Climate of California

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 (Köppen climate classification Csb). Farther inland, the climate becomes more continental, with some areas turning semi-arid (Köppen BSk), with colder winters and markedly hotter summers. Low-lying inland valleys, especially the Central Valley, have a hot-summer Mediterranean climate (Köppen Csa), with subtropical temperatures but a well-defined sunny summer and a foggy winter.

The temperature gradient between immediate coast and low-lying inland valleys in the south is about 7 °F or 3.9 °C in winter, the coast being warmer, and in summer roughly 20 °F or 11.1 °C, the interior being hotter. For example, the average daily high in San Francisco in July and August is between 62 and 68 °F (17 and 20 °C), and in Walnut Creek, some 20 miles (32 km) inland, the average daily high in July and August is 84 °F (28.9 °C): a temperature gain of more than one degree (Fahrenheit) per mile. In Southern California, the temperature differences are approximately 4 °F or 2.2 °C in winter and 23 °F or 12.8 °C in summer. At the coast in Santa Monica, the average high in August is 75 °F (24 °C), while in Burbank, approximately 10 miles (16 km) inland, the average high in August is 95 °F (35 °C): a temperature gain of about two degrees Fahrenheit per mile.

During the cooler winter months (October–March), 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 (72 °F, 22 °C) in all of the western U.S., and Santa Monica averages the warmest winter lows (52 °F, 11 °C) 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, (24 °C/10 °C/17 °C) 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 (Köppen "Csa"/"BSk") as winters are drier there.

The southeastern regions have a hot arid climate (Köppen BWh), 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 120 °F (49 °C). The highest reliably recorded temperature in the world, 134 °F (56.7 °C), was recorded in Death Valley on July 10, 1913. Temperatures of 130 °F (54.4 °C) or higher have been recorded as recently as 2005. The 24-hour average July temperature in Death Valley is 101.8 °F (38.8 °C).

In contrast, the high Sierra Nevada has a much cooler climate with extreme diurnal temperature variation. Bodie, at an altitude of 8,379 feet or 2,554 meters, 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 134 °F (56.7 °C) in Death Valley on July 10, 1913. This is also the highest temperature ever recorded on Earth. The lowest was −45 °F (−42.8 °C) in Boca on January 20, 1937.

Full statistics for selected cities

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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 15 inches (380 mm) to 50 inches (1,300 mm) per year. Some areas of Coast Redwood forest receive over 100 inches (2,500 mm) 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 10 feet (3.0 m) 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 (also called the "southwest 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 (SR 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 (Caltrans) developed a fog warning system along a 13 mi (21 km) section of SR 99.

Tornadoes

Across the state, tornadoes develop in three main regions – the Los Angeles area, the deserts of Southern California, the Central Valley. However, they are typically weaker and shorter-lived than tornadoes in the central United States. Because of their irregular and short-lived nature, the San Diego National Weather Service considers it "nearly impossible to forecast a Southern California tornado before it touches down." The agency utilizes a volunteer group of more than 1,300 volunteers to report local severe weather. The most California tornadoes on a single day in the state was seven, which occurred on November 9, 1982. In July 2004, a twister touched down in Sequoia National Park at an altitude of around 12,156 ft (3,705 m), making it the highest elevation for a confirmed tornado in the United States.

Tropical cyclones

Despite its long coastline, California is not vulnerable to tropical cyclones. Because of the cold California Current from the North Pacific Ocean and the fact that the storms tend to "steer" west, California has only been hit with three tropical storms in recorded history, a storm which came ashore in 1939 and dumped heavy rainfall on the Los Angeles area and interior deserts. The remnants of tropical systems will affect California more commonly, every several years.

Wildfires

Summers in inland California can see temperatures well over 100 °F (38 °C) during the day and less than 0.3 in (7.6 mm) of monthly rainfall, particularly in the southern areas. This makes them prone to wildfires. These can be life-threatening and cause evacuation. Wildfires are less common along the coast because of the cooler, more humid summers, but can occur in autumn when the marine layer is less common, making temperatures warmer and humidity drop significantly.

On occasion, fire whirls or fire tornadoes can develop within wildfires. One such event was rated an EF3, which developed in Redding in Shasta County within the Carr Fire. The fire whirl produced winds over 143 mph (230 km/h), killing four people and injuring six others.

Winds

In February 2017, a winter storm produced a wind gust of 199 mph (320 km/h) at Ward Peak in Alpine Meadows ski resort.

Glossary

The following are terms used to describe local or regional weather events.

"Earthquake weather": Any nonseasonal, uncomfortable weather, typically hot and more humid than usual, and often associated with high and mid-level clouds, is spoken of as "earthquake weather", although there is no scientific basis for that relationship.
Marine layer: The cool, moist layer of air coming in from the ocean which typically includes fog. A standard weather phenomenon along the northern and central California coast from late spring to early fall.
May Gray/June Gloom: A characteristic weather pattern of late spring (May and June) in which a combination of inland heat, off-shore cool water, and prevailing wind patterns bring foggy and overcast weather to coastal regions. From Point Conception northwards the gloom continues until early autumn. (See also: Catalina eddy)
Pineapple Express: A complex combination of events, bringing high levels of torrential precipitation to California. The Pineapple Express occurs when the jet stream dips across California, and warm, humid air from the mid-Pacific (from the vicinity of Hawaii–hence, the name) is drawn towards California. The resulting combination can cause extraordinarily heavy rains. A January 2005 Pineapple Express dropped nearly 25 inches (63.5 cm) of rain in Santa Barbara County.
Santa Ana winds: Santa Anas are hot, high winds that blow from the eastern mountains and deserts towards coastal southern California, usually in the spring and fall. They are the result of air pressure buildup between the Sierra Nevada and the Rocky Mountains. This air mass spills out, is pulled by gravity, and circulates clockwise around the high pressure area, bringing winds from the east and northeast; as the air descends in elevation, it heats up and the humidity plummets to less than 15 percent. It is often said that the air is heated and dried as it passes through the nearby deserts, but according to meteorologists this is a popular misconception (it is actually due to adiabatic compression as the air flows from the higher elevations down to sea level); during Santa Ana conditions, it is typically hotter along the coast than in the deserts. As the Santa Ana winds are channeled through the mountain passes they can approach hurricane force. The combination of speed, heat, and dryness turns the chaparral into explosive fuel for wildfires.
Tule fog: A thick ground fog that settles in the San Joaquin Valley and Sacramento Valley areas of the California Central Valley. Tule fog forms during the mid fall, winter to early spring after the first significant rainfall. This phenomenon is named after the tule grass wetlands of the Central Valley. Tule fog can extend from Bakersfield to Chico. Accidents caused by the tule fog are the leading cause of weather-related deaths in California; visibility is usually less than an eighth of a mile (about 600 feet or 183–200 m), but can be less than 10 feet (3 m).