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. 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.
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. In the extensive mountainous areas of California, winters are severe. The Great Basin and Mojave Desert sections of California are extremely arid. These desert areas are the hottest parts of the state and of the nation. In the Central Valley the frost-free season averages between 240 and 280 days.