foehn, warm and dry, gustywind that periodicallydescends the leeward slopes of nearly allmountains and mountain ranges. The name was first applied to a wind of this kind that occurs in theAlps, where the phenomenon was first studied.

A foehn results from the ascent of moistair up the windward slopes; as this air climbs, it expands and cools until it becomes saturated withwater vapour, after which it cools more slowly because its moisture is condensing asrain orsnow, releasinglatent heat. By the time it reaches the peaks and stops climbing, the air is quite dry. The ridges of the mountains are usually obscured by a bank of clouds known as afoehn wall, which marks the upper limit ofprecipitation on the windward slopes. As the air makes its leewarddescent, it is compressed and warms rapidly all the way downslope because there is little water left to evaporate and absorb heat; thus, the air is warmer and drier when it reaches the foot of the leeward slope than when it begins its windward ascent.

Foehn winds in various parts of the world have local names:Santa Ana andchinook in the North AmericanRockies,ghibliinLibya, andzonda in theAndes ofArgentina.

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