The Föhn effect is characterised by dry and warm winds on the downwind side of high ground. The main warming mechanism originates from the fact that the cooling rate of moisture-laden air as it is forced to ascend over a mountain is lower than the warming rate of drier air as it descends the downslope side. Once the moisture is condensed out of the air into clouds over the mountain itself, it warms up descending the lee side – often becoming notably higher than its original temperature.

Another factor involved in the air warming is that descending air tends to be mostly clear of clouds, allowing the sun to warm the surface and the air that travels over it.

The Föhn effect can vary in duration but when it is prolonged, can result in abnormally high temperatures for the time of year and, in extreme cases, very strong gusts of wind.

Weatherwatch: The Föhn effect and winter warmth in Wales Read more

The term “Föhn” has origins in ancient Greek names for winds over the European continent, where the presence of the Alps and other mountain ranges make such warming winds quite common. Other notorious Föhn winds include downslope flows off the Rockies in North America, named “Chinook” winds, which are known for their rapid snow melting.