= Rain shadow
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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