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A rain shadow is an area of land that lies behind a mountain which gets almost no rainfall. This side of a mountainous area is away from the wind. The mountains block the passage of rain-producing weather systems and cast a "shadow" of dryness behind them.
As shown by the diagram to the right, the incoming warm and moist air is drawn by the prevailing winds towards the top of the mountains. As it does so, it cools, condenses and rain falls before it crosses the top. The air, without much moisture left, goes on over the mountains creating a dry side called the "rain shadow".