Were it not for the fact that they are not really all that common - in this part of the world at any rate - winter anticyclones would surely feature in Keats's "dull catalogue of common things". They are doubly dull: they are dull in the aimless, listless nature of their general character, and often at this time of year they are dull in the hours of sunshine they allow.
An anticyclone, an area of high barometric pressure, is the antithesis of a depression. But unlike a depression which has a clearly defined centre of low pressure, it is difficult to find the corresponding "peak" of an anticyclone; it is more of a "plateau", with the atmospheric pressure almost uniformly high over a large central area, producing very light winds of variable direction.
Anticyclones drift slowly around the weather chart with none of the militant sense of purpose that typifies the behaviour of depressions. In fact, one could say that anticyclones exist merely because there are no depressions in the vicinity at the time.
When accompanied by clear skies, winter anticyclones allow the Earth to lose heat very rapidly at night, so that while the following morning may be bright and sunny, the ground is often hard with frost. Just as easily, however, anticyclonic skies may be dull and overcast, resulting in a winter phenomenon known as anticyclonic gloom.
Both lows and highs are three-dimensional phenomena. The air around a depression spirals inwards towards the centre, where it is forced to ascend in vigorous updraughts, but anticyclones are regions of gentle but widespread descent of air, a phenomenon we know as subsidence. The descending air is subject to compression, which causes it to become warmer than it was, and the consequent drop in the relative humidity sometimes ensures that there is very little cloud.
But subsidence occurs only down to within 1,000 feet or so of the ground. The warm dry air above this, superimposed on colder air near the surface, often in winter results in what we call a "temperature inversion": in effect the warm air aloft acts as a lid on the lower atmosphere and cuts off any tendency for the air to bubble upwards.
Into the air imprisoned below the inversion go the smoke and other pollutants from domestic and industrial sources, reducing visibility and producing haze. Moreover, if the humidity a short distance above the ground is high enough for cloud to form, the cloud is obliged to spread horizontally in a thick uniform sheet just below the discontinuity of temperature - resulting in the depressing condition that we know as anticyclonic gloom.