The Atmosphere: Composition, Structure, and Standard Atmosphere
What you must know
Candidates should be able to describe the atmosphere as a mixture dominated by nitrogen (about 78% by volume) and oxygen (about 21%), with argon, carbon dioxide, water vapour, and trace gases making up the remainder. Water vapour is highly variable (roughly 0–4% by volume) and matters disproportionately for weather. The vertical structure is defined by temperature trend, not by fixed heights: the troposphere (temperature decreases with height), tropopause (a lapse-rate break, the lid on most weather), stratosphere (temperature increases due to ozone absorption), mesosphere, and thermosphere. Know typical tropopause heights: roughly 8 km at high latitudes in winter, 16–17 km in the tropics, and that the tropopause is higher and colder over cold-air lows than over warm ridges.
The standard atmosphere
Memorise the ISA surface values and lapse rates: 15 °C, 1013.25 hPa, 1.225 kg/m³ at mean sea level; tropospheric lapse rate 1.98 °C per 1000 ft (about 2 °C/1000 ft); tropopause at 36,090 ft with temperature −56.5 °C; above that, isothermal to about 65,000 ft. These numbers underpin altimetry, density altitude, and pressure-level charts.
Traps
Questions often swap mass percentage with volume percentage, or claim temperature always decreases with height. Another classic trap is confusing the tropopause with a fixed altitude rather than a thermal boundary. Do not confuse the ozone layer's warming effect with the lapse rate of the troposphere.
Rule of thumb
For a quick check: sea-level pressure falls roughly 1 hPa per 30 ft near the surface, and temperature falls about 2 °C per 1000 ft in the troposphere. Example: at 10,000 ft ISA temperature is about 15 − 20 = −5 °C.