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High Pressure Systems

Anticyclones

The cloudy rainy weather of depressions is due to rising air, which is most pronounced near frontal regions. The anticyclone on the other hand is produced by a large mass of descending air. This subsidence takes place throughout a depth of the atmosphere up to 12km. Such subsidence means that the air is very stable. In addition, winds associated with an anticyclone are usually very light if present at all, especially close to the centre of the high pressure system.

The anticyclone

Subsidence warms the air by compression. Any clouds present quickly evaporate as the temperature of the air rises above its dew point. For this reason, anticyclones usually bring fine, dry and settled weather, particularly in the summer.

Sometimes, subsidence and compression of the air can produce a temperature inversion at one or two thousand metres above the ground. Such phenomena act as caps to rising air heated by the ground under the influence of the Sun, preventing extensive cooling and cloud formation. Unfortunately, if the air is moist below the temperature inversion, a dreary formless layer of cloud can form which becomes difficult to disperse owing to the light winds. Such debilitating weather is common in winter when the Sun's radiation is too weak to burn off the cloud layer.

Winter anticyclones, if clear of cloud, bring with them further problems. A short cloudless day is the forerunner of a long night giving more radiation cooling than a low-angle Sun can counteract the next day. The second night of cooling therefore starts with a lower air temperature than the first. Such conditions, if persistent, can lead to successive nights of frost, which become progressively harder. When the air is particularly moist, cooling at night soon results in fog (see lesson 5). Britain in particular can experience episodes of anticyclonic fog from late September through to May.

Anticyclones move, but not quite in the same purposeful way as travelling depressions. They nudge their way into position and can be incredibly stubborn about leaving, perhaps persisting for weeks, diverting depressions to different routes. Such persistent anticyclones are known as "blocking highs". In winter they can lead to long spells of very cold weather, especially if their air flow comes from Russia and Siberia. In summer they can lead to long hot spells and sometimes drought.

High pressure ridges

A ridge of high pressure is a wedge-shaped extension of an anticyclone or belt of high pressure. The weather associated with ridges is similar to that in an anticyclone. In temperate latitudes, ridges of high pressure often occur between two depressions and move with them. They give rise to intervals of fair weather between the cloud and rain of the low pressure systems.