atmospheric high pressure

atmospheric high pressure Areas of high pressure with central pressure in the range of 1020–45 mb (millibars), occasionally reaching 1060 mb, are known as anticyclones or highs. The isobars are more or less concentric and are widely spaced around the centre of the high and thus, in contrast to depressions, winds are usually light and sometimes rather variable. In general, however, winds normally blow clockwise and outwards in the northern hemisphere (anticlockwise in the southern hemisphere) with a tendency for wind speeds to be greater towards the periphery of the system. High-pressure cells are usually larger (up to 4000 km across), slower moving and more persistent than depressions. Though weather conditions are usually quiet, dry and settled variations can occur over short distances which make forecasting particularly difficult. An example of an area of high pressure can be seen in Fig. 1.

A primary characteristic is the widespread descent or subsidence of air within the troposphere, which results in the air being warmed by adiabatic compression. This subsidence not only warms the air but produces very low relative humidities; the subsided air is thus frequently cloud-free. However, the descending air, because of convection and turbulent currents, rarely reaches ground level, leaving a cooler moist layer some 500 to 1500 m deep immediately above the surface. The boundary between this cooler air and the subsided air aloft is characterized by a temperature inversion (usually referred to as a subsidence inversion), which has a profound influence upon whether conditions by limiting the upward movement of convection currents originating at the ground surface.

Types of anticyclonic high-pressure systems

Anticyclones may be categorized as either ‘warm’ or ‘cold’ anticyclones. Warm highs result from convergence in the upper troposphere and subsidence beneath, producing relatively warm air throughout the troposphere above the subsidence inversion. The zone of high-pressure cells found in the subtropics provides examples of this type of anticyclone, the Azores high is the main centre in the North Atlantic. The high-level convergence responsible for the formation of these cells takes place beneath the subtropical jet stream, which is on the poleward margin of the tropical Hadley cell. The resulting highs are slow-moving, produce long spells of fine weather, and are responsible for some of the world's major deserts such as the Sahara and the Kalahari. Warm highs also form in temperate latitudes and are a result of convergence just ahead of a ridge in the upper westerlies. They are seen on weather maps either as extensions of the subtropical highs and linked to them by a strong ridge of high pressure, or as persistent ‘blocks’. Blocking anticyclones block or disrupt the more normal westerly flow and prevent depressions from following their usual routes. When a blocking high develops over north-west Europe, depressions are deflected further north, and sometimes also to the south, and the Azores high is often absent. These developments often lead to anomalous weather conditions in the European and North Atlantic sector. Blocking highs tend to be particularly persistent over Scandinavia, the North Atlantic between 10° and 20° W, Alaska, and the northwest Pacific in the northern hemisphere, and in the Australian and New Zealand sector in the southern hemisphere.

Cold anticyclones are shallow thermal features, often no more than 3000 m deep, which form over cold surfaces as a result of convergence aloft caused by the contraction of the cold low-level air. They may be found over Antarctica and the Arctic Ocean at any time during the year, and in addition form over northern Eurasia, Greenland, and North America during the winter. A classic example is the persistent winter Siberian anticyclone which dominates the weather of this area and sometimes has a central pressure of 1060 mb. Cold anticyclones or ridges of high pressure also migrate from these source regions in the polar air that is found behind depressions bringing short spells of cool but fine weather before the arrival of the next depression; they often approach Britain from the west or north-west.

Weather

Anticyclones are commonly associated with clear skies and light winds. In late autumn and winter these conditions, together with the long nights of this time of year, lead to a rapid cooling of the lowest few hundred metres of the atmosphere owing to long-wave radiation into space. These are favourable conditions for the formation of frost and fog and, because of the weak sun, the fog may persist in places well into or throughout the following day, leading to a spell of raw grey conditions. The subsidence inversion prevents the upward dispersion of pollutants and consequently there is a decline in air quality. If these conditions last for several days, unpleasant smogs may develop, leading to loss of life, especially among the elderly and those who suffer from respiratory problems. Fog is notoriously patchy, and while some areas, especially low-lying regions, may not see the sun for several days, other areas may have unbroken sunshine, resulting in higher diurnal temperature ranges. If the surface layer of air beneath the subsidence inversion is moist then ‘anticyclonic gloom’ is often a widespread feature. It is characterized by a sky covered by a layer of stratus or stratocumulus which tends to persist because of the weak atmospheric circulation. When this cloud is present, night-time radiational cooling is much reduced and widespread fog is consequently not usually a problem.

In summer, the weather associated with anticyclones is usually fine and cloud-free. During the afternoon small ‘fair-weather’ cumulus clouds may develop beneath the inversion over the land because of the strong insolation, which causes temperatures to rise rapidly. At night, temperatures fall quickly under the clear skies so that towards dawn short-lived mist or fog patches may form. In coastal areas, especially those adjacent to the chilly North Sea in spring and early summer, sea fog may form where the air has passed over cool waters and is cooled to its dew point. In Fig. 1 an anticyclone situated in the southern North Sea allowed continental air to affect much of the British Isles although a cold front remained close to north-western areas for much of the day. The prolonged sunshine, exceeding 13 hours in many areas away from the cloudy and sometimes wet north-west of Scotland, encouraged temperatures to rise rapidly with maxima in excess of 30 °C in many inland areas of England. However, some anticyclones are cloudy and daytime temperatures do not usually rise much above average for the time of year. The cloud is sometimes the result of a ‘decaying’ front moving into the circulation of the high; on other occasions it is the result of moist tropical maritime air having been cooled from its passage north over the North Atlantic and reaching the British Isles from the west or north-west.

Persistent blocking anticyclones can have a profound influence upon the weather, producing anomalous months or seasons. The character of the weather depends upon both the precise location of the high and the season. If a blocking high forms over Scandinavia in winter, this area will experience clear, very cold conditions; areas to the south of the centre will be fed very cold polar continental air, which may reach the British Isles. The North Atlantic depressions are steered to the north and to the south of this high and may influence Britain, especially the south, bringing heavy precipitation which in the continental air is likely to be in the form of snow. A situation such as this occurred in early February 1991 when daytime temperatures remained well below freezing for several days in southern Britain with widespread snowfall. The exceptionally cold winter of 1962–3 was largely the result of blocking highs being located either over Scandinavia or between Norway and Iceland, producing very cold easterly winds, severe frosts and significant snowfalls.

Dry spells over Britain (such as the very pronounced drought of 1975–6 and the series of drought episodes during the period 1988–92) are usually the result of persistent blocking anticyclones close to the British Isles. Hot summers are often the result of slow-moving anticyclones situated either over Scandinavia or to the east or south-east of Britain, with very warm and dry continental air being advected across the country. Cool but relatively dry summers are caused by high pressure persistently reforming to the west of Ireland.

A general lack of blocking action close to Britain produces cool, moist summers and mild winters as the country is subjected to a changeable zonal westerly airflow bringing a succession of frontal depressions across the country.

John Stone

Bibliography

Barry, R. G. (1998) Atmosphere, weather and climate (7th edn). Routledge, London.
Young, M. V. (1994) Back to basics: depressions and anticyclones. Weather, September 1994, 306–11, and November 1994, 362–70.