The Atmosphere: Composition, Structure and Standard Atmosphere
Composition
The atmosphere is a mechanical mixture of gases: about 78% nitrogen, 21% oxygen and 1% argon, with variable water vapour (0–4%), carbon dioxide and traces of others. Candidates must separate the permanent gases from the variable ones. Water vapour is the key variable gas for aviation because it drives stability, cloud and icing, and it is lighter than dry air, so moist air is less dense at the same pressure and temperature.
Vertical structure
Temperature behaviour defines the layers. In the troposphere temperature falls with height at the environmental lapse rate (average 2°C/1000 ft, 6.5°C/km); it contains nearly all weather and water vapour. The tropopause is the boundary where the lapse rate becomes isothermal or reverses; its height varies - roughly 16–18 km over the tropics and 8–10 km over the poles, higher in summer and lower in winter. Above lie the stratosphere (ozone heating, temperature rising with height, stable, good for high-level flight), mesosphere and thermosphere.
Standard atmosphere
ISA assumes sea-level pressure 1013.25 hPa, temperature 15°C, density 1.225 kg/m³, a lapse rate of 1.98°C/1000 ft up to 11 km, then isothermal at −56.5°C. All altimeters, performance charts and Mach/true-airspace relationships are calibrated to ISA.
Traps
Questions confuse the tropopause with a fixed level, or claim temperature always decreases with height. Others mix up the troposphere's weather-bearing property with the stratosphere's stability. Remember that pressure halves roughly every 18,000 ft.
Rule of thumb
Sea-level temperature ≈ 15°C − 2°C per 1000 ft up to the tropopause; if the tropopause is at 36,000 ft, expect about −57°C.