Circulation cells

So why don’t the poles get colder and colder, and the equator hotter and hotter? In nature, systems tend towards equilibrium, that is to say, nature doesn’t like differences, so the Earth system tries to even things out, and it does this by transferring heat from the tropics to the poles.

If the Earth was static, so not rotating, air would rise at the equator (because we know that hot air rises), move toward the poles, sink as it cools, then return the cooler air towards the equator.

A non-rotating Earth: Diagram showing the Earth, with warm air rising at the equator, moving towards the north pole, cooling, and sinking, then the cold air moving towards the equator, creating a circulation cell. In a non-rotating Earth, air would move in a conveyor between equator and poles

But the Earth’s rotation causes some complications, and because of the speed of the Earth’s rotation, we end up with three circulation cells in each hemisphere.

Circulation cells: Diagram of the Earth showing the 3 circulation cells in hemisphere. Warm air rises at the equator and moves towards the poles, this air cools and sinks at around a third of the way towards the poles, then moves back towards the equator at the Earth’s surface, creating the Hadley cell. There is a similar cell in middle third of each hemisphere called the Ferrell cell, and a third called the Polar cell which reaches the poles. A caption reads: Circulating Cells: The Hadley cells have the most regular pattern of movement, an produce extreme wet weather at the equator and extreme aridity on the deserts. The polar cells are the least well-defined. Each hemisphere has three main circulation cells, which redistribute heat from equator to poles (Click to expand)

These circulating cells not only transport heat from equator to pole, but also result in semi-permanent areas of high and low pressure due to the rising and descending parts of the circulation cells, which in turn lead to climatic zones. We’ll learn more about rising and sinking air, and high and low pressure later.

Semi-permanent pressure systems: Diagram showing the northern hemisphere, with bands of high or low pressure running around the Earth due to the rising and sinking air as a result of the circulation cells, which are also shown. Rising air at the equator leads to a low pressure zone, sinking air where the Hadley and Ferrel cells meet leads to a high pressure zone, rising air where the Ferrel and polar cells meet leads to a low pressure zone and sinking air at the North Pole leads to a high pressure zone. Semi-permanent pressure systems in the northern hemisphere

Climate zones: Diagram showing the Earth with semi-permanent pressure zones resulting in climate zones which are visible on the Earths land surface, for example the brown colour of the land across the northern part of Africa where high pressure results in desert due to sinking, dry air, and the very green stripe of land across equatorial Africa as a result of the low pressure zone leading to rising air and wet conditions. Semi-permanent pressure systems result in rainforests and deserts

A projection map of the Earth, colour coded to show the climatic zones, with; equatorial, arid, mediterranean, snow, polar and temperate. Map of global climate zones (Click to expand)

For more information take a look at this article on the Global circulation system

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