03 Structure and physical properties of the layers
Temperature, not composition, is what divides the atmosphere into layers.
- Troposphere (0–12 km): temperature falls from about 15 °C to −56 °C; convection, clouds, rain and all weather occur here; combustion products and dust are trapped in it.
- Stratosphere (12–50 km): temperature rises from −56 °C to about −2 °C because ozone absorbs UV and releases heat; the air is stable and dry, so jet aircraft and balloons operate here.
- Mesosphere (50–85 km): temperature falls again to about −90 °C — the coldest layer; meteors burn up in it.
- Thermosphere (85–600 km): temperature climbs above 1,000 °C because the few molecules absorb intense UV and X-rays; the air is so thin that the heat would not warm an object. Contains the ionosphere; the International Space Station orbits here.
- Exosphere (600 km and above): particles escape into space; merges with the magnetosphere.
Composition of dry air: N₂ 78.08%, O₂ 20.95%, Ar 0.93%, CO₂ 0.04%, with neon, helium, krypton, xenon and variable water vapour (0–4%) and dust that acts as nuclei for condensation.