The magic of the Foehn wind

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stephenprudence
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The magic of the Foehn wind

Post by stephenprudence » Thu Feb 06, 2014 6:30 pm

Most people on here won't have come across a Foehn wind before, maybe some won't have even heard of it, but it's a fairly common theme here in Winter.

The Foehn wind brings warm air down the lee of mountain sides. The range of a Foehn can be quite large, and in my area it can occur in valleys in Wales, and here is pretty much the terminal point. The Foehn can also occur in the lee of the Pennines and Peak district and also Northeastern Scotland.. maybe occasionally in other places with high ground to the south/southwest.

A Foehn wind occurs when cold air travels up one side of a mountain or high hill range, but with the cloud and rain blocked from crossing the mountain range, the air descends down the lee side of the mountain/hill, and picks up warmth via frictional processes. Once the air reaches the ground on the other side, it is suitably warm. Here, because the Welsh mountain ranges are gradually sloping, unlike the high immediate descents of the Alps, the Foehn is less pronounced, so the maximum advance you can have in a Foehn wind is around 6-7C difference from surrounding areas. Typically a Foehn wind here will result in temperatures about 4-5C higher than Liverpool.. though even Liverpool can experience a slight warming from strong Foehns. In a typical Foehn, Manchester might have around 8C whilst Chester has 15C, it is not unusual for this to occur, and it has done numerous times before.

The biggest thing about Foehn winds is the strength of the wind.. I have experienced 60-70mph gusts of a Foehn, a few years ago, but generally the winds are around 40-50mph in gusts, so tend to not be so bad.

The feeling is quite surreal really.. one thing you noticed on the weather station is how quickly the humidity drops, Ive noted the humidity dropping from 71% to 29% in just 15 minutes.. and also the sun is of course shining during the event. With the dry air, and the fairly warm wind, it's difficult to breathe at times, but it does feel surreal.. totally different to your normal winter weather.. in part because of how dry it is.

Foehn winds in Britain are relatively short lived.. on average I may only see a Foehn wind for a maximum of 1-2 hours, though they may repeat throughout the day. They can also happen at night, and did so numerous times in 2007.

The last Foehn I witnessed was on the 15th January.. but it was fairly subdued - only 12.5C resulting, but for no great amount of time.
Heswall, Wirral, UK
USDA equivalent average temperature zone: 9a/RHS zone 3
AHS Heat Zone: 1
Last 5 winter minimums:
2007: -0.1C, 2008: -4.2C, 2009: -5.7C, 2010: -10.5 (record), 2011: -4.9C, 2012: -5.3, 2013: -4.5C (so far)


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flounder
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Re: The magic of the Foehn wind

Post by flounder » Thu Feb 06, 2014 8:12 pm

That's fascinating. Do some more weather facts Stephen please icon_thumleft
my name is flounder, but you can call me.............flounder! (or Gary, just don't call me late for dinner)


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cheshirepalms
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Re: The magic of the Foehn wind

Post by cheshirepalms » Thu Feb 06, 2014 10:13 pm

The best example in the UK I think is on the eastern side of the Cairngorms, where the temperatures could be drastically higher than the (generally milder) western side of Scotland. March has the best potential for this and can lead to places like Aberdeen reaching the high teens.


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stephenprudence
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Re: The magic of the Foehn wind

Post by stephenprudence » Thu Feb 06, 2014 10:18 pm

This would make sense as the Cairngorms are the highest mountain range in the UK.. Aboyne, and Inverness seem to be great examples of places that recieve Foehn, depending on wind direction.

The top three places for Foehn in UK in my experience:

1. Aboyne
2. Inverness
3. Aber (North Wales)
Heswall, Wirral, UK
USDA equivalent average temperature zone: 9a/RHS zone 3
AHS Heat Zone: 1
Last 5 winter minimums:
2007: -0.1C, 2008: -4.2C, 2009: -5.7C, 2010: -10.5 (record), 2011: -4.9C, 2012: -5.3, 2013: -4.5C (so far)


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Troppoz
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Re: The magic of the Foehn wind

Post by Troppoz » Wed Feb 12, 2014 3:17 pm

Stephen the heating of a foehn has nothing to do with friction, and everything to do with adiabatic processes, ie descending air becomes denser and therefore heats up, and the release of latent heat from the condensation of water vapour as the air rises over the windward face..

Youre right though that it is a surreal experience, but even more startling is that foehn winds are thought to create psychosis in some people! :shock: Ive found it not such a pleasant experience mainly due to the extremely low relative humidity of a foehn but thankfully Ive only been in a foehn a handful of times while on holidays in Tasmania.
Sean


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Blairs
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Re: The magic of the Foehn wind

Post by Blairs » Wed Feb 12, 2014 4:44 pm

stephenprudence wrote:This would make sense as the Cairngorms are the highest mountain range in the UK.. Aboyne, and Inverness seem to be great examples of places that recieve Foehn, depending on wind direction.

The top three places for Foehn in UK in my experience:

1. Aboyne
2. Inverness
3. Aber (North Wales)
I had thought Inverness had milder weather due to being on the Moray coast and having the river and canal through it. The Foehn wind makes some sense as the min temp they got in 2010 was -13C...about the same as me 125 miles south. I remember traveling down the A9 at the time and the temp fell to -16C in Daviot and -23 in Aviemore - I had not known how low a car thermometer can go. Ice formed inside the car and that was with the AC on full blast.

There are still lots of mature Cordylines (both red and green) on their riverfront in Inverness as well as large Phormiums. Not many Cordylines survived in Fife/Lothian after 2010 as tall as they were, so perhaps the warming Foehn winds made a difference?


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Exotic Life
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Re: The magic of the Foehn wind

Post by Exotic Life » Wed Feb 12, 2014 6:39 pm

Foehn wind in the alps this weekend, temperatures forecasted to 21/22C some places....

Image


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