Cadillac Desert: The American West and Its Disappearing Water

by Marc Reisner

California is the only state in America with a truly seasonal rainfall pattern—stone-dry for a good part of the year, wet during the rest. Arizona is much drier overall, but has two distinct rainy seasons. Nevada is the driest state, but rain may come at any time of year. If you had to choose among three places to try to grow a tomato relying on rainfall alone, South Dakota, West Texas, or California, you would be wise to choose South Dakota or West Texas, because it rains in the summer there. California summers are mercilessly dry. In San Francisco, average rainfall in May is four-tenths of an inch. In June, a tenth of an inch. In July, none. In August, none. In September, a fifth of an inch. In October, an inch. Then it receives eighteen inches between November and March, and for half the year looks splendidly green. The reason for all this is the Pacific high, one of the most bewildering and yet persistent meteorological phenomena on earth—a huge immobile zone of high pressure that shoves virtually all precipitation toward the north, until it begins slipping southward to Mexico in October, only to move back up the coast in late March. More than any other thing, the Pacific high has written the social and economic history of California.