Sierra Nevada Climates
The climate of the Sierra Nevada varies from hot desert (similar to the Sahara) at its eastern base to an artic-alpine climate only a few miles away on the highest peaks. However, most of the Sierra Nevada is governed either by Mediterranean or microthermal (low temperature) climates.
Mediterranean climates dominate the Sierra below about 6000-7000 ft. These climates are characterized by warm to hot dry summers and cool to cold wet winters.
Microthermal (or Boreal) climates dominate the high Sierra. These climates are characterized by having the average temperature of the coldest month below 27°F. These are the areas of heaviest snowfall in the Sierra.
This map uses the Koeppen climate system. For a more detailed description of each climate type, refer to the key below the map. The State Climate Services of Idaho provides a detailed breakdown of the Koeppen system criteria.
Sources: Adapted from Climates of California, Russell, University of California, 1926 and Atlas of California, 1979. The map in Atlas was too general to be truly useful, and Russell's demarcations (though much more detailed) did not always seem accurate. As a result, I developed my own demarcations based upon these two maps and my own experience in the Sierra, while remaining true to the classifications used in both sources.
These are arid regions where annual evaporation exceeds annual precipitation. There are two main subclasses: BS - semi-arid Steppe climates where precipitation is at least half of potential evaporation; and, BW - true desert climates where precipitation is less than half of potential evaporation.
BWh - Hot Desert. This the climate zone of the Mohave Desert and only touches the eastern base of the Sierra Nevada. It is characterized by extremely hot dry summers (average temperature of warmest month > 28°C (82.4°F)) and dry mild winters. Altitude Range: Below 3,500 ft. Vegetation: Creosote scrub is typical of the Mohave Desert.
Bsh - Hot Steppe. Average temperature of coldest month > 0°C (32°F). This climate zone forms a narrow band along the eastern base of the Sierra Nevada as well as the lowest foothills on the western edge of the southern Sierra. It is similar to BWh, with the exception that it is marginally cooler and experiences slightly higher precipitation. Altitude Range: East Side: 3,500-5,000 ft. West Side < 500 ft. Vegetation: East Side: Joshua Tree scrub and Creosote scrub are common vegetation types. West Side: Annual grasslands.
Bsk - Cool Steppe. Average temperature of coldest month < 0°C (32°F). This climate zone forms a narrow band along the steep eastern slopes of the Sierra Nevada just above Bsh. There is no analog on the western slope. Altitude Range: 5,000-7,000 ft. Vegetation: Joshua Tree scrub (at the very southern end) and pinyon-juniper woodland are common vegetation types.
In California, these are the areas of Mediterranean type climate, with the average temperature of the coldest month < 18°C (64.6°F) and > -3°C (26.6°F), and the average temperature of the warmest month >10°C (50°F). Precipitation occurs primarily between October and April, with little or no rainfall during the summer months.
Csa - Hot Summer Mediterranean. Characterized by hot dry summers, with the average temperature of the warmest month > 22°C (71.6°F). Winters are mild and very little snow falls in this climate zone. Altitude Range: Valley floor to 2,000-3,000 ft. Vegetation: This is the blue oak/digger pine/chaparral zone of the Sierra foothills.
Csb - Cool Summer Mediterranean. Characterized by warm to pleasant dry summers, with the average temperature of the warmest month < 22°C (71.6°F). Winter precipitation is a mix of snow and rain. Altitude Range: 2,000-3,000 ft to 6,000-7,000 ft. Vegetation: This is the ponderosa pine belt of the Sierra Nevada. Sugar pine and incense cedar are common. Black oak is common in the lower portions of this zone. White fir is common in the higher portions of this zone.
These areas are characterized by severe winters and heavy snowfall, with the average temperature of the coldest month < -3°C (26.6°F) and the average temperature warmest month > 10°C (50°F). These climates occur only in the higher areas of Sierra Nevada, typically above 6,000-7,000 ft.
Dsb - Lower Boreal. Pleasant dry summers, cold snowy winters. Average temperature of each of the four warmest months > 10°C (50°F); temperature of warmest month below 22°C (71.6°F). Altitude Range: Typically above 6,000-7,000 ft. to near the timber line at 9,000-10,000 ft. Vegetation: Typified in the Sierra Nevada by the lodgepole pine/red fir association, though the ponderosa pine/sugar pine/white fir association is common at the lower reaches of this climate. Jeffrey pine is common on the east side of the Sierra.
Dsc - Upper Boreal. Cool dry summers (with occasional thunderstorms), very cold snowy winters. Average temperature of less than four months >10°C (50°F); temperature of warmest month below 22°C (71.6°F). Altitude Range: Typically from near the timber line at 9,000-10,000 ft. to about 12,000 ft. Vegetation: Typified in the Sierra Nevada by whitebark pine and limber pine and by the treeless alpine zones above the timber line.
These areas are characterized by extremely severe winters and cool summers where the average temperature of the warmest month < 10°C (50°F).
EH - Polar Highland. Warmest month < 10°C (50°F). Altitude Range: Only in the highest parts of the Sierra, typically above 12,000 ft. Vegetation: Alpine tundra and bare rock.