UK
Foehn effect set to bring possible 25C weather to parts of northern Scotland
An unusual weather effect is likely to bring unseasonably warm temperatures back to the UK this week.
The Foehn effect could see temperatures spike back up in parts of northern Scotland on Wednesday.
Following on from the hottest weekend of the year so far, temperatures have dropped back down to the mid-to-high teens.
But forecasters have warned that parts of northern Scotland could see temperatures climb back up to 24C or even 25C – well above the seasonal average.
This could take place due to the Foehn effect, which is where air on the downwind (or lee) side of a mountain is warmer and drier than air on the other side.
When the air is forced up over a mountain, it cools and condenses to form clouds, bringing rain to the upwind side of a mountain.
When the air then descends down the other side, it’s much drier and that means it ends up warmer.
The size and shape of a mountain, along with the upstream wind speed, temperature and humidity all play a part in how much warmth is seen on the downwind side.
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A number of other factors can help increase the temperature on the downwind side of the mountain as well, such as air being drawn down from aloft, vertical mixing of air at the top of the mountain and radiative warming – due to sunnier conditions on the downwind side.
The Foehn effect can be seen in mountainous regions across the world.
In the areas where the impacts are greatest, local names are given to it.
The Chinook affects the Rockies in North America, while the Zonda impacts the Andes in South America.
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According to the US National Weather Service, a Foehn event in 1972 caused the temperature in the US state of Montana to rise from -48C to 9C, an increase of 57C.
The increase in temperature can have detrimental effects, bringing the risk of avalanches, glacial melting, downstream flooding and wildfires.
For the UK, the prevailing westerly wind means the Scottish Highlands tend to see the most notable Foehn events.
One occurred in January this year, which allowed Achfary to reach a high of 19.9C, provisionally the UK’s highest temperature for January.