- Climate of California
California's climate varies widely, from arid to subarctic, depending on latitude, elevation, and proximity to the coast. Coastal and Southern parts of the state have a Mediterranean climate, with somewhat rainy winters and dry summers. The influence of the ocean generally moderates temperature extremes, creating warmer winters and substantially cooler summers, especially along the coastal areas.
The cool California Current offshore, enhanced by upwelling of cold sub-surface waters, often creates summer fog near the coast, creating a moderate oceanic climate (Köppen climate classification Cfb) on the northern coast and a moderate Mediterranean climate (Köppen Csb) from about Cape Mendocino southward. Further inland, the climate becomes more continental, while remaining semi-arid (Köppen BSk), with colder winters and markedly hotter summers. Low-lying inland valleys, especially the Central Valley, have a hot Mediterranean climate (Köppen Csa), with subtropical temperatures but a well-defined summer dry season and cool, foggy, rainy season.
The temperature gradient between immediate coast and low-lying inland valleys in the south is about 7 °F (4 °C) in winter (the coast being warmer) and in summer roughly 25 °F (14 °C) (the interior being warmer). One exception is worth noting - during the cooler winter months (Oct-Mar) the Coachella Valley, in the far southeastern corner of the State, constantly has the warmest winter temperatures out of any place west of the Rocky Mountains. 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 form Nov. to Apr.  The temperature difference is most dramatic nearer the coast. For example, the average daily high in San Francisco in July and August is 72 °F (22 °C), and in Walnut Creek, some 20 miles (32 km) inland, the average daily high in July and August is 90 °F (32 °C); a temperature gain of approximately one degree (Fahrenheit) per mile. In the south, the figures are approximately 4 °F and 23 °F (2 °C and 13 °C), respectively. At the Santa Monica coast, 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 90 °F (32 °C); a temperature gain over one degree per mile. The temperature gradient is most extreme between Santa Barbara and Death Valley, with temperatures between the two differing by 4 °F and 35 °F (2 °C and 20 °C) in the winter and summer. The extreme southwest, around San Diego, has a semi-arid warm steppe climate (Koppen BSh) as winters are drier there. In San Bernardino the winters get a low of 30 °F (−1 °C) and in the summers a high of 109 °F (43 °C) since it is about 70 miles (110 km) inland of the Pacific Ocean in Southern California.
The southeastern regions have a hot arid climate (Koppen 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 is the hottest spot on the Western Hemisphere. It is common in the summer for temperatures in the valley to reach 120 °F (49 °C). The highest temperature in the Western Hemisphere, 134 °F (57 °C), was recorded in Death Valley on July 10, 1913. Temperatures of 130 °F (54 °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) (1981-2010 NCDC Normals).
City Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Bakersfield 57/39
Death Valley 67/39
Los Angeles 69/50
San Bernardino 67/44
San Francisco 58/46
San Jose 61/42
South Lake Tahoe 42/16
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. Parts of the Valley are occasionally filled with thick fog (known locally as "tule fog").
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 hot summers and nearly frostless mild winters; the higher elevation deserts of eastern California, including the Mojave Desert, Owens Valley, and the Modoc Plateau, are part of the Great Basin region, with hot summers and cold winters. 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.
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 two tropical storms in recorded history, a storm which came ashore in 1939 and dumped heavy rainfall on the Los Angeles Area and interior deserts and Tropical Storm Nora. The remnants of tropical systems will affect California more commonly, every several years.
California's summers are typically hot, and dry. This makes them prone to wildfires. These can be life threatening and cause evacuation. Wildfires are less common along the coast because of the cool humid summers, but can occur in autumn when the Marine layer is less common making it warm and dry.
The following are terms used to describe local or regional weather events.
- 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%. 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's 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 the infamous wildfires the region is known for.
- 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.
- 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).
- 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)
- 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.
- "Earthquake weather:" Any unseasonal, uncomfortable weather, typically hot and more humid than usual, and often associated with high and mid-level clouds, is spoken of (usually jokingly) as "earthquake weather."
Mediterranean climate at Santa Barbara.
Storms over California.
Climate of the United States States
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