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General Circulation

The worldwide system of winds, which transports warm air from the equator where solar heating is greatest towards the higher latitudes, is called the general circulation of the atmosphere, and it gives rise to the Earth's climate zones.

The general circulation of air is broken up into a number of cells, the most common of which is called the Hadley cell. Sunlight is strongest nearer the equator. Air heated there rises and spreads out north and south. After cooling the air sinks back to the Earth's surface within the subtropical climate zone between latitudes 25° and 40°. This cool descending air stabilises the atmosphere, preventing much cloud formation and rainfall. Consequently, many of the world's desert climates can be found in the subtropical climate zone. Surface air from subtropical regions returns towards the equator to replace the rising air, so completing the cycle of air circulation within the Hadley cell.

Although the physical reality of Hadley Cells has been questioned, they provide an excellent means for describing the way in which heat is transported across the Earth by the movement of air. Other circulation cells exist in the mid-latitudes and polar regions. The general circulation serves to transport heat energy from warm equatorial regions to colder temperate and polar regions. Without such latitudinal redistribution of heat, the equator would be much hotter than it is whilst the poles would be much colder.

Without the Earth's rotation, air would flow north and south directly across the temperature difference between low and high latitudes. The effect of the Coriolis force as a consequence of the Earth's rotation however, is to cause winds to swing to their right in the Northern Hemisphere, and to their left in the Southern Hemisphere. Thus the movement of air towards the equator swings to form the northeast and southeast trade winds of tropical regions. Air flowing towards the poles forms the westerlies associated with the belt of cyclonic low pressure systems at about 50 to 60° north and south. In general, where air is found to descend, high pressure develops, for example at the subtropical latitudes and again near the poles. Where air is rising, atmospheric pressure is low, as at the equator and in the mid-latitudes where storms or frontal systems develop.

Global atmospheric circulation