Weather wise

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The Independent Online
SPECTACULAR temperatures have been recorded in southern Britain over the past week, as an area of high pressure centred on western France has sent warm dry air in our direction from Africa and the Canary Islands. In several places, the mercury has soared to a summery 20C - the sort of temperatures we associate with a flaming June, not a foggy February.

Of course, every silver lining has a cloud; skiers are unlikely to be appreciating the warmth, which has cleared the snow from the Scottish slopes and is threatening to do the same in the Alps.

When the prevailing wind direction is from the south, the very warmest places to be in Britain are often on the north coasts of South-west England. Already mild air blows across the Channel, picking up some moisture on the way. As the air is forced over the West Country moors, it rises and cools a few degrees, and some of the moisture condenses out as rain or mist.

Continuing north, the air then descends over the northern flanks of Exmoor, Dartmoor and Bodmin Moor, warming as it falls. The air is now dryer than before, having lost its moisture on the hills. Dry air warms up more easily than moist air, and so there is a net heat gain of 3-5C, good news for the people of Lynmouth and Ilfracombe.

This effect is called a Fohn wind, and this weather feature, on a much larger scale, is common in the northern Alpine valleys of Austria, Switzerland and Bavaria at this time of year, making the snow disappear before your eyes.

Spectacular Fohn winds, locally called the Chinook, affect the eastern slopes of the Rocky Mountains of North America. In 1988, a particularly strong Chinook nearly wiped out the Winter Olympics in Calgary, as temperatures rose from -10C to around 20C in less than one day.

Fohn winds are not the only airflows both consistent and unusual enough to be given a name. The south of France often falls victim to the Mistral, a kind of anti-Fohn that sweeps south down the Rhone valley in winter, bringing unusually cold air to the Mediterranean coast.The Harmattan blows ochre Saharan dust all over West Africa and sometimes into Europe and onto freshly-washed cars and washing lines. And the Texas Norther can see temperatures plummet in this sub-tropical state to below freezing in January and February. The same wind, which changes its name to Tehuantepecer as it crosses the Rio Grande, brings a chill to the Mexican plains.

The Arabian Khamsin, or Sirocco, blows hot dry air from the south-east. Khamsin winds can bring temperatures of 50C or more across the Middle East and North Africa. Fortunately, for anyone experiencing such outlandish temperatures, the humidity will probably be close to zero.

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