Climatic Variations and the California Raisin Industry

Abstract

IN the central San Joaquin Valley of California, raisins are made from grapes by processing them on paper trays under the open sky in the vineyards. Drying of the raisin grapes begins after harvest in late August and lasts until approximately the second week in October. The critical drying period is the month of September, during which the grapes may be damaged if they are moistened by rains of as little as 0.25 millimeters. The whole crop can be destroyed by amounts of rain in excess of 6.4 millimeters. Although the amount of rain received on any one day is critical, temperature, humidity, and solar radiation conditions in the period immediately after the rain are decisive in mitigating damage. When rain threatens, the trays are rolled and stacked in the vineyards as a protective measure, but the large number of trays and the wide area over which the trays are spread make such protection a costly enterprise. This unique approach to the processing of raisin grapes is apparently predicated on the assumption that the probability of a wet day during the critical drying period is low. This expectation is probably justified because the region is ordinarily dry until mid-September after which the probability of adverse raisin-drying weather has been shown to increase.1 However, during the period 1920 to 1950, Septembers were very dry and hence less hazardous for the open-air drying of grapes in comparison with Septembers from 1890 to 1920.2 It is immediately apparent that temporal variations in the probability of occurrence of adverse weather that run counter to the expectations, fostered by the anomalously good raisin-drying weather in the period of 1920 to 1950, could result in substantial monetary losses, because the decision-making process will incorporate climatic expectations that may no longer be valid. In this article I propose, first, to ascertain whether the ideal raisin-drying weather between 1920 and 1950 has continued and, if not, what variations have occurred. Second, I will evaluate the impact of the variations on the raisin industry.