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Weatherwatch: A fast jet stream causes serial depression Weatherwatch: A fast jet stream causes serial depression
(about 14 hours later)
The storms repeatedly swinging in from the Atlantic bringing the damaging winds and rain are being driven by an unusually strong jet stream.The storms repeatedly swinging in from the Atlantic bringing the damaging winds and rain are being driven by an unusually strong jet stream.
This wind normally blows across from North America between the cold Arctic air and the warm Atlantic at about 100 miles an hour, meandering about like a river, bringing alternate cold and warm air masses over Britain.This wind normally blows across from North America between the cold Arctic air and the warm Atlantic at about 100 miles an hour, meandering about like a river, bringing alternate cold and warm air masses over Britain.
But when it speeds up, and it has been reaching 200 miles an hour, the jet stream gets straighter. This speed causes greater than normal turbulence in the lower atmosphere, and creates low pressures along its edge like a series of whirlpools. It is this succession of depressions that have been causing such havoc.But when it speeds up, and it has been reaching 200 miles an hour, the jet stream gets straighter. This speed causes greater than normal turbulence in the lower atmosphere, and creates low pressures along its edge like a series of whirlpools. It is this succession of depressions that have been causing such havoc.
Pressure below 950 is rare in Britain but the centre of the low on Christmas Eve was 936.8 at Stornaway in the Outer Hebrides. This is close to the all time record of 925.4 at Ochertyre, near Crieff, Perthshire on 26 January, 1884. Pressure below 950 is rare in Britain but the centre of the low on Christmas Eve was 936.8 at Stornoway in the Outer Hebrides. This is close to the all time record of 925.4 at Ochtertyre, near Crieff, Perthshire on 26 January, 1884.
Other storms in the series have not been so violent but have brought a lot of rain. Not that extreme low pressure is always necessary for a strong wind. The St Jude Storm of October 28 last year had a low of 967.6 but still managed hurricane force winds.Other storms in the series have not been so violent but have brought a lot of rain. Not that extreme low pressure is always necessary for a strong wind. The St Jude Storm of October 28 last year had a low of 967.6 but still managed hurricane force winds.
One other feature of a fierce jet stream is that the low pressures form quickly, grow deeper very fast, and take only a couple of days to reach Britain from the other side of the Atlantic. It should all slow down soon.One other feature of a fierce jet stream is that the low pressures form quickly, grow deeper very fast, and take only a couple of days to reach Britain from the other side of the Atlantic. It should all slow down soon.
• This article was amended on 6 January 2014 to correct the spelling of Stornoway and Ochtertyre.
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