2.4.1 Climate, Atmospheric Circulation and Biome Distribution

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Intense solar heating at the equator warms the air near the surface

The warmed air becomes less dense and rises, creating a zone of LOW pressure at the surface

As the rising air cools, water vapour condenses, forming large cumulonimbus clouds and heavy rainfall

The now-dry air moves poleward at high altitude

By approximately 30° latitude, the air has cooled enough to become denser and it descends

The descending dry air creates a zone of HIGH pressure and arid conditions at approximately 30°

This cell explains why the world's major hot deserts are found at similar latitudes in both hemispheres

At the surface, some of this descending air flows back towards the equator

The combination of high temperatures and very high precipitation supports the most productive terrestrial biome on Earth

Surface winds returning towards the equator are deflected by the Coriolis effect, blowing from the northeast in the Northern Hemisphere

Some of the air that descends at approximately 30° moves poleward along the surface rather than returning to the equator

Surface winds in this cell are deflected by the Coriolis effect to blow from west to east, bringing moisture from the oceans

At approximately 60°, this warmer air meets colder air moving from higher latitudes

The warmer, less dense air is forced upward over the denser cold air, creating a zone of LOW pressure

As this rising air cools and condenses, it produces moderate to high precipitation

This cell is unusual because it is partly driven by the cells on either side rather than being entirely self-sustaining

The moderate temperatures and regular precipitation in this zone support temperate forests and grasslands

This cell is characterised by the most variable and changeable weather conditions of the three cells

Western Europe experiences frequent rainfall because moisture-laden winds blow onshore from the Atlantic across these latitudes

At the poles, the air is extremely cold and dense, causing it to sink

The sinking air creates a zone of HIGH pressure at the surface

Cold surface air moves towards lower latitudes, deflected by the Coriolis effect to blow from the east

Despite the presence of ice and snow, precipitation here is very low because the descending air is dry

At the boundary of this cell (approximately 60°), rising air from the convergence of warm and cold air masses completes the circulation

The very low temperatures and low precipitation at high latitudes support only tundra and polar ice

Antarctica receives less precipitation annually than most of the world's hot deserts

Heavy snowfall can occur in boreal forests near 60° because water vapour carried from mid-latitudes condenses as the air rises at this boundary

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