The magic of the Foehn wind
Posted: 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.
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.