An anticyclone is a region of high atmospheric pressure, with winds flowing outwards from the center. Due to the Earth’s rotation, the Coriolis effect deflects the winds, causing them to rotate clockwise in the northern hemisphere and counterclockwise in the southern. As air moves away from the center, more is drawn down from above, so that anticyclones are associated with descending air. The term is the opposite of cyclone, which means an area of low pressure into which air from higher-pressure areas flows. Anticyclones, and high pressure in general, are normally associated with dry weather and light winds and for this reason, household barometers will indicate “fine” weather when the pressure is high.
Generally, an anticyclone has a roughly circular shape. Air pressure variations are shown on meteorological maps and charts using isobars: lines connecting points of equal pressure. Anticyclones can be seen as sets of concentric, approximately circular, isobars with the pressure rising toward the center. More elongated high-pressure areas can occur; these are known as high-pressure ridges.
Anticyclonic conditions are prevalent in the subtropical regions, resulting from global atmospheric circulation patterns. Moist air near the Equator is heated and rises, spreading northward and southward, and descending at latitudes of about 30 degrees north and south of the Equator, forming large anticyclones. This type of high-pressure area is known as a subtropical anticyclone. Since most of the moisture has been precipitated out of the air at lower latitudes, the descending air is very dry and so the subtropical regions tend to be arid; in fact, most of the world’s deserts are found in these regions.
These large anticyclones are a more or less permanent feature of the Earth’s climate. Several distinct semi-permanent high-pressure systems exist in the subtropical regions and have been given names, such as the Bermuda-Azores High and the Pacific High. Although semi-permanent, they are subject to seasonal movement. For example, the Bermuda-Azores High is typically centered off the southeast coast of North America during the summer, but moves east during the autumn and winter to settle over the mid-Atlantic. Smaller, more transitory anticyclones can form over temperate areas, generally bringing warm, sunny weather during the summer and cold, clear weather during the winter.
Anticyclones also form over the poles by the cooling of air near the surface. Cold, dense air flows outwards to be replaced by air from above, resulting in the typical anticyclonic pattern of descending air flowing outwards from the high-pressure center. These anticyclones are strongest during the winter months, with the Siberian High producing some of the highest barometric pressure readings on the planet.
Major high-pressure systems like the Bermuda-Azores High and the Pacific High have a big influence over the climate of surrounding areas. Although anticyclones themselves are associated with calm, dry conditions, they can bring storms and wet weather to adjacent areas. For example, during the summer months, winds from the Bermuda-Azores High bring moist air from the Atlantic to the southeast United States, resulting in high rainfall. This high-pressure system also has a major influence over the paths of hurricanes.
In temperate regions, anticyclones tend to be associated with good weather — dry, sunny conditions — however, they can also have adverse effects. A persistent anticyclone can delay or prevent seasonal precipitation, bringing drought to areas dependent on this rainfall for agriculture. Anticyclonic conditions can also worsen air pollution in urban areas, where the descending air and light winds slow down the dispersal of pollutants.