The Atmosphere: Composition, Structure and Standard Atmosphere
Composition and vertical structure
Know the dry-air mix by volume: roughly 78% nitrogen, 21% oxygen, and about 1% argon plus trace gases. Water vapour is variable (0-4%) and matters far more than its share suggests, because it drives latent heat release, cloud formation and density changes. Aviation questions often test the variable constituents: carbon dioxide and ozone are tiny fractions but ozone in the stratosphere absorbs ultraviolet radiation, while the troposphere holds nearly all water vapour and weather.
Temperature defines the layers. In the troposphere temperature falls with height (average about 2°C per 1000 ft, or 6.5°C per km) and this layer contains about 75% of the mass and almost all weather. The tropopause is the boundary; it is highest over the equator (around 50,000-55,000 ft) and lowest over the poles (around 25,000-35,000 ft), and it is a favourite exam trap. Above it the stratosphere is isothermal then warming with height, giving smooth air and good visibility. Then come the mesosphere and thermosphere.
Standard atmosphere and traps
The ICAO Standard Atmosphere is a reference, not a forecast: mean sea level pressure 1013.25 hPa, temperature 15°C, density 1.225 kg/m³, lapse rate 1.98°C per 1000 ft up to 11 km (36,090 ft), where temperature is -56.5°C. Candidates confuse the tropopause height in the standard atmosphere (36,090 ft / 11 km) with the real tropopause. They also forget that pressure, temperature and density all decrease with height, but density decreases more slowly than pressure because temperature also falls.
Worked example
Standard sea-level temperature is 15°C and the lapse rate is 2°C per 1000 ft. At 20,000 ft the standard temperature is 15 - (20 × 2) = -25°C. If the actual temperature there were -10°C, the air is warmer than standard, so pressure levels are further apart and true altitude is higher than indicated.