Global atmospheric circulation — Geography, 11–13
Why warm air rises in some places and sinks in others, shaping broad patterns of wind and rainfall.
Air moving around Earth
The Sun heats Earth unevenly: the equator receives more direct energy than the poles. Warm air tends to rise, cool air tends to sink, and this creates large circulation cells between low and high pressure. Earth’s rotation bends the moving air, producing broad wind belts rather than one simple north-to-south flow.
Why this pattern exists
Without circulation, the equator would keep heating and the poles would keep cooling, making Earth’s temperature differences even greater. Moving air transfers energy around the planet and helps balance that heating. The circulation model explains why some latitude bands are usually wet and others usually dry, even though local weather still changes.
Worked example: rising air
At the equator, strong sunlight warms the ground and the air above it. The warm air expands, becomes less dense and rises, leaving lower pressure near the surface. As it rises, it cools; water vapour can condense into clouds, so equatorial regions often receive heavy rain. Higher up, the air moves away before sinking elsewhere.
The simple-wind trap
It is reasonable to imagine air travelling straight from the hot equator to the cold poles. The real system is more complicated because rising and sinking air form several cells, and Earth’s rotation changes direction. Also, mountains, oceans and seasons disturb the broad pattern, so it predicts tendencies, not tomorrow’s exact weather.
Where it is used
This idea helps explain why rainforests cluster near the equator and many deserts lie around 30° north or south. It supports long-term planning for farming, water supply and wind-energy sites. Weather forecasters then add local evidence, such as sea temperatures and pressure systems, to make shorter-term predictions.
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