föhnwind

Föhnwind, also written föhn wind, is a warm, dry downslope wind that occurs on the leeward side of mountain ranges. It is most commonly associated with the Alps, but similar winds occur in other regions with significant topography, such as the Pyrenees, the Rocky Mountains and the Andes. The term originates from the German word Föhn, describing the wind that descends the lee slope.

The phenomenon arises when moist air is forced to rise over a mountain barrier. As it ascends, the air cools and its moisture condenses, often producing precipitation on the windward side. After passing the crest, the air descends on the leeward side, where it is compressed and warms adiabatically. Since much of the moisture has been removed on the windward side, the descending air is relatively dry, and its warming produces a noticeable rise in temperature and a drop in humidity in the lee region. The wind itself is typically strong and downslope-directed, accelerating as it descends.

Regional patterns vary with the terrain and prevailing wind, but föhn winds are a common driver of sudden warm spells in mountain valleys. They can rapidly melt snow, alter local weather, and affect outdoor comfort and avalanche risk. In different regions, related downslope winds are given local names—Chinook in North America, and other names in various mountain ranges—though the basic mechanism remains the same.

Forecasting relies on understanding air flow over mountains, moisture supply, and pressure systems that set up the required lifting and downslope compression.