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.