Abstract

ABSTRACT The apparent effect of cloud seeding on the average 24-hr precipitation in the Santa Catalina Moun tains during the two programs of the 7-year-long Arizona experiment was found to be a 30% loss of rain (P = 0.06). Considering rainy days only, the apparent effect is a 34% loss of rain (P = 0.03). On South-East days the apparent loss was 40% (P = 0.03). The analysis of the diurnal varia tion in the amountsof hourly precipitation brought out two suggestions: («) more active silver iodide enters the clouds through seeding at their bases than at the -6°C level; («i) the population of experimental days in cludes two categories with opposite responses to seeding: augmentations of rain in one case and losses in the other. These suggestions require independent confirmation. This is a sequel to the earlier paper (1) primarily concerned with the broad question whether "local" cloud seeding with silver iodide, intended to augment the rainfall over a limited area, can in fact affect the precipitation at relatively large distances. The specific object of study was the possible effect of cloud seeding over the Santa Catalina Mountains in Arizona (2-4) on the 24-hr precipitation at Walnut Gulch, some 65 miles to the SE. It was found that the effect averaged over all the 212 experimental days of two experimental pro grams, 1957-60 and 1961, 1962, and 1964, is represented by the apparent 40% loss of rain at Walnut Gulch (P = 0.025). Larger apparent losses of rain, some highly significant, were found for experimental days on which Walnut Gulch was downwind from the seeding site (but not on upwind days), and also on "second days" of the randomized pairs (but not on "first days"). The study of the diurnal variation in the hourly precipitation amounts on several categories of days showed two pronounced maxima of rain on not^seeded days, one at about 1800 Mountain Standard Time (MST) and the other, more pronounced, at about midnight. On seeded days, these maxima were hardly detectable. While confirming the possibility of widespread effects of local cloud seeding, the above results stimulated our interest in the apparent effects of the same seeding of the 24-hr pre cipitation that fell in the Santa Catalina Mountains, the in tended target of the seeding operations.