anticyclone

anticyclone As the name suggests, an anticyclone is the opposite of a cyclone. Whereas cyclones are represented by lower than normal atmospheric pressure at the Earth's surface, anticyclones have higher than normal pressure. Where cyclones are dominated by air moving upward between low-level regions of convergence and upper level regions of divergence, anticyclones are exactly the opposite, with downward motion between upper convergence and lower divergence. Where cyclones have positive vorticity, anticyclones have negative vorticity; that is, they rotate in a clockwise sense in the northern hemisphere, and anti clockwise in the southern hemisphere.

Permanent examples of anticyclones exist in the subtropics, where a belt of anticyclones girdles the world at latitudes between about 20 and 40 degrees. In satellite pictures (Fig. 1) the visible manifestation of an anticyclone is a lack of cloud. Most anticyclones are very large systems, thousands of kilometres across, and deep enough to reach the tropopause. Winds in anticyclones are light; air near the surface therefore moves little and has sufficient time to adopt uniform characteristics. As a result, anticyclones are ideal source regions for air masses.

As a result of the deep downward motion in anticyclones, the air through most of the troposphere becomes much warmer and drier than usual. If temperature is measured at different altitudes, the typical profile through an anticyclone can be separated into three regimes (Fig. 2). The uppermost region, the subsiding region, is dominated by the downward motion; this region is warm and dry and the temperature decreases with height. Below this there is a shallow layer, the inversion layer, in which the downward motion ceases. At the bottom of this layer lies air which has not been moved from upper levels, whereas the air at the top has undergone considerable subsidence. Subsiding air is warmed as it is compressed by ever-increasing pressure; the further it subsides the more it is warmed. The air at the top of the inversion layer has subsided from high altitudes and therefore is warmer by several degrees than the air at the bottom. Although the inversion layer is usually shallow (rarely more than a few hundred metres), it has enormous significance because it is extremely stable. Under average conditions the temperature decreases at a rate of about 6.5 °C per kilometre in the troposphere. In the inversion layer, temperature increases with height, often at a rate of more than 10 °C per km. The great stability of this layer means that it acts as an impenetrable lid on the bottom layer, the mixed boundary layer. This boundary layer is dominated by turbulence caused by friction at the Earth's surface. The mixing in the boundary layer is always present. The mixed layer has a depth of up to about a kilometre, although it is shallower at night when there is no heating of the surface to produce convection. The inversion layer exists as the boundary between the downward-moving air in the anticyclone and the turbulent boundary layer.

Between the inversion layer (at about 1 km altitude) and the tropopause (at 10 to 15 km) clouds do not usually develop. However, within the boundary layer shallow clouds can form, particularly if the surface is moist. Shallow layers of cloud often form overnight in anticyclonic regions, but usually evaporate in the morning when heat from the Sun warms the boundary layer. Anticyclones are usually associated with warm, sunny weather and no precipitation, although clear skies during the hours of darkness can lead to very cold nights. Unfortunately, pollution released into the boundary layer is not easily dissipated and cannot be dispersed either above the inversion layer or (because winds are usually light) over large geographical areas. These are the circumstances under which dangerous smog levels are encountered in large cities such as Los Angeles and which in 1953 caused the deaths of thousands in a few days in London.

Anticyclones in middle latitudes are commonly of two types. The first is the ridge of high pressure which exists between depressions moving from west to east steered by the upper-level jet stream. These anticyclones give temporary respite from the wet and windy conditions associated with the depressions. They are transient features which pass in a day or two. The other type is the ‘blocking’ anticyclone or ‘block’. This is a large quasi-stationary system, similar in structure to a subtropical anticyclone but occurring much further poleward. When these blocks occur they persist for days or even weeks, and because they are located in the latitudes where depressions usually travel from west to east they block the passage of the depressions and force them to divert either north or south of the block (Fig. 3). These blocking anticyclones can provide long periods of stable weather which changes little from day to day, making weather forecasting relatively easy. It is, however, particularly difficult to forecast the onset or the end of a block.

Anticyclonic weather is usually benign. However, the very rapid changes in temperature and humidity in the inversion layer affect the transmission of microwaves used in television and telephone communications. The paths of the transmitted beams are bent to produce ‘anomalous propagation’, which causes the signals to travel much further than is normal, producing weak or disturbed television signals.

Charles N. Duncan

Bibliography

Ahrens, C. D. (1994) Meteorology today. West Publishing Co., St. Paul, Minnesota.
Palmen, E. and and Newton, C. W. (1969) Atmospheric circulation systems. Academic Press, London.