The Atmosphere: Composition, Vertical Structure and the ICAO Standard Atmosphere
This domain covers what the air is made of, how it is layered, and the ICAO Standard Atmosphere that every altimetry and temperature question is referenced to. The exam asks short, numerical or definitional items: standard temperature at a level, which layer holds the weather, or the standard tropopause figure.
Composition
- Nitrogen about 78% and oxygen about 21% by volume, argon about 0.93%, carbon dioxide about 0.04%.
- Water vapour is the only constituent that varies strongly in space and time - from near zero to about 4% - and it is the gas that matters for cloud, fog and latent heat.
- Ozone is concentrated in the stratosphere and absorbs ultraviolet radiation; condensation nuclei are required for every cloud droplet and fog particle.
- Pressure, density and temperature all decrease with height in the troposphere, but at different rates.
Vertical structure
- Troposphere: heated from below, temperature falls with height, contains roughly three-quarters of the mass and almost all water vapour and weather.
- Tropopause: an isothermal dividing layer, higher over the tropics (about 55,000 ft) and summer, lower over the poles (about 25,000 ft) and winter.
- Stratosphere: isothermal then warming by ozone absorption; extremely stable, very dry, free of significant weather - the reason high cruise levels are smooth.
- Mesosphere cools again to the coldest temperatures in the atmosphere; the thermosphere warms strongly with increasing height.
ICAO Standard Atmosphere
- Mean sea level: 15°C, 1013.25 hPa, density 1.225 kg/m³, relative humidity nil.
- Temperature lapse 1.98°C per 1,000 ft (6.5°C/km) up to 11 km, then constant at 216.65 K (-56.5°C) - a tropopause at 36,090 ft.
- Pressure halves roughly every 5,000 m (about 18,000 ft).
Pressure altitude and the three settings
- QFE is station pressure; with it set the altimeter reads height above the aerodrome reference point. QNH is pressure reduced to mean sea level using the ISA, so the altimeter reads elevation on the ground.
- QNE is the standard 1013.25 setting used at and above the transition altitude, where the altimeter reads flight levels.
- Near the surface 1 hPa is about 27 ft (8 m); at high level the same hectopascal is nearer 50 ft.
Worked example
An aerodrome at elevation 1,000 ft reports QNH 1003 hPa. An altimeter left at 1013.25 reads 1,000 ft plus (1013.25 − 1003) × 27 ft ≈ 1,000 + 277 = about 1,280 ft. The real pressure is below standard, so pressure altitude exceeds elevation. Reverse the arithmetic to obtain QNH from a known elevation and an indicated altitude.
Traps that cost marks
- Quoting the standard tropopause as 36,000 ft, or applying the lapse rate above 11 km.
- Confusing QFE (height) with QNH (elevation) when a question asks for terrain clearance.
- Claiming carbon dioxide is the main variable absorber - it is water vapour.