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Weather: Heigh-ho, the wind and the rain

A family of depressions crossing the country has been responsible for the winds and torrential rain that have been causing such havoc. The cause of those depressions may be the same jetstreams that blew Steve Fossett's balloon a third of the way round the world.

The easy bit to understand about the current weather is why heavy winds and torrential rain so often occur together, and why they are associated with areas of low pressure. There are two principal air masses fighting for space in the skies above us. Tropical maritime air - warm and moist - drifts over, in a northeasterly direction from the Atlantic, while polar maritime air - cold and dry - drifts southwards from the Arctic. The heavy, cold air moves beneath the lighter, warm air, forcing it upwards and chilling it. As it cools, it can no longer contain so much water vapour, and rain results. For heavy or prolonged rain, a steady supply of moist air is needed. High winds greatly assist such a supply.

Winds, however, are associated with pressure differences, and the current high winds have been caused by a series of low-pressure areas chasing each other across the North Atlantic.

What has been going on is a common pattern. Depressions are gregarious creatures; they are most often sighted in groups of three or four, each following a path slightly south of the one before, as the south-moving polar air pushes it in that direction. Eventually the cold polar air forms a wedge of high pressure at its southernmost point, and puts an end to the process. The windiest and rainiest days in England tend to come when a low-pressure area follows a path that takes it past the north of Scotland, then down into the North Sea. The build-up of waters in the south of the North Sea caused by a northerly wind add more moisture to the air, and fuel the storms that follow.

How the depressions arise is a more difficult question, but it seems likely that the jetstream of strong, high-level winds plays an important role. Jetstreams are belts of strong winds occurring above about 8km, which meander around the globe, constantly adjusting their path because of the Coriolis effect, generally heading clockwise around the globe, but wobbling between a north-easterly and a south-easterly direction.

While the temperature of the air near the ground is affected mainly by the heat radiated from the earth itself, the movement of the resulting air masses can be greatly affected by the motion of the high-level jetstreams, and the interaction between high-level and low-level weather systems poses one of the biggest problems for the meteorologist. High-level strong winds must play a part in the formation of weather systems, though it is far easier predicting their paths once they are formed than predicting their formation in the first place. On their own, the jetstreams are a considerable boon to the round-the-world balloonist. Richard Branson was reported to have had 70 weather stations around the world monitoring jetstreams for him, in case he got into his balloon before it blew away. Steve Fossett no doubt had similar back-up.

While he was enjoying his high-speed crossing of the Atlantic, however, the same jetstreams that were blowing him along so well were also forming the depressions that have just brought the devastation of high winds and storms to parts of this country. What is it they say about ill winds?