limit of most of the weather
All of those sit just below the tropopause, not above it. That is why a cruising level chosen relative to the tropopause is a real operational decision and not a piece of trivia.
Temperature across the globe, and the reversal at 8 km
In the troposphere, temperature increases from the poles to the equator, which is what you would expect. In the lower stratosphere it increases from the equator to the poles in summer, and reaches its maximum in mid latitudes in winter.
Joshi makes the consequence explicit, and it is a favourite question. Because temperature keeps falling with height right up to a much higher tropopause at the equator, and stops falling much lower down over the poles, above about 8 km the poles are warmer than the equator. That is the reversal of temperature and density above 8 km.
Atmospheric hazards at altitude
As operating altitudes rise, two things become a real concern. Ozone, because above 50 000 ft normal concentrations exceed tolerable limits and the air has to be filtered before it enters the cabin. In practice the heat of the compressor system breaks most of it down to an acceptable level anyway. And cosmic radiation, which is not normally hazardous, but during solar flare activity a lower flight level may be necessary.
The International Standard Atmosphere
Temperature and pressure vary with time and position, both horizontally and vertically. That is a problem, because an altimeter has to be calibrated against something and an aircraft has to be performance tested against something. So aviation agreed on an imaginary average atmosphere and calibrates everything against it.
It exists for exactly two reasons: the calibration of aircraft pressure instruments, and the design and testing of aircraft. Joshi adds performance evaluation, which is the same idea applied daily.
The ICAO International Standard Atmosphere
| Quantity | Standard value |
|---|---|
| Air | Dry |
| MSL temperature | +15°C, 288.15 K |
| MSL pressure | 1013.25 hPa |
| MSL density | 1225 g per cubic metre |
| Acceleration due to gravity | 980.665 cm per second squared |
| Lapse rate to 11 km, 36 090 ft | 0.65°C per 100 m, 1.98°C per 1000 ft |
| 11 km to 20 km, 65 617 ft | Constant at −56.5°C |
| Above 20 km | Rise of 0.1°C per 100 m, 0.3°C per 1000 ft, to −44.5°C at 32 km |
ISA is defined all the way from −5 km to 80 km, but between mean sea level and 20 km is the only part you need. For calculations up to the tropopause everyone uses a lapse rate of 2°C per 1000 ft.
ISA temperature at a level
Reduce the sea level temperature by 2°C for every 1000 ft:
At 18 000 ft: 15 − (2 × 18) = −21°C.
Above 36 000 ft the ISA is isothermal, so the answer is −57°C no matter how much higher you go.
ISA deviation
To work out true altitude, and to assess performance data, you need to know how far the real day differs from the standard one. Subtract the standard from the real:
At 18 000 ft with an actual temperature of −27°C: −27 − (−21) = ISA −6.
Practise the arithmetic
Work these out before you look at the answers. This is exactly the form DGCA uses.
| Height | Actual temp | ISA temp | ISA deviation |
|---|---|---|---|
| 5 000 ft | +9°C | +5°C | ISA +4 |
| 10 000 ft | −10°C | −5°C | ISA −5 |
| 25 000 ft | −30°C | −35°C | ISA +5 |
| 39 000 ft | −62°C | −57°C | ISA −5 |
Standard pressure levels, and the flight levels they sit at
Upper air charts are drawn at fixed pressures, not fixed heights, and the back questions in both books assume you know which flight level each one corresponds to in the standard atmosphere. Learn this table. It is the most quoted piece of arithmetic in the whole subject.
| Pressure level | Approx height | Flight level | ISA temperature |
|---|---|---|---|
| 1013 hPa | Mean sea level | — | +15°C |
| 850 hPa | 5 000 ft | FL50 | +5°C |
| 700 hPa | 10 000 ft | FL100 | −5°C |
| 500 hPa | 18 000 ft | FL180 | −21°C |
| 400 hPa | 24 000 ft | FL240 | −33°C |
| 300 hPa | 30 000 ft | FL300 | −45°C |
| 250 hPa | 34 000 ft | FL340 | −53°C |
| 200 hPa | 39 000 ft | FL390 | −57°C |
Once you have the table, a whole family of questions becomes one step. Asked to judge a temperature of −15°C at the 700 hPa level, read 700 hPa as FL100, where ISA is −5°C, so the air is 10 degrees below standard, which is cold for that level.
Finding the freezing level
The same arithmetic run backwards gives the freezing level, and it comes up constantly in the icing chapter later.
You need to gain 8 degrees to reach zero, and temperature rises 2°C per 1000 ft as you descend, so descend 8 ÷ 2 = 4 000 ft. The freezing level is FL100.
Lapse rate, inversion and isothermal
Three words describe what temperature can do with height, and DGCA uses all three.
- A positive lapse rate is the normal case, temperature falling with height.
- A negative lapse rate means temperature rising with height. That is an inversion, and the stratosphere is the largest one in the atmosphere.
- Isothermal means temperature unchanged with height, which is what the ISA does between 11 km and 20 km.
The Jet Standard Atmosphere
ISA values work well below 30 000 ft but are less satisfactory above it. For space flight, very high altitude flying and engine manufacture, a simplified standard called the Jet Standard Atmosphere is used instead:
- Mean sea level temperature +15°C, 288.15 K, the same as ISA
- Lapse rate a flat 2°C per 1000 ft
- There is no tropopause, so the cooling never stops
That last point is the entire difference worth remembering. JSA keeps cooling for ever, ISA goes isothermal at 36 090 ft.
To determine true altitude and for the assessment of performance data it is necessary to determine the temperature deviation from the ISA at any specified altitude.Oxford ATPL Vol 9, 1.11
To find ISA Deviation: Subtract ISA value from Actual value. i.e. ISA Deviation = (Actual − ISA)IC Joshi, Aviation Meteorology, chapter 1