THE BLOG

Turn on the Foehn

21/08/2014 16:00 BST | Updated 20/10/2014 10:59 BST

Since it's been so chilly of late (thanks to a northerly wind) I will try to warm you up with tales of a meteorological phenomenon nicknamed the Snow Eater.

More properly called the Foehn - or Föhn in Europe, it was named after a Roman wind god and is nowadays synonymous with German hairdryers. Pretty apt for a warm, strong dry wind that blows down the side of a mountain.

Rainshadow effect

Here's how it works (the wind, not the hairdryer). When moist air is forced up a mountain it cools with elevation and condenses to form clouds and then rain. Most of the rain falls on the windward side of the mountain, so by the time the air reaches the leeward side it has lost most of its moisture.

This is the rainshadow effect and explains why eastern parts of the UK, sheltered from the prevailing westerly wind by hills and mountains, are much drier and often warmer too. It also explains why western areas such as the Lake District are best explored in waterproof clothing.

MORE FROM THE WEATHER CHANNEL UK: 50 of the world's most beautiful coastlines [http://uk.weather.com/story/travel/50-worlds-most-beautiful-coastlines-20140616]

But it's the air rushing down the other side of the mountain that we're really interested in. As this drier air descends down the leeward slope it warms, and because drier air warms more quickly than moist air it reaches a higher temperature than when it began its mountain journey.

Balmy Aberdeenshire

In northeast Scotland this can lead to temperatures 5 to 10 degrees higher than areas to the west of the Cairngorms and Grampian mountains. Elsewhere the effect is more dramatic, in the Alps the Foehn wind can raise temperatures by as much as 30C in just a matter of hours.

So far, so good. In the UK the Foehn warms up places to the leeward side of hills and mountains and allows central Europe to enjoy a warmer climate as moist winds off the Mediterranean Sea blow northwards over the Alps. But it's not all good news, as well as warmer and drier weather, Foehn winds can lead to serious natural disasters such as droughts, wildfires, avalanches and floods.

Snow no more

I mentioned earlier the nickname Snow Eater, given because this warm dry wind can make snow melt, evaporate or even sublimate thanks to the combination of rising temperature and low humidity. This sudden and dramatic temperature change can lead to avalanches and floods.

For cloud spotters the strong and gusty Foehn winds can produce some spectacular cloud formations on the leeward side of the mountain - including my favourite, lenticularis cloud. But the gusts cause problems for mountaineers, not to mention aircraft flying near to mountain ranges.

While mostly a winter or spring phenomenon, Foehn winds can occur at other times of the year and elsewhere in the world, as long as there's a mountain range of course. As with most weather terms, these winds have different names in different locations, such as the Chinook in North America's Rockies and Cascades, the Zonda in the Argentinean Andes and the Berg wind in South Africa.

Ill wind blows no good

Whatever you want to call it, prolonged spells of Foehn winds have been blamed for increases in migraines, asthma, accidents, crime and suicide rates. At the moment most of the evidence is anecdotal but it's not unexpected that gusty, hot, dry winds would have some effect on us and our health.

Still, I reckon that chilly unseasonal northerly winds at the end of August (to be renamed Augtober any day now) are also having an effect on our health, not to mention my summer wardrobe plans. The good news is that temperatures should head towards more normal values after the Bank Holiday weekend. Let's just focus on that and ignore the unsettled (ie wet) weather heading our way.