foehn

A foehn, also spelled föhn wind, is a warm, dry downslope wind that occurs on the lee side of a mountain range. It is typically associated with rapid warming and a marked drop in humidity as air descends the windward barrier and compresses.

The mechanism involves moisture-laden air rising over a mountain barrier on the windward side, cooling as it ascends and releasing much of its water vapor as precipitation. The latent heat released during condensation partially offsets this cooling, so the air crest is warmer than the surrounding air. As the air descends on the leeward side, it is compressed and warms adiabatically, losing most of its remaining moisture. The result is a strong, warm, and dry wind that can raise temperatures by many degrees within hours and lower relative humidity significantly.

Foehn winds are particularly associated with the European Alps, where they are common in many valleys. The term is used in German-speaking regions as well as in English, and the phenomenon has parallels in other regions under different names. A related wind in North America is the Chinook; similar downslope warming winds occur in the Andes and other mountain ranges, often with local names.

Impacts of foehn winds include rapid snowmelt on slopes, shifts in local temperatures, and effects on crop phenology and weather-sensitive activities. They can contribute to avalanche risk by rapidly warming and drying the snowpack, and they influence microclimates within valleys and foothill regions.