An area of diminished precipitation on the lee side of mountains. There are marked rain shadows, for example, east of the coastal ranges of Washington, Oregon, and California, and over a larger region, much of it arid, east of the Cascade Range and Sierra Nevadas. Precipitation on the northern Oregon coast is around 100 in. (2.5 m) per year; east of the coastal ranges in the Willamette Valley it is about 40 in. (1.0 m); and east of the Cascades it drops to around 10 in. (0.25 m). All mountains decrease precipitation on their lee; but rain shadows, as is shown by annual totals, are sometimes not marked if moist air often comes from different directions, as in the Appalachian region.