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24 WATER RESOURCES OP CALIFORNIA. CHAPTER III. CLIMATE. The California year is distinctive from that in most other states of the Union, in having hut two well-defined seasons, summer and winter. This occurs because the transitional periods, spring and autumn, are brief and devoid of special features other than that they are inter­mediate between the more clearly defined, seasons of summer and win­ter. The summer, or growing period, is long, warm and without heavy rains; the winter is the dormant period, or interval of retarded growth in the annual cycle of plant life, and normally is short, cool, and at times stormy. The greater portion of the waters precipitated upon the lands of California fall during this season of winter. The covert of encircling mountains and the proximity of an ocean that borders the state with nine hundred miles of coast line, so modifies California’s climate that only moderate seasonal fluctuations of tem­perature occur over most of its area. Any great extremes of heat and cold that do transpire are confined principally to the high mountain , or arid areas. On the low lands generally, the mean monthly temper­atures show departures from the average for the entire year, markedly less than similarly compared heat measurements for the adjoining states or those located eastward and included in the same latitude. California is exempt from hurricanes and tornadoes, and though the mountainous regions experience days of intense cold, blizzards are unknown over the valley areas. Favored of nature through immunity from devastating tempest, rigorous cold, and enervating heat, California’s climate is heralded the world over. The outstanding features of the state’s climatic regime are the rains of winter and the sunshine of summer. During the winter months, the state is swept by moisture-laden winds that traverse large areas in their journey from one locality to another; while in the summer or dry season, similar winds may blow, but they are rainless and serve only to modify the mounting temperatures that ensue from continuous sun­shine. This distinct division of the year into a short season of inter­mittent drenching rains and a longer season of warmth and sunshine, determines that, more and more, in the years to come, this peculiarity of climate will influence the activities of man in this state. As greater numbers of people elect to live within its borders, water will be needed in increasing amounts for every activity, and all of California’s waters originate in the precipitation concentrated in a few months of the year. O f greatest economic importance therefore, among the climatic phe­nomena, are the moisture-carrying winds that visit the state at annually recurrent intervals. In blowing over the land areas, these winds pre­cipitate varying amounts of water along the way as they are cooled, and deflected or diverted by local topography. The shelter of knolls, of hills or mountains, or of ridges or spurs, may lessen the amounts reaching leeward areas, while increased quantities may fall on more Plate I IL L U S T R A T IV E C L IM A T O L O G Y O F A G R IC U L T U R A L L A N D S