Dry West

bioclimatic region, United States

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Last updated

June 25, 2026

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The Dry West is a bioclimatic region in the United States characterized by arid or semiarid conditions, receiving less than 10 inches (250 mm) of rain annually in arid areas and 8 to 15 inches (200 to 380 mm) in semiarid regions. This dryness is primarily due to a combination of factors: the midlatitude high-pressure belt, tropical continental air masses, and the rain shadow effect from mountain ranges like the Sierra Nevada.1

Summer winds blow from the continent outward, limiting the influence of moist Pacific subtropical air. As these winds descend from high interior plateaus, they become warmer and drier. This results in large arid zones in the American Southwest, extending from the Columbia River basin to Guadalajara, Mexico, and along the Pacific coast from San Diego, California, to Mazatlán, Mexico. The Great Plains also experience semiarid conditions. Desert plants and animals have unique adaptations to survive these harsh conditions, such as water storage, deep root systems, reduced water loss, and dormancy.12

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United States
The Dry West
In the United States, to speak of dry areas is to speak of the West. It covers an enormous region beyond the dependable reach of moist oceanic air, occupying the entire Intermontane area and sprawling from Canada to Mexico across the western part…
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In the United States, to speak of dry areas is to speak of the West. It covers an enormous region beyond the dependable reach of moist oceanic air, occupying the entire Intermontane area and sprawling from Canada to Mexico across the western part…
Read More