Snow and Avalanche Glossary

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Foehn

Also: Fohn, Chinook
Locally: Santa Ana (S California), Mono (Sierra Nevada), East Winds (Cascades)
Colloquial: Snoweater

A warm, dry wind descending the lee side of a mountain range. The snowpack can diminish at a very rapid rate as a result, from both sublimation and melting. A foehn or chinook generally develops when there is a large scale pressure difference across the mountains with low pressure on the lee side.

As air rises on the windward side it cools at a certain (dry) rate until it becomes saturated. It then cools at a slower (saturated) rate. As soon as the air crosses the crest of the mountains and begins to sink it is no longer saturated and warms at the higher dry rate. So it can warm more as it sinks than it was cooled by rising on the windward side.

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