The Meteorological Factors Associated with the Precipitation Effects of the Swiss Hail Suppression Project

Abstract

During 1957-63 a randomized experiment to test the efficacy of cloud seeding to suppress hail in the southern Alps was conducted using ground-based silver iodide generators. While the results from the standpoint of hail suppression were negative, there appears to have been a definite overall effect in increased precipitation on days with seeding. Further, Neyman and Scott discovered that the precipitation effect was strongly positive on days selected for the experiment by one forecaster, and somewhat negative on those selected by another. This paper is an attempt to identify the factors responsible for this difference. An analysis of the meteorological conditions assoiated with the experhnent showed that, in general, the conditions favoring convective predpitafion, i.e., southerly flow and upper-level instability, were also associated with positive seeding effects. To some extent the difference between forecasters referred to above was due to accidental differences in the random selection of seeding opportunities among cases of southerly and northerly flow. Not all the difference was explained in this way. The conditions favoring large amounts of precipitation and large seeding effects were studied. For instance, the experimental days were typed according to surface pressure pattern, and in two of the weather types strong positive seeding effects were present. The necessity of caution in attributing to seeding differences which might be due to other factors is discussed in the light of the possibility of the process of random selection producing unequal samples from meteorological types having widely different physical characterhfics.