A large-scale atmospheric disturbance in a region of low atmospheric pressure characterized by a roughly circular ground plan, a center towards which ground winds move, and at which there is an upward air movement, usually spiraling. Above the center, in the upper troposphere, there is a general outward movement. The direction of spiraling is counterclockwise in the northern hemisphere, clockwise in the southern hemisphere, owing to the Coriolis effect. A cyclone forms when a cold air mass moving south (north) from the Arctic (Antarctic) meets a warm air mass moving north (south) from the tropics.
Anticyclones, which occur in regions of high pressure, are characterized by an opposite direction of spiral.
See also depression.
Related category ATMOSPHERIC PHENOMENA AND STRUCTURES
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