Concept Notes 1: Atmospheric Composition, Thermal Structure & Insolation
1. Composition of the Atmosphere
The atmosphere is a mechanical mixture of gases, water vapor, and aerosols held gravitationally to Earth:
- Major Gases: Nitrogen ($N_2$ ~78.08%), Oxygen ($O_2$ ~20.95%), Argon ($Ar$ ~0.93%), Carbon Dioxide ($CO_2$ ~0.04%).
- Variable Constituents: Water vapor (0% in dry deserts to 4% in humid tropics; drives precipitation), Dust particles, sea salt, pollen (hygroscopic condensation nuclei).
2. Atmospheric Thermal Layers
[ EXOSPHERE ] --> Up to 10,000 km, ultra-thin helium and hydrogen gas.
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[ THERMOSPHERE ] --> 85 - 600 km, temperature rises to 1500°C; contains Ionosphere (radio reflection, auroras).
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[ MESOSPHERE ] --> 50 - 85 km, coldest layer (-90°C), meteors burn up.
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[ STRATOSPHERE ] --> 12 - 50 km, temperature increases with altitude; contains Ozone Layer (O3); smooth flight.
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[ TROPOSPHERE ] --> 0 - 12 km (18 km at equator, 8 km at poles); 75% atmospheric mass; Normal Lapse Rate 6.5°C/km.
3. Solar Radiation, Insolation & Heat Budget
- Insolation: Incoming shortwave solar electromagnetic radiation ($0.2–4 \mu m$).
- Terrestrial Radiation: Longwave infrared radiation ($4–100 \mu m$) emitted back by the warmed Earth surface.
- Greenhouse Effect: Natural warming mechanism where atmospheric $CO_2$, water vapor, and methane absorb outgoing longwave terrestrial radiation, keeping Earth's mean temperature at +15°C instead of -18°C.