The causes of the foehn effect in the lee of mountains [image credit: Depunity @ Wikipedia]
Foehn (Föhn) winds are an interesting climatic phenomenon that cause rapid temperature changes. They’re well-known in Alpine regions but not so much in the UK.
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The highest ever temperature recorded for the UK this late in the year was early on Sunday morning in the Scottish Highlands, reports LBC News.
A Met Office observation post in Cassley, in Sutherland, north Scotland, recorded a temperature of 16.8C at 3am on Sunday morning – this was well above the average nighttime temperature for that area in late December of zero degrees.
However given how early in the morning the high temperature was recorded, most people would have slept through it.
Met Office forecaster Alex Burkill said it was the highest UK temperature recorded this late in the year.
He said: “It is pretty exceptional. We have never recorded a temperature that high this late in the month. What makes it more unusual was the temperature came at 3 am.”
According to meteorologists, Sunday’s record is down to a process called the Foehn effect – which the Met Office says is “a change from wet and cold conditions one side of a mountain, to warmer and drier conditions on the other (leeward) side”.
In the UK, the most notable Foehn events tend to occur across the Scottish Highlands where the moist prevailing westerly winds encounter high ground along Scotland’s west coast.
This results in a marked contrast in weather conditions across the country with the west being subjected to wet weather, while the lower-lying east enjoys the warmth and sunshine of the Foehn effect.
Overnight, mild air from Africa pushed across the UK. As the air travelled from the south-west over the UK, it rose up and lost its moisture. This meant that the other side of the hills ended up with drier air.
There is no indication that the warm nights will last, the Met Office said.
Full report here.
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Screen shot from the video…
Tallbloke's Talkshop 
Reblogged this on Climate- Science.press.
The weather system from Africa blew in from the south west over Northern Ireland and Western Scotland, as the latest chart still shows.
Wikipedia:
North-East Scotland, south-westerly winds create a föhn effect bringing relatively warm temperatures on the lee side of the Grampians and Cairngorm mountain ranges. The reverse occurs when south-easterly winds create a föhn effect to the North-West of Scotland, with the air drying out and warming up as it crosses the Grampians and Cairngorms from east to west. With the prevailing wind direction in the UK being from the west or south west, the föhn effect in Scotland is more common in the North-East of the country, with the west of Scotland being much wetter.
https://en.wikipedia.org/wiki/Foehn_wind#Local_examples
It’s interesting how MET-O videos and web pages explaining the Foehn effect have dropped all reference to pressure in recent years.
TB – Wikipedia still refers to it, e.g. here:
Isentropic draw-down is the draw-down of warmer, drier air from aloft. When the approaching winds are insufficiently strong to propel the low-level air up and over the mountain barrier, the airflow is said to be ‘blocked’ by the mountain and only air higher up near mountain-top level is able to pass over and down the lee slopes as foehn winds. These higher source regions provide foehn air that becomes warmer and drier on the leeside after it is compressed with descent due to the increase in pressure towards the surface. [bold added]
https://en.wikipedia.org/wiki/Foehn_wind#Isentropic_draw-down
Maybe they’ll want to get rid of that now 😆
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THE CAUSES OF FOEHN WARMING
IN THE LEE OF MOUNTAINS
by Andrew D. Elvidge and Ian A. Renfrew
It can increase the risk of avalanches or floods
(Barry 2008), melt glaciers, and contribute to the
disintegration of ice shelves (Cook et al. 2005;
Kuipers Munneke et al. 2012).
Click to access elvidge_renfrew_causes_foehn_warming_BAMS_2016.pdf
Their cartoon on page 3 is similar to the one in this post except it has four versions of foehn phenomena (but they say the ‘fourth mechanism (radiative heating), found to be unimportant here (at most 0.1 K)’).
I am sure that Climate Change will be blamed.
Note the spin to add spurious significance – “this late in the year”, but it doesn’t beat the January record or the existing December record. So later in the Winter has been warmer, before ‘climate change’ was a thing. Keep keeping records long enough and every winter day should eventually have a record high of over 18C, regardless of ‘climate change’. It’s just random chance.
From a much better video than the MetO effort:
There’s the compression. Note the leeward warming rate is greater than the earlier cooling.
And here’s the video:
[Added these items to the blog post]
I HAVE to say – What a load of Nonsense!. I have it on good authority from Family elders that we’ve had , in the INNER MORAY FIRTH Area, Temps over 17C between Christmas & NewYear in the early 70’s and again in the early 80’s
In so far as the Highlands are concerned, stretches from South,-West of Glasgow, right up to John o’Groats and EAST to PeterHeid – a HUGE area with Much and varied Climatic experiences. Most inhabitants DO NOT live HIGH UP in the hills.and are happy … I could go on – but this XR rubbish etc is doing my head in -whaever I listen to seems to have “Her” in the report…… just saying..
Happier New Year to all!
Daily Mail: Thousands of customers are left without TV as Freeview goes down across huge swathes of England and Wales because of ‘high pressure weather’
— Bad weather affecting Wales, south west, south east, East Anglia, Midlands
— Parts of Yorkshire has also been experiencing poor signal since Sunday
— Freeview confirmed ‘high pressure’ is the cause of the problem in most places
By LARA KEAY FOR MAILONLINE
PUBLISHED: 13:20, 30 December 2019
High pressure weather conditions are interfering with Freeview signal in Wales, south west England, south east England, East Anglia, parts of the Midlands and Yorkshire.
https://www.dailymail.co.uk/news/article-7837229/Weather-high-pressure-interfering-Freeview-TV-signal-UK-viewers.html
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‘Bad weather’? High pressure normally gives clear weather.