The Atmosphere

The atmosphere is the envelope of gases held around the Earth by gravity. For a pilot it is both the medium that supports the aeroplane and the source of its weather. Its pressure, temperature, density and humidity all vary from place to place and with height, and each of them changes what the altimeter reads, how much lift the wing produces and how much thrust the engines deliver.

Because the real atmosphere never stands still, aviation measures it against an agreed model, the International Standard Atmosphere (ISA). Altimeters, airspeed indicators and performance charts are all calibrated to it, so a working knowledge of the model and of how the real air departs from it underpins altimetry, performance and much of meteorology.

Composition of the atmosphere

Dry air is a mixture of gases whose proportions by volume are:

Turbulent mixing keeps these proportions constant up to about 60 km; above that the gases begin to separate by molecular weight. The exception is ozone, whose concentration varies with height and place. The constancy matters physiologically: oxygen is still 21 per cent of the air at 40,000 ft, and hypoxia at altitude comes from the fall in total pressure, and so in the partial pressure of oxygen, not from any change in the mixture (see hypoxia and hyperventilation).

Water vapour is not counted in the table because its share varies, from almost nothing in polar or desert air to a few per cent in warm, moist air near the surface. Meteorologically it is the most important gas of all. It forms cloud, fog and precipitation, and when it condenses it releases latent heat that drives convection (see humidity and water vapour).

Ozone and greenhouse gases

The greenhouse gases, chiefly water vapour, carbon dioxide and methane, are almost transparent to the short-wave radiation arriving from the sun but absorb the long-wave radiation emitted by the Earth's surface. The atmosphere is therefore heated mainly from below, by the ground and the sea, which is why temperature normally falls with height in the lowest layer. Cloud acts in the same way at night, which is why an overcast night stays warmer than a clear one (see temperature and heat exchange).

Ozone is a form of oxygen with three atoms per molecule. Most of it lies in the stratosphere, where it absorbs ultraviolet radiation from the sun; that absorption heats the upper stratosphere and explains why temperature there rises with height. Ozone is toxic: it irritates the lungs, causes headaches and impairs night vision. Its concentration increases above the tropopause, and where the tropopause is low, as at high latitudes in winter, ozone-rich air can reach normal cruising levels. Jet aircraft reduce it in the cabin air supply, partly through the heat of compression and on many types with catalytic converters.

The troposphere

The troposphere is the lowest layer, extending from the surface to the tropopause. Its defining feature is that temperature decreases with height, on average by about 2 °C per 1,000 ft. It holds about three quarters of the mass of the atmosphere and almost all its water vapour, so nearly all cloud and weather forms within it.

Because it is heated from below, the troposphere is continually stirred by convection and turbulence. Its depth depends on how warm the air near the surface is: warm air expands and pushes the top of the layer higher. The troposphere is therefore about 16 to 18 km deep over the equator, about 11 km in mid-latitudes and about 8 km over the poles. Horizontally, tropospheric temperature increases from the poles towards the equator.

The tropopause and stratosphere

The tropopause is the boundary between the troposphere and the stratosphere, where temperature stops falling appreciably with height. In practice it is taken as the lowest level at which the lapse rate falls to 2 °C per km (0.2 °C per 100 m) or less. A climbing crew sees it on the temperature display: the outside air temperature falls steadily, then stops and stays almost constant.

The table holds the point examiners like: the higher the tropopause, the colder it is, because the temperature has kept falling through a deeper troposphere. The tropopause is higher in summer than in winter, and it does not slope smoothly from equator to pole but drops in steps. The main breaks lie near 40° and 60° latitude, and the jet streams lie close to them. On WAFS significant weather charts the tropopause height appears as a flight level in a rectangular box.

The tropopause usually marks the upper limit of most weather and of significant cloud, the level of the strongest winds, and the zone where clear air turbulence is most common. A cumulonimbus meets it as a lid: the updraught spreads into an anvil, although the strongest tops can overshoot into the lower stratosphere.

The stratosphere extends from the tropopause to about 50 km. Temperature first stays roughly constant with height and then increases, warmed by the ozone absorbing ultraviolet light. Air whose temperature is constant or rising with height is very stable, so vertical motion is suppressed, and the stratosphere is dry and almost free of cloud. Its horizontal temperature pattern differs from the troposphere's: in summer the lower stratosphere is warmer over the poles than over the equator, where the tropopause is high and very cold, and in winter it is warmest in mid-latitudes. Above the stratosphere lie the mesosphere and thermosphere, far above any aeroplane.

The International Standard Atmosphere

Instruments and performance data need a single reference, so ICAO defines the ISA. It is an idealised atmosphere with fixed values:

The FAA states the same model in its own units: 29.92 inHg and 15 °C at sea level, with pressure falling about 1 inHg and temperature about 2 °C (3.5 °F) per 1,000 ft in the lower atmosphere.

Below the tropopause the ISA temperature is 15 °C minus 2 °C per 1,000 ft: −13 °C at FL140, −1 °C at 8,000 ft. The ISA also fixes the pressure at every height, so each standard pressure level used for upper-air charts has an approximate flight level:

A flight level is itself a surface of constant pressure: flight level zero is the 1013.2 hPa isobaric surface, and successive levels are 500 ft apart in the standard atmosphere. A chart for the 300 hPa level therefore describes conditions at about FL300, whatever the true height of that surface on the day. The pressure altimeter converts pressure to height with the ISA relationship, which is why every departure of the real air from the model becomes an altimeter error (see altimeter settings, altitude and height).

View full schematic

ISA deviation

ISA deviation is the actual temperature minus the ISA temperature at the same pressure altitude. It is written as ISA +10 or ISA −5, for example, and it is the figure performance charts, cruise tables and density calculations are entered with.

• At 24,000 ft the ISA temperature is 15 − (2 × 24) = −33 °C. An outside air temperature of −35 °C is therefore ISA −2: colder than standard, even though both numbers are negative.
• Above 36,090 ft the ISA temperature remains −56.5 °C. At FL450 it is still −56.5 °C, not the −75 °C that extending the lapse rate would give.

A warm deviation lowers air density, so the aeroplane performs as if it were higher. A handy rule is that density altitude rises by about 120 ft for each degree Celsius above ISA: at a 5,000 ft pressure altitude and +30 °C, ISA +25, the aeroplane behaves as though it were at about 8,000 ft. A cold deviation improves performance but makes the altimeter over-read, by about 4 ft per 1,000 ft for each degree below ISA. In jet cruise a large warm deviation reduces the maximum altitude and raises fuel burn.

Air density and the gas laws

Density is mass per unit volume, expressed in kg/m³ or g/m³. Air behaves very nearly as an ideal gas, whose pressure, volume and temperature are linked by the combined gas law, PV/T = constant, or in meteorological form P = ρRT, where ρ is density, R the gas constant for air and T the absolute temperature in kelvin (°C + 273, so ISA sea level is 288 K). Rearranged, density is proportional to pressure and inversely proportional to absolute temperature:

• Raising the pressure compresses the air and increases its density.
• Heating the air makes it expand, which reduces its density.
• Adding water vapour reduces density, because water molecules are lighter than those of nitrogen and oxygen.

As a rough guide, density changes by about 1 per cent for a 3 °C change of temperature or a 10 hPa change of pressure. With height, the fall in pressure outweighs the fall in temperature, so density falls steadily:

Density also varies with latitude. At the surface it increases towards the poles, because the air is colder. At about 26,000 ft it is roughly the same at all latitudes, and above that level it decreases towards the poles.

Density drives performance. Lift and drag depend on ½ρV², so in thin air the wing needs the same indicated airspeed but a higher true airspeed, the take-off run lengthens, the climb gradient falls and the maximum take-off mass may be limited. Engines and propellers lose thrust as well. High aerodromes such as Denver and Nairobi, and hot ones such as Bahrain, are the classic cases. Lower density also explains why upper winds are stronger than surface winds: for the same pressure gradient the geostrophic wind is inversely proportional to density, so at 20,000 ft, where density is about half, it is roughly double (see wind forces and geostrophic wind).

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.