California State Climate
Spanning nine degrees of latitude and almost 15,000 feet (4,500 meters) of elevation, California has five distinct climates. They include the Mediterranean, for which the state is famous, as well as desert, mountain, continental, and subarctic. The state is noted for its mild, rainy winters. Unlike almost all of the rest of the United States, summers are dry. They are characterized by mild temperatures in the 60s and 70s F (16°–26°C) along the coast, quickly climbing to three-digit numbers inland (100°+ F/38°+ C), especially in the deserts that dominate the state's southern third. A great influence on the state's climate is the cool California Current off the coast, which creates summer fog.
Mediterranean: This climate is distinguished by cool, wet winters and dry, warm-to-hot summers. It extends along the coast from Cape Mendocino to San Diego, and inland through most parts of the Central Valley. The coastal variation features frequent fog or overcast skies in summer, which keeps daily temperatures mild. Freezing temperatures are rare. The inland variation usually has very hot summers, and winters that can bring intense, ground-hugging "tule fog," as well as rain.
Continental: Although it follows the Mediterranean norm of rainless summers, the southern part of the state's vast Central Valley has a semi-arid steppe climate like that found on the Great Plains. Rainfall is lower than with the Mediterranean climate, summers can be decidedly warmer, and winter freezes occur more frequently.
Desert: California's two major deserts, the high-altitude Mojave and the lower-altitude Sonoran, are both very dry and subject to summer temperatures above 100°F (38°C). In winter, the Mojave is cool, often reaching freezing temperatures, while the Sonoran is warmer, with daytime temperatures in the 70s F (21°–26° C). Winter brings snow to the Mojave. Occasional summer thunderstorms occur in the Sonoran. Desert conditions prevail east of the Sierra Nevada because of the rain shadow formed by that range. The northeastern part of the state is part of the Great Basin, an extensive high desert.
Mountain: The state's major mountain ranges undergo significant snowfall in winter and rain in spring. Like the rest of the state, the mountains experience summer drought, although occasional thunderstorms occur.
Subarctic: California has extensive areas in the Sierra Nevada above 10,000 feet (3,000 meters) where climatic conditions resemble those just south of the Arctic Circle: very cold winters and warm, brief summers. The same conditions prevail on 14,179-foot (4,322-meter) Mt. Shasta in the state's far north.
Temperatures rise according to elevation and distance inland. Average January temperatures range from the mid-60s F (17°C to 19°C) along the central and southern coasts to 10-20°F (-12° to -7°C) in the High Sierra. Average July temperatures range from the low 60s to 70s F (17°–23°C) along the north coast to Central Valley temperatures in the 90s F (32°–37°C) to desert temperatures in the 100s F (38°+ C).
California's all-time extreme temperatures span 179°F (82°C). Its all-time highest temperature of 134°F (57°C) was recorded at Death Valley on July 10, 1913, and its all-time lowest temperature of -45°F (-43°C) was recorded at Boca on January 20, 1937.
When floods occur, it is in winter or early spring, the seasons of heaviest rainfall. Areas most vulnerable to major flooding are in the Central Valley, especially near the Sacramento and American river flood plains. The more common forms of flooding are flash floods created by heavy rainfall in coastal areas and near the Transverse Ranges in Southern California. In the Bay Area, high tides can combine with "stalled" rainstorms to create local flood conditions. Dams and diversion pumps have curbed most major flooding, but flash flooding of minor creeks and streams still poses a threat in urban and suburban areas where plant cover has been removed.
Drought is a recurring part of the state's climatic cycle, often lasting for years at a time. The cause is thought to be high-pressure areas that "park" over the state, sometimes for years, forming a barrier that prevents rainstorms from entering the state. The state's last major drought was from 1987 to 1992. Beginning in 2007, the state has experienced successive years of drought-like rain and snowfall levels.
Winter varies from extremely mild on the central and southern coasts to bitterly cold in the Sierra Nevada. Snow is rare anywhere along the coast and in the Central Valley. The heaviest snowfalls are in the Sierra Nevada, followed by the northern and southern mountains. Snowfalls in the high (Mojave) desert are comparatively light. Rain is heaviest in winter and California's annual grasses respond by carpeting the state in green.
Spring is marked by the end of the rainy season, although wet weather can occur into May. The state is lushly green and rivers and streams run high. Temperatures are cool in early spring, then warm almost to summer highs in June.
Summer is the hottest season, although in June, cool air along the Pacific, called the Marine Layer, keeps coastal cities 10°–20°F (up to 5°C) cooler than inland cities. The state's annual drought, a feature of its Mediterranean climate, begins in summer. Seasonal grasses die, turning a gold color on the hillsides.
Statewide, autumn is characterized by warm days and cool nights. The rainy season usually begins in late October. Snow is unusual in early autumn, but begins by mid-November in most years.
Temperatures rise according to elevation and distance inland. Average January temperatures range from the mid-60s F (17°–19°C) along the central and southern coasts, and the southeastern low desert to 10-20 degrees F (-12° to -7°C) in the High Sierra. About half of the state, including the north coast, Central Valley and high desert, average daytime temperatures in the mid-50s F (12° –14°C). Foothills and northern mountains average in the 30s F (0°–3°C).
Average July temperatures range from the mid-70s to mid 80s F (24°–29°C) along the central and southern coasts, and the low 60s to 70s F (17°–23°C) along the north coast. Central Valley temperatures are in the 90s F (32°–37°C), while desert temperatures are in the 100s F (38°+ C). The Sierra, Cascade, and Klamath Mountains are generally cooler, in the high 70s F (25°C).
The state's average rainfall, regardless of region or climate, is 17.28 inches. Rain falls most heavily in the state's northwest and decreases as one moves south and east. Northwestern California receives up to 50 inches (1,300 mm) of rain per year, with some redwood forests in the area receiving more than 100 inches (2,500 mm). Moving south, coastal areas receive decreasing rainfall: San Francisco, 20.4 inches (518 mm); Los Angeles, 14.77 inches (375 mm); and San Diego, 9.9 inches (251 mm). In the northern Central Valley, Redding receives 33.52 inches (851 mm) of rain while Bakersfield in the south receives 6.49 inches (165 mm). Barstow in the desert and Bishop in the Sierra rain shadow receive, respectively, 4.64 inches (118 mm) and 5.02 inches (128 mm).
Snow is rare on the coast and in the Central Valley. The Sierra routinely receives up to 350 inches (8,890 mm) of snow at some of its higher elevations. The snowpack later becomes a source of drinking water for much of the state. Snow in the Klamath and Cascade Mountains can also be heavy. Snow in the southern mountains is less heavy due to the "wringing out" of moisture that occurs as storms move over the state.
Humidity is highest along the coast and decreases inland. Although Los Angeles (79 percent morning/65 percent afternoon) and San Francisco (84 percent morning/62 percent afternoon) have high relative humidities, their coastal location controls temperatures and keeps humidity from becoming uncomfortable. Humidity is low inland (66 percent morning/39 percent afternoon at Bakersfield; 54 percent morning/20 percent afternoon at Bishop), giving the state a reputation for "dry heat" in summer versus the hot, clammy heat of more humid U.S. climates.
The state is one of the sunniest in the United States, averaging seven to eight hours of sunshine daily in winter, and up to 14 hours daily in dry inland regions. Averages on the coast are somewhat less because of fog and rain: six to seven hours in winter; nine to 10 hours in summer.
California divides into five regions:
Micro-climates: Unlike continental climates, where temperature and humidity can be remarkably consistent over a huge area, California's complex geography and proximity to cold ocean water creates "micro-climates." Micro-climates are variations in temperature that occur as one moves inland. For example, coastal San Francisco has an average summer temperature of 68°F (20°C) while suburban Walnut Creek, 20 miles (32 km) east, has a summer average of 87°F (31°C), a temperature gain of approximately one degree F (0.56°C) per mile (1.6 km). Micro-climates become less pronounced further inland, where temperatures remain more uniform over a greater area.
Tornadoes: Although California is 1,400 miles from "Tornado Alley," it experienced 303 tornadoes from 1950 through 2004. Only 21 of the storms (7 percent) reached F2 force, which is defined on the Fujita Scale (F0–F6, with F6 being an "inconceivable tornado") as a "significant tornado" with winds up to 157 miles per hour, and the ability to tear roofs off frame houses, demolish mobile homes, and uproot large trees.
Tropical cyclones: The state has been hit by the remnants of tropical cyclones (called hurricanes on the East Coast) several times in its history, but only two delivered memorable amounts of warm, wet, intense rainfall by the time they reached California: 14.76 inches (374.9 mm) in 1976 at San Gorgonio from Hurricane Kathleen, and 11.6 inches (294.6 mm) in 1939 at Mt. Wilson from an unnamed storm.
Hail occurs in winter in the state's more northerly areas. It is rare in Southern California. Hail pellets are small compared to those in other areas where hail occurs, and damage from them is usually negligible.
Blizzards are unknown in coastal California. However, in the high mountains, such as the Sierra Nevada, Klamath and Transverse Coastal ranges, heavy snowfall and wind can create blizzard conditions.
Thunderstorms are unusual but not rare in California. The aftermath of tropical Mexican storms can bring them on, as well as the onset of the monsoon season in summer in the Sonoran Desert.
Wildfires can be extreme in California, consuming tens of thousands of acres at a time. The state's summer drought conditions combine with high temperatures and low humidity to create ideal conditions for large fires.
Mudslides: Heavy winter rains on slopes that have been denuded by summer and autumn wildfires often create large mudslides that can block highways or destroy hillside homes.
High Winds: The state's most famous wind is the Santa Ana, a hot, almost gale-force wind that comes in autumn when low pressure over the coast pulls warm inland air over and down the mountains north of Los Angeles. As the air descends from the mountains, compression heats it. Other parts of the state experience similar winds under similar conditions.
Smog, trapped particles from car exhaust, atmospheric dust, and industrial emissions, is created when an inversion layer, an overlap of cool air, traps warmer air below it and prevents it from rising and dispersing. The Los Angeles Basin has the state's severest smog because of its inversion layer of cool ocean air hemmed in by mountains.
-World Trade Press