A mountain’s dry side

Two places separated by a mountain range can receive different amounts of rain. The contrast often arises because the mountains change the temperature of air moving across them. The side facing the wind is called windward; the opposite side is leeward.

When moist air meets a mountain slope, it is forced upward. Air pressure decreases with height, so the rising air expands and cools. If cooling brings it to the point where water vapor condenses, clouds form and precipitation may fall. The windward slopes consequently receive water from air that has traveled inland from the sea.

After crossing the ridge, the air descends. It encounters higher pressure, becomes compressed, and warms. Warming reduces its relative humidity, making cloud formation less likely. In addition, some of its moisture has already fallen on the other side. The leeward region may therefore lie in a rain shadow: an area that is relatively dry because of the mountain barrier.

This pattern helps explain sharp changes in vegetation across a mountain range. Forest may cover a moist slope while grassland or sparse shrubs occupy the drier side. The contrast depends on the prevailing wind direction, so knowing which slope faces east or west is insufficient to identify the wetter one.