The rain shadow of several mountain ranges, chiefly the Sierra Nevada Mountains and the Rocky Mountains, keeps Western North America much drier than the humid locations of Eastern North America, which are more often affected by moist air from the Gulf of Mexico and the Atlantic Ocean.
Moisture condenses at high altitude, leaving air masses arid or semi-arid after passing over mountain ranges. You will see this in regional climate patterns in the U.S. The Central Valley of California and the Mojave Desert are both relatively dry as they are in the rain shadow of the Coast Ranges. The Great Basin is extremely dry, too, as it sits in the rain shadow of the Sierra Nevada Mountains. Even the Great Plains see higher aridity due to the Rocky Mountains to the west, resulting in long dry stretches throughout the year.
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