Present Climate of the Contiguous Western States

Because of its wide latitudinal range, the proximity of the Pacific Ocean, and the presence of long, north-south mountain ranges, the Western States have an enormous variety of climatic areas. These include hot, dry deserts in the Basin and Range, a Mediterranean climate along the southern Pacific Border, rainforests in the northern Pacific Border and Alaska, and tundra in Alaska’s far north. Even individual states can have tremendous diversity—depending on which of the many Köppen system maps you refer to, the state of Washington alone contains as many as eight different climate types.

The Köppen Climate Map

Wladimir Köppen developed a commonly used system of climate categorization based on the kinds of vegetation that areas sustain. He defined 12 climate types, many of which are familiar: rainforest, monsoon, tropical savanna, humid subtropical, humid continental, oceanic, Mediterranean, steppe, subarctic, tundra, polar ice cap, and desert. Updated by Rudolf Geiger, it has been refined to five groups each with two to four subgroups.

Figure 9.8: Mean annual temperature for the contiguous Western states.

With such diverse climate types, a wide range of temperatures can be experienced throughout the West (Figure 9.8). Generally, temperatures tend to decrease northward (and also from west to east), with cooler temperatures at higher elevations and across the West’s north-south mountain ranges. Temperatures in coastal areas are moderated by the Pacific Ocean and, in the northwest, by the Rocky Mountains, which prevent cold Arctic air from reaching the coast. Average lows and highs in Southern California range from 3° to 46°C (37° to 114°F) inland in Death Valley and 9° to 24°C (49° to 76°F) on the coast in San Diego. Statewide average lows and highs in Oregon run from -3° to 28°C (26° to 82°F), while in Washington, temperature ranges from -1° to 32°C (29° to 89°F). Nevada experiences average temperatures spanning from 4° to 40°C (39° to 104°F)

Figure 9.9: Mean annual precipitation for the West Coast.

The West’s spectacular mountain ranges (apart from those in Alaska) run from north to south. These ranges—the Coastal Range, the Cascades, the Rockies, and the Sierra Nevada—create a pronounced east-west precipitation gradient across the Western states. The overall effect is to produce dry rain shadows on the eastern sides of the West’s mountain ranges, and wet areas on the western sides (See Figure 9.6). This effect is most pronounced from Northern California up through Washington, since the jet stream is often located over this area—especially in winter—and brings moist ocean air inland. As an example of how extreme this precipitation gradient can be, Olympic National Park in Washington receives over 190 centimeters (75 inches) of rain annually on average, whereas communities only 400 kilometers (250 miles) to the east in Washington receive only 18 to 20 centimeters (7 to 8 inches) annually. As the most arid state in the US, Nevada receives only about 24 centimeters (9.5 inches) of rainfall a year (Figure 9.9).