Source: USA TODAY research by Brian Brinch; graphic by John Herne

How mountains influence rainfall patterns

As air ascends mountains, such as the Washington Cascades, it is forced to rise. The rising air cools, condenses, and drops rain on locations situated on the windward slopes, like Seattle. When the air descends the back side of the mountain toward Spokane, it is compressed, warming and drying it out. This sinking, dry air produces a rain shadow, or area in the lee of a mountain with less rain and cloudcover.