<P> Oregon's climate is generally mild . West of the Cascade Mountains, winters are chilly with frequent rain, while light snowfall occurs a few days a year; temperatures can get very cold, but only occasionally, as the result of Arctic cold waves . The high desert region of the state is much drier, with less rain, more snow, colder winters, and hotter summers . An oceanic climate (also called "marine west coast climate") predominates in Western Oregon, and a much drier semi-arid climate prevails east of the Cascade Range in Eastern Oregon . Major factors determining Oregon's climate include the large semi-permanent high pressure and low pressure systems of the north Pacific Ocean, the continental air masses of North America, and the Cascade mountains . Oregon's population centers, which lie mostly in the western part of the state, are generally moist and mild, while the lightly populated high deserts of Central and Eastern Oregon are much drier . </P> <P> Precipitation in the state varies widely: some western coastal slopes approach 200 inches (5,100 mm) annually, while the driest places, such as the Alvord Desert (in the rain shadow of Steens Mountain) in eastern Oregon, get as little as 5 inches (130 mm). </P> <P> The Pacific Ocean, the moisture - laden air above it, and the storms moving from it over the Oregon coast, are major factors in the state's precipitation patterns . As humid ocean air flows east from the ocean and encounters the Coast Range, it rises steeply, cools, and loses moisture through condensation, which produces heavy rain . The heaviest precipitation in the state occurs at 2,000 to 4,000 feet (610 to 1,220 m) above sea level in these coastal mountains . At lower elevations along the coast, orographic precipitation is less intense but still produces 60 to 80 inches (1,500 to 2,000 mm) a year . </P> <P> In the Willamette Valley east of the Coast Range, storms blowing from the Pacific retain enough moisture to drop from 35 to 45 inches (890 to 1,140 mm) annually in the most heavily populated part of the state . East of the valley, the storm air rises again as it meets the Cascade Range, cooling once more and forming condensate at elevations often as low as 3,000 feet (910 m). Since volcanic peaks in the range are quite high--more than 11,000 feet (3,400 m) in the case of Mount Hood--most of the remaining Pacific moisture falls here in the form of rain or snow . </P>

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