Climate

The Pacific Northwest experiences a wide variety of climates. An oceanic climate ("marine west coast climate") occurs in most coastal areas, typically between the ocean and high mountain ranges. An Alpine climate dominates in the high mountains. Semi-arid and arid climates are found east of the higher mountains, especially in rainshadow areas. The Harney Basin of Oregon is an example of arid climate in the Pacific Northwest. Humid continental climates occur inland on windward sides, in places such as Revelstoke, British Columbia. A subarctic climate can be found farther north, especially in Yukon and Alaska.
Under the Köppen climate classification, a cool-summer version of the dry-summer mediterranean (Csb) designation, is assigned to many areas of the Pacific Northwest as far north as central Vancouver Island and the Gulf Islands, including cities such as Victoria, British Columbia, Seattle, and Portland.^[35] These zones are not associated with a typical mediterranean climate, and would be classified as Temperate Oceanic (Cfb), except dry-summer patterns typical to the Pacific Northwest meet Koeppen's minimum Cs thresholds. Other climate classification systems, such as Trewartha, place these areas firmly in the Oceanic zone (Do).^[36]

Ecoregions

Much of the Pacific Northwest is forested. The Georgia Strait–Puget Sound basin is shared between British Columbia and Washington, and the Pacific temperate rain forests ecoregion, which is the largest of the world's temperate rain forest ecozones in the system created by the World Wildlife Fund, stretches along the coast from Alaska to California. The dryland area inland from the Cascade Range and Coast Mountains is very different from the terrain and climate of the coastal area due to the rain shadow effect of the mountains, and comprises the Columbia, Fraser and Thompson Plateaus and mountain ranges contained within them. The interior regions' climates largely within eastern Washington, south central British Columbia, eastern Oregon, and southern Idaho are a northward extension of the Great Basin Desert, which spans the Great Basin farther south, although by their northern and eastern reaches, dryland and desert areas verge at the end of the Cascades' and Coast Mountains' rain shadows with the boreal forest and various alpine flora regimes characteristic of eastern British Columbia, northern Idaho and western Montana roughly along a longitudinal line defined by the Idaho border with Washington and Oregon.