An anticyclone is a region of high atmospheric pressure relative to the surrounding air, generally thousands of kilometres in diameter and also known as a high or high-pressure system. Anticyclones appear on weather charts as a series of concentric, widely spaced isobars of 1000 mbs and above. The roughly circular closed isobar at its central region indicates the area of highest pressure.
The centre of an anticyclone has a characteristic pattern of air circulation, with subsiding air and horizontal divergence of the air near the surface. The name anticyclone comes from the circulatory flow of air within the system; anticyclonic circulation has a local circulation that is opposed to the Earth's rotation. Winds, generally light, circulate around the high pressure centre in a clockwise direction in the Northern Hemisphere and anticlockwise in the Southern Hemisphere.
The subsiding air compresses as it descends, causing adiabatic warming. The eventually warmer and drier air suppresses cloud formation and thus anticyclones are usually associated with fine weather in the summer and dry, cold, and sometimes foggy weather in the winter. Calm settled weather is usually synonymous with anticyclones in temperate latitudes. Anticyclones are typically relatively slow moving features.
However, mid-latitude anticyclones can be divided into warm and cold anticyclones (continental highs). Subtropical anticyclones are usually warm and quasi permanent features of the Earth's general circulation (e.g. the Azores high). In mid-latitudes anticyclones are often located beneath the leading edge of ridges in the upper-air westerlies, where they may be associated with blocking weather patterns.