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Thursday, 7 January 2010

Why is it so cold?

The Big Chill is being blamed on a change in the position of the jet stream – the current of air that moves from west to east. In a normal British winter – when conditions are mild and soggy – the jet stream lies over northern Europe, at an altitude of between 35,000 to 50,000 feet.

During these grey winters, Britain’s prevailing winds come from the west and south west, and bring with them warm and moist air from the sub-tropical Atlantic. But since mid December, the weather patterns high in the atmosphere have changed.  The jet stream has shifted south hundreds of miles and is now positioned over North Africa.

The warm westerlies that usually keep away the snow are instead giving the Mediterranean an unusually mild winter. What wind the UK has experienced has blasted in from the Arctic, or from across the cold land masses of Siberia and Eastern Europe.

Helen Chivers, of the Met Office, said: ‘Because the jet stream is so far south, we have now got two areas of high pressure bringing cold weather to Britain.

‘One is over Greenland, and the other is over Russia. We are stuck in an area of low pressure between them and we are getting northerly winds from the Arctic and northern Scandinavia.’

No one knows why the jet stream has shifted so far south. Some believe its location may be linked to the emerging El Nino weather phenomena – where the surface temperatures of the Pacific periodically get warmer.

The Met Office said yesterday that the cold snap has nothing to do with climate change – but is part of the normal ebb and flow of normal local weather.

Read more:

BBC Weather forecast for the week ahead - broadcast 3rd January 2010

Posted by D.Drake

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