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Why is the eastern side of the Cascade Range much drier than its western side?
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Pacific mountain system
Coast Mountains along the Torres Channel, an arm of Atlin Lake, northwestern British Columbia, Canada.
The eastern side of the Cascade Range is significantly drier than the western side because the mountains act as an orographic barrier to moist air moving inland from the Pacific Ocean.12
As this moist air encounters the range, it is forced to rise, cool, and release the majority of its moisture as precipitation on the windward, western slopes. By the time the air descends on the eastern side of the mountains, it has been depleted of its moisture, creating a phenomenon known as a rain shadow. Consequently, while the western slopes may receive over 100 inches of precipitation annually, areas immediately to the east experience a drastic decrease, sometimes receiving less than 8 inches of precipitation per year.1342