Weather wise

OUR weather in the second half of this week is being dominated by an anti-cyclone (high pressure area) that yesterday settled directly above Britain. In the summer, we often think of high pressure as a portent of fine weather, but at other times of year, the situation is far less clear.

High pressure means heavier air, and there are two main things that can bring that about. First, cold air is denser than warm air, and second, dry air is heavier than damp air (since the specific gravity of water vapour is only about two-thirds that of air). So an area of high pressure may correspond to cold air, or dry air, or both.

This doesn't sound like a recipe for guaranteed fine weather, but there is another important process involved with anticyclones that does have a warming effect. In a summer anti-cyclone, air is piled up in the troposphere by high altitude winds. As the air descends, it is compressed by the pressure of the air above it, and it warms as the compression energy is converted to heat. (This is the process known as adiabatic warming.) Under such conditions, clouds tend to disappear as the air warms, resulting in fine, sunny summer days - though the chilling effect of the upper, cold air makes occasional thunderstorms also characteristic.

In summer, high pressure areas are usually formed of maritime tropical or continental tropical air. The winter anticyclone is a different beast, more likely resulting from continental polar wind. Its cooling effect can easily lead to cloud and fog.

Yesterday, in the South-east, we saw a beautifully clear and bright morning, but by lunchtime the sky had completely clouded over. This is again quite typical of some sorts of anticyclonic weather. What happens is that turbulence near ground level, caused by the land being warmed by the sun, and the air at ground level being warmed by the land and rising, lifts moist air high enough to condense and form cloud.

The main characteristic of high pressure, however, at whatever time of year, is its tendency to lead to stable weather conditions. High pressure areas, consisting of heavy air, are more difficult to budge than their flimsy low pressure colleagues.