Less common than the LOW pressure systems associated with the ‘Icelandic Low’ which are prevalent over UK.

HIGH PRESSURE systems often linked with ‘AZORES High’ area.

UK Weather dominated by westerly airstream (both at the surface and in the upper troposphere - see later notes on Rossby Waves)

Circulation over UK is ‘anticyclonic’ for 25% of the year.

Term coined by Galton in 1861 to describe cells of high pressure with roughly concentric isobars. Generally large (over 3000km across), slower moving and more persistent than depressions, with light variable winds which tend to diverge from the centre of the feature.

Normally, with relatively clear skies, the weather is quiet, dry and settled.

Dominated by subsidence throughout the troposphere (and particularly between 1.5 and 6km) Air doesn’t normally sink all the way to the ground, but to approximately 0.5-1.5km above the surface where it meets air rising by convection and turbulence from the ground. Air descending is dry.

Air warms at the DALR, and relative humidity is reduced. Atmosphere becomes stable and clouds dissipate.

Subsidence suppresses upward motion and prevents most precipitation.

SUMMER:

Fine weather, light winds, clear skies apart from ‘fair weather’ cumulus which develop during the day. Daytime temperatures high due to insolation. Nights cool due to radiational cooling. Mists in morning dissipate quickly, although sea fog may develop where air passes over cool sea and is cooled to dew point.

WINTER:
Clear, stagnant and stable conditions. Long nights favour formation of fog and frost. Temperature inversions. If surface air is moist, a layer of stratus or stratocumulus may form. Doesn’t easily disperse due to weak circulation, so cloud may persist for several days: known as ‘anticyclonic gloom’.

2 types of anticyclone:

WARM ANTICYCLONE

Result from convergence in the upper troposphere. Develop chiefly in sub-tropics and mid-latitude regions, e.g Azores high.

COLD ANTICYCLONE

Thermal in origin, developing over continental interiors e.g Siberia, N. Canada.

Result from persistent radiational cooling of the land. Lower troposphere is cold: in contact with ground below. Cloudless skies and very low temperatures (-20° C to -50° C)

Also:

BLOCKING ANTICYCLONE

Occurs when a cell breaks away from the Azores high, and establishes itself at higher latitudes for a period of several days. This disrupts usual westerly flow, sometimes for weeks and this results in a marked change from usual temperatures, precipitation and sunshine.

‘Blocking highs’ are warm anticyclones, so influence extends into upper troposphere, diverting the jet stream (see Fig 11.5) from zonal (west-east) pattern to meridional (north-south) pattern.

Leads to exceptional periods of weather e.g very cold winter of 1962/3, drought of 1976.