In California, much of the climatic variation results from the patterns of global weather systems. Precipitation is greater in the northern part of the State than elsewhere (fig. 3). However, prominent mountain ranges in California and western Nevada also have an important influence on moisture distribution in the region. Temperatures are cooler in the higher altitudes of the mountains. As eastward-moving, moist, unstable air masses rise up the western slopes of the mountains, the air is cooled and water vapor condenses and falls as rain, snow, or ice. When these air masses descend the eastern slopes, they become warmer and more stable and thus retain most of the remaining moisture. Consequently, precipitation amounts are much greater on the western slopes of the north-south-trending mountain ranges of western and eastern California, whereas semiarid to arid conditions prevail east of the mountains, as in much of Nevada and in central and southern California.
Storms that bring moisture to the region are most frequent in winter; about 80 percent of the annual precipitation falls between October and April. The extreme northern part of California has slightly wetter summers than the rest of the segment. However, amounts of precipitation vary greatly from year to year; for example, from 1860 to 1980 the average annual precipitation of Sacramento, Calif., was 18 inches, but precipitation ranged from 35 to 195 percent of the average annual precipitation (fig. 4), or from about 6 to 35 inches per year. Fog occurs frequently on the coast and provides some additional moisture that is used primarily by vegetation.
Mountain ranges that parallel the coast also affect temperature distribution. Seaward of the mountains, temperature is moderated by the ocean, and the range between daily high and low temperatures is usually less than 20 degrees Fahrenheit. Winters are cool, but they are not generally cold in coastal areas, although temperatures drop sufficiently in the coastal part of northern California to cause some frost and a dormant season for plants. Summers in coastal areas are mild, but temperatures occasionally become hot in southern California. In contrast, the valleys east of the coastal mountains experience much greater temperature extremes. In these valleys, summer daytime temperatures can be greater than 90 degrees but fall to 55 degrees or less at night. Winters in the interior valleys are relatively mild, and freezes are uncommon.
Temperature ranges in the mountains of western California and eastern Nevada, as well as in the desert parts of Nevada and southern California, are much greater than in other parts of Segment 1. In the mountains and deserts very little moisture is in the air to absorb the rays of the sun, or to retain heat at night. Consequently, solar radiation is intense during the day, but the heat stored in the ground is released rapidly after sunset. Temperature extremes are hotter and colder in the desert than in other lowland areas elsewhere in the segment, but mountainous areas are warm in summer and extremely cold in winter.