Anticyclones are high-pressure regions with outward winds that rotate clockwise in the northern hemisphere and counterclockwise in the southern hemisphere. They are associated with dry weather and light winds, and their shape is roughly circular with pressure rising towards the center. Subtropical anticyclones are permanent and result in arid regions, while smaller, transient anticyclones can bring warm, sunny weather in summer and cold, clear weather in winter. Anticyclones can also have adverse effects, such as delaying or preventing seasonal rainfall and worsening air pollution in urban areas.
An anticyclone is a region of high air pressure, with winds flowing outward from the center. Due to the Earth’s rotation, the Coriolis effect deflects winds, causing them to rotate clockwise in the northern hemisphere and counterclockwise in the southern hemisphere. As air moves away from the center, more is drawn in from above, so that anticyclones are associated with descending air. The term is the opposite of cyclone, meaning a low pressure area where air flows from higher pressure areas. Anticyclones, and high pressure in general, are normally associated with dry weather and light winds and for this reason home barometers will indicate ‘good’ weather when the pressure is high.
Generally, an anticyclone is roughly circular in shape. Changes in barometric pressure are shown on weather maps and charts using isobars – lines connecting points of equal pressure. Anticyclones can be viewed as sets of concentric, roughly circular isobars, with pressure rising towards the centre. More elongated high pressure areas may occur; these are known as high pressure ridges.
Anticyclonic conditions are prevalent in subtropical regions, resulting from global atmospheric circulation patterns. Moist air near the equator warms and rises, spreading north and south, and descending at latitudes 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 from the air at lower latitudes, descending air is very dry and therefore 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. In the subtropics, several distinct semi-permanent high pressure systems exist and have been given names, such as Bermuda-Azores High and Pacific High. Although semi-permanent, they are subject to seasonal movement. For example, the High Bermuda-Azores is typically centered off the southeastern coast of North America during the summer, but moves east during the fall and winter to settle in the mid-Atlantic. Smaller, more transient anticyclones can form over temperate zones, generally bringing warm, sunny weather during the summer and cold, clear weather during the winter.
Anticyclones also form over the poles by cooling near-surface air. Cold, dense air flows outward to be replaced by air from above, resulting in the typical anticyclonic pattern of descending air flowing outward from the high-pressure center. These anticyclones are strongest during the winter months, with the High Siberian producing some of the highest barometric pressure readings on the planet.
Major high pressure systems such as the Bermuda-Azores High and the Pacific High have a large influence on the climate of the surrounding areas. While anticyclones themselves are associated with calm, dry conditions, they can bring storms and rain to adjacent areas. For example, during the summer months, Upper Bermuda-Azorean winds carry moist air from the Atlantic to the southeast United States, causing heavy rainfall. This high-pressure system also has a large influence on hurricane paths.
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 rainfall, leading to drought in areas dependent on this rainfall for agriculture. Anticyclonic conditions can also worsen air pollution in urban areas, where descending air and light winds slow down the dispersion of pollutants.
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