Climatology: atmosphere, pressure belts & global winds

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.

The seven global pressure belts

Differential heating creates alternating thermal and dynamic pressure belts:

1. Equatorial Low (Doldrums, 0°–5°) — thermally induced; intense surface heating drives convection, calm or variable winds, heavy convectional rainfall. It coincides with the Inter-Tropical Convergence Zone (ITCZ).
2. Sub-tropical Highs (~30°N/S, Horse Latitudes) — dynamically induced by the subsidence of air descending from the upper troposphere; clear skies and the world's hot deserts (Sahara, Atacama, Kalahari) lie beneath them.
3. Sub-polar Lows (~60°N/S) — dynamically induced where warm tropical and cold polar air converge; cradle of mid-latitude cyclones along the polar front.
4. Polar Highs (~90°N/S) — thermally induced by intense cold and subsidence.

The distinction between thermally induced (equatorial, polar) and dynamically induced (sub-tropical, sub-polar) belts is a high-frequency MCQ.

Planetary winds and the Coriolis force

Winds blow from high to low pressure but are deflected by the Coriolis force—to the right in the Northern Hemisphere, left in the Southern (Ferrel's Law)—which is zero at the equator and maximum at the poles, and which underlies Buys Ballot's Law. The three planetary wind systems are:

• Trade winds blow from the sub-tropical highs to the equatorial low as the NE trades (N hemisphere) and SE trades (S hemisphere); their convergence drives the ITCZ.
• Westerlies blow from the sub-tropical highs toward the sub-polar lows; in the Southern Hemisphere's open-ocean band they intensify into the Roaring Forties, Furious Fifties and Shrieking Sixties.
• Polar easterlies flow from polar highs to sub-polar lows.

Tri-cellular circulation

Vertically these resolve into three cells per hemisphere: the Hadley cell (rising at the equator, sinking at 30°), the Ferrel cell (mid-latitude, thermally indirect) and the Polar cell. This 1856 model by William Ferrel, refined by the Norwegian (Bergen) school's polar-front theory of 1922, explains why deserts sit at 30° and rain belts at the equator and 60°.

The seasonal migration

Because the overhead Sun migrates between the Tropics of Cancer (21 June) and Capricorn (22 December), the entire belt system shifts roughly 5° toward the summer hemisphere. This migration explains the Mediterranean climate (westerly rain in winter, sub-tropical-high drought in summer) and is the planetary backdrop to the Indian monsoon reversal, where the ITCZ (the monsoon trough) is dragged north over the Gangetic plain in July. Upper-air features—the sub-tropical jet stream and the easterly (tropical) jet—modulate these surface winds and are direct prelims material.