The Atmosphere: Composition, Structure, Temperature and Pressure
Composition
The atmosphere is a mechanical mixture of gases: about 78% nitrogen, 21% oxygen, and roughly 1% argon plus trace gases. Water vapour is highly variable (0–4% by volume) and is the key gas for weather because it supplies latent heat and cloud. Carbon dioxide (~0.04%) matters for radiation. Aerosols give condensation nuclei.
Vertical structure
Temperature defines the layers. In the troposphere temperature falls with height (average lapse rate 6.5 °C/km) and this is where all weather occurs; it is capped by the tropopause, higher over the equator (~16–18 km) and lower over the poles (~8–10 km). Above it the stratosphere has an isothermal lower part then warms with ozone absorption to the stratopause. The mesosphere cools again to the mesopause, then the thermosphere warms strongly.
Pressure
The atmosphere is held by gravity, so pressure and density decrease with height - but not linearly: pressure halves roughly every 5.5 km. Pressure is measured in hPa (mb); mean sea-level value is 1013.25 hPa. The tropopause acts as a lid because the temperature inversion stops vertical motion.
Traps
Questions often confuse the standard lapse rate with the environmental one, or ask where ozone is concentrated (stratosphere, not troposphere). Another trap: the tropopause is higher (colder) over the equator - students sometimes reverse this. Remember nitrogen is the most abundant gas, not oxygen.
Rule of thumb
Pressure halves about every 5.5 km; temperature falls about 2 °C per 300 m in the standard troposphere.