November anticyclones

Dull, drab, damp days
by Philip Eden

Weather forecasters have a name for it: anticyclonic gloom. It describes exactly the sort of weather much of the UK has had during the last week, when a large high pressure system covers the country, but the weather stays unremittingly drab and grey. You may have a hazy memory from school geography lessons that low pressure brings rain whereas high pressure means good weather. However, this is often not true especially during the winter half of the year and the recent spell is an excellent illustration. True, it was settled in the sense that changes in the weather were small and slow, much of the UK stayed dry, and the wind was light. But what happened to the sunshine?

When the anticyclone (high pressure systems are known as anticyclones in the trade) first established itself across the country during the middle of last week most of us enjoyed a couple of days of bright sunshine although it was very frosty at night. During Thursday and Friday (Nov 15 and 16), however, moist air from the Atlantic Ocean penetrated the anticyclone's circulation, forming a layer of cloud about half a mile above the ground; the winds aloft which blow sluggishly in a clockwise manner around a 'high' carried this cloudsheet to all parts of the British Isles.

When this happens in summer the cloud layer is usually broken up by turbulent air motion - convection currents, to be precise - caused when the lowest layers of the atmosphere are warmed up during the daytime. In late autumn and winter the sun is comparatively weak and nights are long, and the layer of air near the ground quickly becomes colder than the air above it. When the wind blows this cold zone is soon dispersed, but when there is little or no wind it is often strongly developed. This reversal of the usual fall of temperature with height above the ground is called a "temperature inversion" and it acts as a sort of atmospheric lid, trapping pollution, cloud, and mist in the cool layer of air near the ground.

This explains the particularly depressing nature of antiyclonic gloom: the overcast sky is accompanied by haze and mist, rising pollution levels, poor or indifferent visibility, and a moist, heavy atmosphere. There may even be a damping of drizzle in the air, but heavy rain is very unlikely. Such weather may last several days, until the high pressure system collapses, or until a weak front introduces cleaner air from outside the anticyclone's circulation.