Chapter Fifteen CHAPTER FIFTEEN Anticyclones The horizontal motion of the air in an anticyclone has a major influence on its properties An area of convergence is one in which the horizontal inflow of air is greater than the horizontal outflow at the same level An area of divergence is one in which the horizontal outflow exceeds the horizontal in.t10w Both of these motions are accompanied by compensatory vertical motion FORMATION Convergence at upper level increases the quantity and total weight of air within a limited area This causes barometric pressure to rise at the surface The surface air circulates anticyclonically round the high pressure area with the usual outflow (divergence) of air across the isobars towards areas of lower pressure This outflowing surface air is replaced only by dry air descending vertically from above The downward motion of the air is known as "subsidence" The high pressure system will be maintained so long as the convergence at upper level exceeds the surface divergence When the convergent flow of air at upper level ceases to feed the subsidence the system will decline SOME GENERAL PROPERTIES The air at upper level is dry and is warmed by compressional heating as it subsides into regions of higher pressure Thus in the central area of a well developed anticyclone the air at a height of 500 m is generally warmer than the air below This increase of air temperature with height is called an inversion and is a very stable condition, in which any vertical ascent of air is arrested at a low level Since ascending air is one essential condition for the formation of cloud and rain, it follows that the central area of an anticyclone is generally associated with fine dry weather TYPES OF ANTICYCLONES Anticyclones can be classified into two types; cold anticyclones and warm anticyclones The classification is based upon the temperature of the surface air 130 Chapter Fifteen Cold Anticyclones Cold anticyclones are ones in which the air is colder than the air in the surrounding area They build up over large land masses in winter, notably over Siberia where the pressure may reach 1,050 hPa or more, but less frequently over the continent of North America The process of formation is the reverse to that of a thermal low and may be described therefore as a thermal high in which the air becomes cooled through a long period in a cold region The column of air over the cold surface contracts vertically leaving a deficiency of air at upper level (an upper low) The resultant inflow increases the total quantity and weight of air in the area and barometric pressure increases at surface level (See Figure 15.1) The layer of cold air produced is very shallow and lies in the lowest to kilometres Above this level the air is at "normal" temperatures This means that these anticyclones not appear on higher level charts In late winter, a ridge of high pressure protruding from the Siberian high can give easterly winds over the British Isles and sometimes a prolonged period of intensely cold weather If the air is dry the weather will be clear, bright and frosty but if enough moisture is picked up from over the North Sea it will be dull and foggy 131 Chapter Fifteen Transitory Cold Anticyclones These commonly form within a polar maritime air mass in the temperate latitudes and in the rear of a family of depressions Weak ones sometimes develop into intense centres when they move over cold land in the winter In the summer such systems are generally short-lived An anticyclone or ridge separating successive depressions of a family normally moves with the depressions In winter, near coasts where the air is moist layer type cloud spreads below the subsidence inversion and accumulates smoke pollution and dust causing anticyclonic gloom particularly in and near large industrial areas Under these conditions artificial light is sometimes necessary during daylight hours in order to carry on with normal work indoors Inland, where the air is dryer, clear skies at night can cause intense radiation cooling and there is every likelihood of frost or radiation fog Warm Anticyclones These are anticyclones in which the surface is warmer than the surrounding air but at higher levels the air in these anticyclones is actually colder than the surrounding environment at corresponding levels These features can be identified on charts through the full depth of the troposphere These anticyclones are formed by the air motion in the upper atmosphere Permanent Warm Anticyclones These are the oceanic highs in the subtropical belts These systems are very stable and the weather is generally fine with little or no cloud and good visibility Their average seasonal movement north and south with the sun lies somewhere between latitudes 30o and 40° in both hemispheres; but the mean positions of the centres for any particular month vary considerably from year to year The subtropical highs are the source regions of the tropical maritime air flowing into higher latitudes to meet the polar front When the Azores high is well developed and extends well to the north of its average summer latitude we are likely to have a very fine and settled summer in the British Isles so long as the situation is maintained Conversely, if the high is relatively weak and positioned in a lower than average latitude for the season, our summer will probably be very unsettled with many invading depressions and more than average rainfall Temporary Warm Anticyclones These may reach Great Britain as ridges moving along the northern perimeter of the Azores high, or sometimes as separate centres which start as ridges and then 132 Chapter Fifteen break off from the main system In the British Isles they are more a feature of summer than winter and can persist for long periods Temporary warm anticyclones may also develop as a result of continued subsidence within a temporary cold high which remains stationary for a long period A warm high often gives bright fine weather, especially over land in summer There is a possibility of sea fog at any time of the year especially in the spring and early summer In autumn and winter light winds of maritime origin can bring radiation fog QUESTIONS Describe (with the aid of a diagram) the circulation of air resulting in upper convergence Describe, in sequence, the processes which result in the formation of an anticyclone Differentiate between warm and cold anticyclones Explain how a large anticyclone may govern the speed and direction of movement of a depression Describe the general properties of an anticyclone 133