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Cyclone, closed system of winds revolving around a low-pressure area. The air rotates counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. In the tropics, particularly in the Indian Ocean (where they are sometimes known as typhoons), cyclones bring severe tropical storms with winds of 200–300 mph (320–480 kmph). In temperate regions, where they are more common, they are often known as lows, or depressions, and generally bring rain, snow, or strong winds. A cyclone develops when a mass of warmer and lighter tropical air meets a mass of colder and heavier polar air (a polar front). The two masses flow in opposite directions and roughly parallel to each other, but under certain conditions the disturbance of the front may develop into a more serious turbulence. The rotating masses of cold and warm air mutually reinforce their circular movements, giving rise to winds of 30–40 mph (48–64 kmph) toward the center, and producing widespread clouds and rainfall.

See also: Hurricane; Tornado.

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21st Century Webster's Family Encyclopedia21st Century Webster's Family Encyclopedia - Cretinism to Davis, David