An anticyclone is a weather phenomenon defined as a large-scale circulation of winds around a central region of high atmospheric pressure. In the Northern Hemisphere, the winds rotate clockwise around the center of the anticyclone, while in the Southern Hemisphere, they rotate counterclockwise. Anticyclones are associated with clear skies and cooler, drier air. They can also cause fog to form overnight within a region of higher pressure. The evolution of an anticyclone depends on variables such as its size, intensity, and extent of moist convection, as well as the Coriolis force. Anticyclones can be seen as sets of concentric, approximately circular, isobars with the pressure rising toward the center. They are most common in subtropical regions, resulting from global atmospheric circulation patterns. Anticyclones also form over the poles by the cooling of air near the surface.