Global Atmospheric Circulation
The three-cell model
Earth's atmosphere transfers heat from the equator to the poles through three pairs of convection cells in each hemisphere — the Hadley, Ferrel and Polar cells. They form because the equator receives the most concentrated solar radiation, while the poles receive it at a low angle and lose more heat to space than they gain.
Hadley cell (0°–30°)
Intense solar heating at the equator warms air, which rises in deep convection — creating a zone of low pressure and heavy rainfall called the Inter-Tropical Convergence Zone (ITCZ). The rising air diverges aloft, cools, and sinks at around 30° N/S. Here, descending dry air creates a belt of high pressure — explaining why the world's hot deserts (Sahara, Atacama, Australian) cluster around this latitude.
Ferrel cell (30°–60°)
Air moves polewards along the surface from 30° toward 60°. At ~60° it meets cold polar air at the Polar Front, rises, and creates a zone of low pressure with frontal rainfall (this is why the UK at ~52° N is wet and changeable).
Polar cell (60°–90°)
Cold dense air sinks at the poles → high pressure → flows back toward 60°.
Prevailing winds
Surface winds are deflected by the Coriolis effect:
- 0°–30°: north-easterly Trade winds (NH); southeasterly (SH).
- 30°–60°: Westerlies dominate (UK weather arrives from the west/SW).
- 60°–90°: Polar easterlies.
Why this matters
The cells determine the locations of climate biomes — equatorial rainforest, hot desert, temperate, polar — and explain rainfall patterns, the UK's prevailing SW winds, and the timing of monsoons.
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