Oregon has a generally temperate climate, but there are marked regional variations. The Cascade Range divides the state into two broad climatic zones: the western third, with relatively heavy precipitation and moderate temperatures, and the eastern two-thirds, with relatively low precipitation and more extreme temperatures.

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.

The climate of Oregon is moderated by the Cascades which 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 inches (94 cm). In the drier areas rainfall is less than 8 inches while in the upper west coast rainfall is about 200 inches.