GEOG101: World Regional Geography

In the United States, precipitation generally decreases as we move from east to west due to the Pacific Mountain system, which creates a rain shadow effect. The Pacific Mountains cast a rain shadow that limits precipitation in much of the western half of the United States, including eastern Washington and Oregon, the Great Plains, and the Desert Southwest. Figure 4.4 illustrates the rain shadow effect.

Moist air from the ocean is blown onshore and rises as it follows the slope of the mountains. As the moist air rises, it cools, and the moisture precipitates as rain and/or snow. By the time the air begins to descend on the leeward side, it is dry. We will see that the rain shadow effect impacts other regions too. The East coast lacks a coastal mountain range that would prevent moisture-laden air from reaching the interior.