A warm, dry wind descending the leeward side of a mountain is called a foehn wind, or föhn. It forms when air loses moisture on the windward slope and then warms as it descends on the opposite side.
As moist air rises over a mountain, it expands and cools. Water vapor condenses into clouds and falls as rain or snow, releasing latent heat. Once the air crosses the range, it descends into higher pressure, becomes compressed, and warms. Because much of its moisture has already been removed, the resulting wind is unusually dry.
Foehn winds are strongly associated with the Alps, especially Switzerland, Austria, and southern Germany. Similar winds have regional names elsewhere, including chinook in the Rocky Mountains, zonda in Argentina, and Santa Ana in Southern California, although not every locally named wind has exactly the same formation mechanism. The word foehn ultimately derives from Latin favonius, a mild west wind.