The Atmosphere: Composition, Structure and Standard Atmosphere
What the exam tests
This domain checks atmospheric composition, vertical structure and the ISA, plus your ability to convert between altitude, temperature and pressure.
Core rules
- Dry air is about 78% nitrogen, 21% oxygen, 1% argon and 0.04% carbon dioxide; water vapour is variable and strongly affects density.
- The troposphere contains most weather and is where temperature falls with height at the environmental lapse rate. The tropopause is a stable layer above it; its height varies from roughly 36,000 ft in ISA to lower near the poles and higher near the equator.
- ISA sea level is 15°C, 1013.25 hPa and 1.225 kg/m³. Temperature falls 1.98°C per 1,000 ft to 36,090 ft, then remains -56.5°C.
- Pressure falls with height; near sea level, about 1 hPa per 30 ft is a useful planning approximation.
Worked example
At 5,000 ft in ISA, temperature = 15 - (5 × 1.98) = 5.1°C. Pressure is approximately 1013 - (5,000/30) = 846 hPa. The temperature result is the ISA value; the pressure result is only an approximation and is not a substitute for an altimeter setting.
Traps
Mixing per-1,000-ft and per-km lapse rates; assuming a fixed tropopause; ignoring humidity when comparing density; confusing QFE with QNH.