The Atmosphere: Composition, Structure and Standard Atmosphere
This domain tests the physical baseline on which every altimetry and performance question rests: what air is made of, how temperature and pressure change with height, and how the ICAO Standard Atmosphere defines the reference values.
Composition
Dry air is 78.08% nitrogen, 20.95% oxygen, 0.93% argon and about 0.04% carbon dioxide by volume. Water vapour is the variable component, from near zero to roughly 4% in warm humid air. It is the only gas that changes phase at normal atmospheric temperatures, so it drives cloud, latent heat release and most weather.
Vertical structure
The troposphere holds about 75% of the mass and almost all the water vapour, with a mean environmental lapse rate of 6.5°C/km. The tropopause lies near 16-18 km over the equator but only 8-10 km over the poles; it is marked by an abrupt lapse-rate change and is where jet streams concentrate. Above it are the stratosphere (isothermal, then warming through the ozone layer), mesosphere and thermosphere.
ISA reference
At MSL the ISA gives 15°C, 1013.25 hPa and density 1.225 kg/m³. Temperature falls at 6.5°C/km to 11 km (-56.5°C), stays constant to 20 km, then rises 1°C/km to 32 km.
Worked example
ISA temperature at 5,000 ft: 5,000 ft = 1,524 m, so 15 - (1.524 × 6.5) = 15 - 9.9 ≈ 5°C. At 20,000 ft (6,096 m) the tropopause has already been crossed, so the ISA value is the constant -56.5°C, not 15 - 39.6.
Traps
Do not confuse the ISA lapse rate with the actual ELR; do not apply 6.5°C/km above 11 km; remember tropopause height changes with latitude and season, which changes cruise temperatures and turbulence.