Anticyclones (MAR Rev. 05/12/01)

UNIVERSITY OF TRINIDAD & TOBAGO

ANTICYCLONES

An anticyclone or high is characterised by its centre of high pressure around which are roughly

concentric isobars and by its clockwise (NH) circulation of winds. Anticyclones are three

dimensional features and their structures vary with height.

Apart from their pressure and wind patterns, highs and lows show other important differences. In

general, highs cover much larger areas, move slower and are more persistent than lows. Their

pressure gradients are less steep, especially near their centres, so the winds are characteristically

light and variable near their centre. A new high forms almost always either as an extension of an

existing high or as a centre near the original, eventually replacing it.

Subsidence

For an anticyclone there is a horizontal divergence of air at the earths surface. Air must move

downwards to compensate for the horizontal divergence. Air moving downwards or subsiding

warms adiabatically. Since the air temperature is rising the relative humidity of the air is lower. If

subsidence is persistent an inversion may form.

Anticyclones can be divided into two categories, warm anticyclones and cold anticyclones. The

classification is based on temperatures in the lower troposphere.

Cold Anticyclones In a cold anticyclone the cold dense air is largely confined to the lowest 3 km. Above this level

temperatures are near normal. The high pressure is mostly caused by the cold dense air near the

surface.

Height Normal

Cold High

(Shaded area represents layer of cold air)

Temperature

Above 3 km temperatures are normal and there is no excess pressure. A cold anticyclone is a

shallow feature and cannot be found on upper air charts.

Examples

The polar anticyclone is an example of a cold anticyclone as are the semi-permanent

anticyclones which form over continents in winter. The seasonal anticyclones are the source

regions of polar continental air masses.

Anticyclones (MAR Rev. 05/12/01)

In winter temperatures are low (-20C to -50C). Temperatures are generally above freezing in

summer, but stratiform cloud or fog will form at sea and in coastal areas.

Weather Over Britain

High pressure commonly occurs in the cold polar air which flows behind a south moving cold

front An anticyclone does not often form, but a ridge will quite often develop. The weather it

brings is typical of polar air, but it is modified by the presence of a temperature inversion.

Warm Anticyclones In the middle and lower layers of the troposphere in a warm anticyclone the air is warmer, level

for level than in the surrounding air. There is often cold air in the upper troposphere and lower

stratosphere.

Height Normal

Cold High

(Shaded area represents layer of cold air)

Temperature

A warm anticyclone is accompanied by high pressure on upper level charts even into the

stratosphere.

Examples

Typical examples are the anticyclones forming over sub-Tropical oceans. These anticyclones

form as a consequence of the general circulation of the atmosphere.

The result of the uneven distribution of insolation over the earths surface is that low pressure

develops at the equator and relatively high pressure near the poles. At the equator there is surface

convergence and so air rises and diverges towards the poles at higher level. In the Northern

Hemisphere the northward flow of air is increasingly deflected by Coriolis force to the right, i.e.

towards the west.

At about 35N to 40N the winds are in equilibrium and flow westerly. This blocks the northward

flow of air and so air collects to the south of this forming the anticyclones.

The weather near the Azores is quiet with broken Cu and Sc.

Weather over Britain Most of our spells of warm dry weather in the summer are associated with warm anticyclones. If

subsidence extends near to the ground it gives widespread clear skies so radiation fog may form.

This fog may be persistent in winter, but usually clears in the summer.