rainshadow

A rainshadow is a dry or semi-arid region on the downwind side of a mountainous barrier, created when moist air rises over the windward slope and releases much of its moisture as precipitation. As the air ascends, it cools and condenses water vapor, producing clouds and rain on the windward side. Descending on the leeward side, the air warms and its relative humidity decreases, reducing the likelihood of precipitation and creating arid or semi-arid conditions.

The extent and intensity of a rainshadow depend on several factors, including wind direction and strength, the amount of available moisture, the height and shape of the mountains, and regional climate patterns. Some rainshadows are broad and persistent, while others are partial or seasonal; local weather variations can erode or intensify the effect in certain years.

Prominent examples include the rainshadow created by the Cascade Range in North America, which leaves much of eastern Washington and Oregon drier; the Sierra Nevada contributes to arid conditions in the Great Basin and parts of the Mojave Desert; the Andes generate a desert along the western coast of northern Chile (the Atacama) while the eastern side receives heavy rainfall in the Amazon basin; and the Himalayas block monsoon moisture to the north, contributing to arid ranges on the Tibetan Plateau and parts of Central Asia.

Rainshadows influence regional climate, vegetation, soil moisture, agricultural potential, and water resources. They are a key concept in climate, hydrology, and land-use planning, helping explain why adjacent regions can have starkly different climates despite proximity.