A region of relatively high atmospheric pressure, frequently thousands of kilometres across, often formed as a response to convergence in the upper atmosphere, and also known as a high. As air near ground level flows into an anticyclone, its absolute vorticity decreases, causing divergence, and the descent of air (Tan and Curry (1993) Monthly Weather Rev. 121). Anticyclonic circulation is clockwise in the Northern Hemisphere, and anticlockwise in the Southern Hemisphere.

Cold anticyclones

(continental highs) form as continental interiors, such as Siberia, lose heat in winter through terrestrial radiation, and cool the air above. They are marked by subsidence, which inhibits cloud formation and maximizes radiative cooling, making them self-sustaining. Keegan (1958) J. Meteorol. 15.

Subtropical anticyclones

are warm, and form due to subsidence below convergence resulting from the westerly sub-polar jet stream at the northern limit of the Hadley cell. The semi-permanent subtropical anticyclone over the North Atlantic (the Azores high) strongly influences weather and climate of much of North America, western Europe, and north-west Africa (Davis et al. (1997) J. Climatol. 10, 4). Subtropical anticyclones bring stable atmospheric conditions, and fine, hot, dry weather. In mid-latitudes, anticyclones often locate beneath the leading edge of ridges in the upper-air westerlies, where they may be associated with blocking weather patterns; see Wiedmann et al. (2002) J. Climate 15, 23.