A rain shadow is a region with reduced precipitation downwind of a topographic barrier. Air forced up a mountain expands and cools. If it reaches saturation, condensation and precipitation can remove some of its water vapour on the windward side.
Air descending on the other side is compressed and warms, lowering its relative humidity when its water-vapour content changes little. This favours a drier leeward region. The effect depends on the wind direction, moisture supply and storm structure: a mountain range does not permanently prevent every possible route by which moist air can reach its far side.
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