The Coriolis effect modifies global air circulation so that there are six atmospheric circulation cells, three in each hemisphere. At the equator, air behaves as described in the section on atmospheric circulation above. Warm air rises, creating a low pressure cell, and then moves toward the poles at the top of the troposphere. As the air advects poleward, it is deflected by the Coriolis effect—to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. By about 30°N or 30°S, the air has deflected and cooled quite a bit and meets the relatively cool air flowing from the higher latitudes toward the equator. This combined batch of air is relatively cool and dense, so it sinks, creating a high pressure zone. At the ground surface, some of the air circulates back toward the equator to complete the first convection cell. The pattern of air rising at the equator, sinking at the poles, and advecting in between describes the two cells, known as Hadley cells, that circulate air from near the equator to 30°N and 30°S.
Atmospheric circulation cells set the framework for the climate of a region. In locations where the air is rising or sinking, there is little wind. Low pressure zones are the site of a lot of precipitation, and high pressure zones experience more evaporation than precipitation. In the first set of atmospheric circulation cells (the Hadley cells), rising air at the equator causes a great deal of rain and little wind. Early mariners called this region the doldrums because their sailing ships could be becalmed for weeks. The sinking air at 30°N and 30°S is relatively warm since much of it came from the equator and the high pressure cell it creates causes evaporation. On land, these latitudes are where many of the world's great deserts are located, including the Sahara in Africa and the Sonoran Desert in North America. At sea, these regions were named the horse latitudes by Spanish mariners because the lack of wind would sometimes delay their ships for so long that they would run out of water and feed for their livestock, which included horses; the dead horses would be disposed of over the side of the ship.
As the air in the circulation cell moves back toward the equator along the ground, it is deflected by the Coriolis effect. This forms the trade winds, which are referred to as the northeasterly trades in the Northern Hemisphere and the southeasterly trades in the Southern. The trade winds were given their names because they provided a reliable thrust for sailing ships engaging in trade and commerce.
The next two atmospheric circulation cells are located in the middle latitudes. Some of the air that sank at 30°N and 30°S latitude moves along the ground toward the pole. This air is deflected by Coriolis, creating the westerly winds or westerlies (named for the direction they are coming from). At about 50° to 60°N and 50° to 60°S, this air meets up with air coming from the poles and the whole mass rises, often leading to precipitation. Where these two very different air masses come together—one mass indirectly from the equator and the other directly from the pole—is the polar front. An
^^ Ferrel cell
Polar cell cr
^^ Ferrel cell
The six-cell model of global air circulation, showing the locations of high and low pressure cells and the directions of the major wind belts on the Earth's surface.
air mass is a large body of air—hundreds of thousands of square miles in area and several miles thick—with similar temperature and humidity throughout (although these features change somewhat with altitude). Because of the arrangement of the different air masses, weather at the polar front is extremely variable, as it is over much of North America and Europe. After the air at the low pressure cell has risen, some of it moves toward the equator. At about 30°N and 30°S, this air meets the air coming from the equator and sinks, completing the second set of circulation cells, known as the Ferrel cells.
Back at the polar front, the air that didn't travel toward the equator moves towards the pole. By the time it reaches the polar region, it is extremely cold and dense, so it sinks. At the surface, it advects toward 60° latitude, completing the third set of circulation cells, known as the polar cells. Winds in the polar cells are deflected by the Coriolis effect, creating the polar easterlies.
In reality, bands of high and low pressure do not encircle the globe but center primarily over the oceans. Over landmasses, which are more abundant in the Northern Hemisphere, many factors can weaken the effects of circulation cells.
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