The Atmosphere: Composition, Structure and Standard Atmosphere
Composition
Dry air is close to 78% nitrogen, 21% oxygen and 1% argon, with trace gases including carbon dioxide. Water vapour is variable, up to about 4% by volume in the tropics, and it is the gas that matters most for weather. Aerosols and dust act as condensation nuclei.
Vertical structure
Temperature behaviour defines the layers. The troposphere cools at roughly 2°C per 1000 ft (6.5°C/km) on average, contains about 75% of the mass and almost all the water vapour, and ends at the tropopause. The tropopause is higher over the equator (around 50,000–55,000 ft) and lower over the poles (around 25,000–35,000 ft); it is a favoured level for jet streams and clear air turbulence. Above it the stratosphere warms with height because ozone absorbs ultraviolet radiation, giving a stable layer that suppresses vertical motion. The mesosphere cools again, and the thermosphere warms strongly.
Standard atmosphere
ISA assumes sea-level pressure 1013.25 hPa, temperature 15°C, density 1.225 kg/m³, and a lapse rate of 1.98°C per 1000 ft up to 36,090 ft, then isothermal at −56.5°C. These values let altimeters and performance charts share one reference.
Traps
Questions often confuse the layer that contains most water vapour (troposphere) with the layer that contains the ozone layer (stratosphere). Do not assume the tropopause is a fixed height. Remember that the standard lapse rate is a defined average, not the actual atmosphere.
Worked example
At 20,000 ft in ISA, temperature is 15 − (1.98 × 20) = 15 − 39.6 ≈ −24.6°C. Pressure at that level is about 466 hPa, roughly half of sea-level pressure.