Climatology: atmosphere, pressure belts & global winds
Atmospheric composition and structure, the global pressure-belt system, planetary winds, and the mechanisms that drive them—core UPSC physical-geography climatology.
Composition of the atmosphere
The atmosphere is a mechanical mixture of gases held by gravity. By volume in dry air, nitrogen constitutes 78.08 percent, oxygen 20.95 percent, argon 0.93 percent and carbon dioxide approximately 0.04 percent (≈420 ppm as measured at Mauna Loa Observatory, 2024). Permanent gases dominate to about 80 km; variable constituents—water vapour (0–4 percent), ozone, dust and aerosols—control weather and radiation. Water vapour, concentrated below 5 km, supplies latent heat and is the atmosphere's principal greenhouse agent; carbon dioxide and ozone are the others examiners pair with the Stefan-Boltzmann and greenhouse logic.
Vertical layering
The atmosphere is layered by its temperature gradient, not its density:
- Troposphere (0–8 km at poles, 0–18 km at the equator): temperature falls at the normal lapse rate of 6.5 °C per 1,000 m. All weather occurs here. Its ceiling is the tropopause.
- Stratosphere (up to ~50 km): temperature rises because the ozone layer (concentrated 15–35 km) absorbs ultraviolet radiation. The ozone maximum and the 1987 Montreal Protocol that protects it are recurrent prelims hooks. Jet aircraft cruise in the lower stratosphere for its stability.
- Mesosphere (up to ~80 km): temperature falls again to roughly −90 °C at the mesopause, the coldest part of the atmosphere; meteors burn here.
- Thermosphere / ionosphere (80–400 km): temperature rises sharply; ionised layers reflect radio waves and host the aurora.
- Exosphere: merges into space.
Insolation and the heat budget
The Earth intercepts solar energy as insolation; the angle of incidence, day length and atmospheric transparency govern how much reaches the surface. Of incoming shortwave radiation, roughly 35 percent is reflected (planetary albedo), 14 percent absorbed by the atmosphere and 51 percent absorbed by the surface. The surface re-radiates as longwave terrestrial radiation, which water vapour and CO₂ trap—the greenhouse effect. Averaged over the year the planet is in radiative balance, but it shows a surplus between 40°N and 40°S and a deficit poleward. This latitudinal energy imbalance is the engine of every wind, ocean current and storm—and the conceptual spine of this lesson. Heat is redistributed poleward by atmospheric circulation (about two-thirds) and ocean currents (about one-third), preventing the tropics from overheating and the poles from freezing solid.
The processes of heat transfer—radiation (Sun to Earth), conduction (surface to adjacent air), convection (vertical) and advection (horizontal)—must be distinguished precisely; UPSC has tested advection (e.g., the warming of Western Europe by air moving over the North Atlantic Drift) directly.