Foehn is a term often brandished by skiers but seldom used correctly.

The Foehn, or Föhn effect occurs over mountain ranges around the world but the phrase was originally coined in reference to winds that blow over the Alps. Pronounced fø-n, the word is derived from the Latin word favonius, meaning spring breezes.

In short the phenomenon happens when moist air is generated over a large body of water and blown inland. These fast moving systems, heavy with moisture are forced to ascend to a higher, cooler altitude when they reach mountain ranges. The damp air cools releases it’s moisture via precipitation, in summer this manifests in sudden and sustained rainfall and in winter in the form of heavy snowfall.

Whilst this sounds like good news for skiers, there is also a darker side to the weather system. Once the air has deposited it’s moisture on the rise over the mountains it descends on the other side of the mountain, or the lee, the air warms. This warm, dry air can be quite pleasant in summer, but in the winter, particularly in the second half of the season, it causes havoc in resorts that lie in the lee. The fast moving dry air is responsible for rapid temperature rise, melting snowfall at a ferocious rate, casing snow conditions to deteriorate and the avalanche risk to increase rapidly.

Foehn winds on the downward side of the valley are known for their strong warm gusts and are said to cause madness - although this has never been proven!

View the Ski Club’s in depth weather forecasts and snow reports.

These types of storms explain how some resorts in the Alps can see increased snowfall whilst resorts on the Lee side of a mountain, in the shadow of the Foehn, see none of the benefits of a Foehn wind. Foehn winds are typically blown in from the Atlantic and over France or up the Mediterranean where they rise sharply over the Alps.

Earlier this season a Foehn and was in part responsible for blocking off the resort of Zermatt from the outside world. Following a strong retour d’est at the start of the season the warm winds caused havoc with the snowpack resulting in avalanches over major transport links. Roads were at a standstill for one of the busiest transfer days of the season.

Foehns of the world

As we noted earlier the Foehn effect was first identified and named in the Alps, but Foehn winds blow over any mountain range.

One of the strongest Foehn effects can be found in the North American and Canadian Rockies, this Foehn is known colloquially as the Chinook. Warm air laden with moisture sucked up from the Pacific Ocean moves east where it collides with the Rocky Mountains almost as soon as it reaches land. The speed at which the moist air rises, cools and then releases it’s moisture is seen in few other places on Earth.

The resulting warm wind in the shadow of the range is responsible for the largest temperature change over a 24hour period ever recorded. The US National Weather Service reported a temperature increase of an incredible 57 °C; from -48 to 9 °C.

Not all Foehn winds are too far from home though. As well as affecting the Scottish highlands, our tallest domestic mountains, the Foehn effect is welcomed in the North East of England where the Pennines force cold noise Atlantic air to ascend and release water. Once it has done this it dries and warms and descends to the East, commonly this can be called a rain shadow. Unfortunately the same courtesy is not extended to the entirety of England - it would certainly be welcomed in some parts!