Climate of California
The climate of California varies widely from hot desert to alpine tundra, depending on latitude, elevation, and proximity to the Pacific Coast. California's coastal regions, the Sierra Nevada foothills, and much of the Central Valley have a Mediterranean climate, with warmer, drier weather in summer and cooler, wetter weather in winter. The influence of the ocean generally moderates temperature extremes, creating warmer winters and substantially cooler summers in coastal areas.
Temperature range
The cool offshore, enhanced by upwelling of cold sub-surface waters, often creates summer fog near the coast, creating a warm-summer Mediterranean climate. Farther inland, the climate becomes more continental, with some areas turning semi-arid, with colder winters and markedly hotter summers. Low-lying inland valleys, especially the Central Valley, have a hot-summer Mediterranean climate, with subtropical temperatures but a well-defined sunny summer season and a foggy winter season.The temperature gradient between immediate coast and low-lying inland valleys in the south is about in winter, the coast being warmer, and in summer roughly, the interior being hotter. For example, the average daily high in San Francisco in July and August is between, and in Walnut Creek, some inland, the average daily high in July and August is : a temperature gain of more than one degree per mile. In Southern California, the temperature differences are approximately in winter and in summer. At the coast in Santa Monica, the average high in August is, while in Burbank, approximately inland, the average high in August is : a temperature gain of about two degrees Fahrenheit per mile.
During the cooler winter months, the Coachella Valley regularly has the warmest winter temperatures out of any place west of the Rocky Mountains. East Los Angeles, the Gateway Cities, and parts of the San Gabriel Valley average the warmest winter high temps in all of the western U.S., and Santa Monica averages the warmest winter lows in all of the western U.S. Palm Springs, a city in the Coachella Valley, averages high/low/mean temperatures of 75 °F/50 °F/63 °F, respectively during the period of cooler weather from November to April. The extreme southwest, around San Diego, has a borderline mediterranean/semi-arid or steppe climate as winters are drier there.
The southeastern regions have a hot arid climate, similar to that of the Sahara Desert. In the northern portion of the Mojave Desert on the east side of the state is Death Valley, which has recorded temperatures among the highest in the world. It is common in the summer for temperatures in the valley to surpass. The highest reliably recorded temperature in the world,, was recorded in Death Valley on July 10, 1913. Temperatures of or higher have been recorded as recently as 2005. The 24-hour average July temperature in Death Valley is.
In contrast, the high Sierra Nevada has a much cooler climate with extreme diurnal temperature variation. Bodie, at an altitude of, has never had a freeze-free month in a record dating back to 1895, and can expect only about 60 minima above freezing annually.
The highest temperature ever recorded in California was in Death Valley on July 10, 1913. This is also the highest temperature ever recorded on Earth. The lowest was in Boca on January 20, 1937.
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Precipitation
The prevailing westerly winds from the oceans also bring moisture, and the northern parts of the state generally receive higher annual rainfall amounts than the south. California's mountain ranges influence the climate as well: moisture-laden air from the west cools as it ascends the mountains, dropping moisture; some of the rainiest parts of the state are west-facing mountain slopes. Northwestern California has a temperate climate with rainfall of to per year. Some areas of Coast Redwood forest receive over of precipitation per year.The Central Valley has a wide range of precipitation. The northern parts of the Central Valley receive substantially greater precipitation from winter storms which sweep down from the Pacific Northwest, while the southernmost regions of the Central Valley are near desert-like because of a lack of precipitation.
The high mountains, including the Sierra Nevada, the Cascade Range, and the Klamath Mountains, have a mountain climate with snow in winter and mild to moderate heat in summer. Ski resorts at Lake Tahoe, Mammoth Lakes, and Mount Shasta routinely receive over of snow in a season, and some years, substantially more – leading, for example, to annual ski races on the Fourth of July.
On the east side of the mountains is a drier rain shadow. California's desert climate regions lie east of the high Sierra Nevada and southern California's Transverse Ranges and Peninsular Ranges. The low deserts east of the southern California mountains, including the Imperial and Coachella valleys and the lower Colorado River, are part of the Sonoran Desert, with minimal frost in the winter; the higher elevation deserts of eastern California, including the Mojave Desert, Owens Valley, and the Modoc Plateau, are part of the Great Basin region, which has a more continental climate. During the summer months, especially from July through early September, the region is affected by the Mexican Monsoon, which drives moisture from the tropical Pacific, Gulf of California, and/or Gulf of Mexico into the deserts, setting off brief, but often torrential thunderstorms, particularly over mountainous terrain.
The ENSO cycle has a huge effect on rainfall and snowfall patterns in California, especially during the winter and spring seasons. During the El Niño phase, the jet stream is located south through California, allowing for warmer temperatures and more heavy rains to occur, particularly in the southern portions of the state. During the La Niña phase, the jet stream is much further north, and therefore the far northern portions of California are wetter, while the southern half stays cool and dry.
Fog
Incidences of dense fog in Los Angeles have been decreasing over time; researchers surmise the change may be a consequence of both the urban heat Island effect and decreased air pollution.In the Central Valley during the winter months and in times of clear skies and light winds, the moisture from the Pacific can produce the tule fog, which is exceptionally thick fog that is visible on satellite imagery. The tule fog, named after the local tule grass wetlands, can reduce visibility to near zero, making it the primary cause of weather-related traffic accidents in the state. On November 3, 2007, a sudden patch of dense fog along California State Route 99 resulted in a traffic crash involving 108 vehicles in Fresno, which killed two people and at least 36 injuries. Following the accident, the California Department of Transportation developed a fog warning system along a section of SR 99.