The climate of California is characterized by cool to mild winters and, except in the high mountains, warm to hot summers. The year is divided into a wet season and a dry season. Precipitation falls mainly during the period from October to April. The mountain slopes facing westward are usually wetter than the slopes facing eastward because the moisture-bearing winds from the Pacific are forced to condense and precipitate their moisture as they rise over the mountains. In general, northern California has lower temperatures and greater precipitation than southern California. However, climatic and weather conditions in the state vary greatly from place to place and from year to year.
The prevailing winds of all of California are the westerlies, so-named because they blow from the west toward the east. The westerlies not only bring winter storms and eagerly awaited precipitation to the state, but throughout the year they drive the nation’s largest wind-power facilities.
Located at Altamont, east of San Francisco Bay, and in Tehachapi and San Gorgonio passes, in southern California, the largest windfarms supply several hundred thousand residents with electricity when the winds are greater than 23 km/h (14 mph). The dry Santa Ana wind, a reversal of the prevailing westerly pattern to an easterly or northeasterly wind, occurs predominantly in southern California and in the fall of the year when high pressure builds over the interior deserts and flows offshore to cells of low pressure. In the coastal areas north of Point Conception, July temperatures average 16°C (60°F). January temperatures are between 4° and 10°C (40° and 50°F).
Precipitation increases from 380 mm (15 in) near Point Conception to more than 1,800 mm (70 in) at Crescent City, near the Oregon border. Fogs are frequent along the coast, especially in summer. South of Point Conception the coastal areas are drier and have a greater range of average temperatures. Rainfall averages only 310 mm (12 in) at Los Angeles and 250 mm (10 in) at San Diego. Average January temperatures are between 10° and 16°C (50° and 60°F). July averages are generally between 21° and 27°C (70° and 80°F), but much higher temperatures, even in the upper 30°s C (lower 100°s F) occur during summer.
In the Central Valley, average temperatures are 27°C (80°F) in July and 7°C (45°F) in January. Precipitation varies from more than 760 mm (30 in) a year in the valley’s northern part to less than 150 mm (6 in) at its southern end. In the extensive mountainous areas of California, winters are severe. The western slopes of the Klamath Mountains, the wettest part of the state, receive more than 2,500 mm (100 in) of precipitation yearly. Many peaks in the Sierra Nevada support small glaciers and thus appear snowcapped throughout the year, and in some locations the snowfall exceeds 13,000 mm (500 in), the equivalent of 1,300 mm (50 in) of rain.
The Great Basin and Mojave Desert sections of California are extremely arid. In Death Valley, precipitation averages less than 50 mm (2 in) a year, and in some years it never rains. These desert areas are the hottest parts of the state and of the nation. July temperatures in Death Valley average in the upper 30°s C (lower 100°s F), and the highest temperature (57°C/134°F) ever recorded in the United States was taken there.
In the Central Valley the frost-free season averages between 240 and 280 days. This long period permits the cultivation of many crops that are sensitive to frost damage. Elsewhere in California the growing season ranges from more than 320 days along the southern coast to less than 120 days in the northern valleys. © Emmanuel Buchot and Encarta
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