Thunderstorms occur 2 to 5 times a year, chiefly in winter, on the coast of northern California, and 10 to 15 times a year, chiefly in summer, in the Great Basin. In the intervening Central Valley, 5 to 10 thunderstorms occur per year, mostly in late spring; a secondary maximum in early autumn is particularly important in starting forest fires. Correlations between numbers of days with thunderstorms at adjacent Weather Bureau stations are quite low; the highest found is 0.70 for summer thunderstorm days at Sacramento and Blue Canyon, 60 mi apart.

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1 This study is part of the research performed at the Pacific Southwest Forest and Range Experiment Station, Forest Service, U. S. Department of Agriculture, under a cooperative project contract with the California Department of Natural Resources, Division of Forestry, on methods of reducing the incidence of lightning-caused forest fires.