Why is one side of the mountains so dry?
Moist wind rises up the windward side of a range, cools, and drops its water as rain and snow. The air that sinks down the far side has already been wrung out, and it warms as it descends. That dry side is in the mountains' rain shadow.
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Wet side, dry side
In Washington, the Hoh Rain Forest on the west side of the Olympic Mountains gets around 140 inches of rain a year. Sequim, on the northeast side, gets roughly 16. The same air passes over both; the mountains in between take the water.
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Deserts made by mountains
Many deserts sit in rain shadows. The Sierra Nevada and other ranges block Pacific moisture from the Great Basin and Death Valley. The Andes do the same for Patagonia's dry steppe, and the Himalaya help keep the Tibetan Plateau dry.
The warm wind on the dry side
Sinking air warms by compression, so the lee side of a range often gets warm, dry, gusty winds. They have local names: chinook in the Rockies, foehn in the Alps, and in Southern California the Santa Ana is a close cousin.
Questions people ask
- Which side of a mountain gets more rain?
- The windward side, facing the prevailing wind. On the West Coast that's usually the west side.
- Why is Death Valley so dry?
- It sits behind several mountain ranges that wring Pacific moisture out of the air before it arrives, and it's low, so sinking air warms even more.