Rain Shadows: Why Portland Is Wet and Eastern Oregon Is Dry

Portland averages roughly 36 inches of precipitation per year. Bend, Oregon, just 130 miles east across the Cascades, gets about 11. Same latitude, same ocean, wildly different climate.

That's a rain shadow. Pacific moisture slams into the western slopes of the Cascades, rises, cools, and drops its water as rain or snow. By the time the air reaches the eastern side, it's dried out.

The same phenomenon explains why Seattle is famously wet while Denver — also a mile high, also surrounded by mountains — averages under 15 inches annually. Different mountain orientation, different rain shadow.