The Atmosphere: Composition, Vertical Structure and Standard Atmosphere
The atmosphere is a mechanical mixture of gases, not a chemical compound: roughly 78% nitrogen, 21% oxygen, 0.93% argon and 0.04% carbon dioxide by volume, with water vapour variable from near zero to about 4%. Candidates must know that water vapour and aerosols are the variable constituents, and that ozone concentrates in the stratosphere where it absorbs ultraviolet radiation and produces the inversion that caps the troposphere.
Vertical structure
The troposphere holds about 75% of the mass and almost all the water vapour; its depth varies from roughly 8 km at the poles to 16-18 km over the equator, and it is deeper in summer than winter. The tropopause is the boundary, marked by a sharp change of lapse rate to near isothermal, and it is where jet streams and much clear-air turbulence are found. Above it lie the stratosphere (isothermal then warming), mesosphere (cooling to the coldest temperatures in the atmosphere) and thermosphere.
Standard atmosphere
ISA assumes sea-level pressure 1013.25 hPa, temperature 15 °C, density 1.225 kg/m³, a tropospheric lapse rate of 1.98 °C per 1000 ft (6.5 °C/km) up to 11 km, then isothermal at −56.5 °C. These values underpin altimeter and performance calculations.
Trap: questions often swap the lapse rates or ask where temperature stops falling - the answer is the tropopause, not the stratopause.
Worked example: ISA temperature at 20 000 ft = 15 − (20 × 1.98) = 15 − 39.6 = −24.6 °C.