A "rain shadow" is a dry area on the leeward side of mountains where less precipitation occurs.

A rain shadow forms when moist air ascends a mountain range, cooling and releasing precipitation on the windward side. As the air moves over the peak and descends on the leeward side, it becomes warmer and drier, resulting in significantly reduced rainfall. This effect creates arid or semi-arid conditions, often contributing to the formation of deserts in these regions. Rain shadows are observed in many U.S. locations, such as the eastern side of the Sierra Nevada in California, where the air loses moisture on the western slopes, leaving eastern areas much drier.