in contrast to depressions, anticyclones only involve one type
of air mass which usually cover large areas and do not have any fronts. they are
high pressure systems in which the air moves downwards towards the earth's
surface. as the air descends, the molecules become compressed, the pressure
increases and it warms. when air is warming, any moisture in the atmosphere is
evaporated so no clouds can form. the sky is clear. anticyclones can be very
large, typically at least 3,000 km wide. once they become established, they can
give several days of settled weather. winds are very gentle or even calm in an
anticyclone, move clockwise, and this is shown on a synoptic chart by wide
spaced isobars.
British anticyclone weather
in Britain in summer an anticyclone will mean heat waves during
the day. at night, however, as there are no clouds, heat will be quickly lost.
the ground will cool sufficiently to cause condensation of water vapour in the
descending warm air and mist or heavy dew may form. this will clear quickly in
the morning sun. after a few days, a layer of hot air builds up at ground level,
which eventually will give rise to thunderstorms, ending the anticyclone.
in winter the longer nights combined with clear skies leads to
intense cooling of the land. there is an increased risk of dew, frost and
thicker, more extensive fog patches which may be slow to clear or even persist.
under very calm conditions, both frost and fog may persist for
several days. an anticyclone's very stable conditions and little air movement
means that pollution is trapped at low levels, resulting in very poor air
quality such as smogs.
