A foehn wind is a dry, descending wind that warms as it moves down the lee side of mountains.
Moist air rises on a mountain’s windward side, cools and often produces clouds and precipitation. After crossing the ridge, the air descends on the sheltered side. Compression raises its temperature, so the wind arriving there can be much warmer and drier than the air on the windward slope.
The name comes from the Alpine Föhn, a well-known example in the Alps. Similar winds have different regional names, including the chinook in the Rocky Mountains and the Canterbury northwester in New Zealand.
Foehn conditions can rapidly reduce humidity and visibility and may accelerate snowmelt. They are not simply any warm mountain wind; the descent and drying process are central to the definition.