A rain shadow (or more accurately, precipitation shadow) is a dry region on the surface of the Earth that is leeward or behind a mountain with respect to the prevailing wind direction. A rain shadow area is dry because, as moist air masses rise to top a mountain range or large mountain, the air cools and water vapor condenses as rain or snow, falling on the windward side or top of the mountain. This process is called orographic precipitation. The effect of the process is the creation, on the leeward side, of an area of descending dry and warming air, and a region that is quite arid.
Good examples are in the deserts of the Basin and Range Province, which includes the dry areas east of the Cascade Mountains of Oregon and Washington known as the Great Basin, and Chile's Atacama, the most arid place in the world. The aptly-named Death Valley is another good example; it is behind both the Coastal Range of California and the Sierra Nevada range, and is one of the driest places on the planet. Hawaii also has rain shadows, with some areas of the islands being desert, much to the surprise of many tourists.
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