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Oregon has a generally temperate climate, but there are marked regional variations. The Cascade Range separates the state into two broad climatic zones: the western third, with relatively heavy precipitation and moderate temperatures, and the eastern twothirds, with relatively little precipitation and more extreme temperatures. Within these general regions, climate depends largely on elevation and land configuration.

In January, normal daily mean temperatures range from more than 45°F (7°C) in the coastal sections to between 25°F (–4°C) and 28°F (–2°C) in the southeast. In July, the normal daily means range between 65°F (18°C) and 70°F (21°C) in the plateau regions and central valleys and between 70°F (21°C) and 78°F (26°C) along the eastern border. Oregon's record low temperature, –54°F (–48°C), was registered at Seneca on 10 February 1933; the all-time high, 119°F (48°C), at Pendleton on 10 August 1898.

The Cascades serve as a barrier to the warm, moist winds blowing in from the Pacific, confining most precipitation to western Oregon. The average annual rainfall (1971–2000) in Portland was 37 in (94 cm); rainfall elsewhere varied from less than 8 in (20 cm) in the drier plateau regions to as much as 200 in (508 cm) at locations on the upper west slopes of the Coast Range. In the Blue Mountains and the Columbia River Basin, totals are about 15 in (38 cm) to 20 in (51 cm). In Portland, fog is common, and the sun shines, on average, during only 39% of the daylight hours—one of the lowest such percentages for any major US city. From 300 in (760 cm) to 550 in (1,400 cm) of snow falls each year in the highest reaches of the Cascades.