Grossversuch IV: “Extended Area” Effects on Rainfall

Abstract

During the years 1977?82 a randomized single area hail suppression experiment was conducted in Central Switzerland, testing the Soviet concept of beneficial competition. For 37 seed and 50 no-seed days, rainfall data, both in and outside of the target area, have been analyzed for a possible seeding effect. ?Moving grid? analyses within a range of 100 km of the seeding center showed no statistical evidence of a seeding effect for 24 h-values. For the experimental unit (9 h), an area ?upwind? (with respect to the mean wind direction) of the target showed a statistically significant increase in rainfall of 118% on seed days, although no ?upwind? effect should be expected. The seed ensemble in that area is dominated by four days with large rainfall amounts. Hence it must be concluded that, despite randomization, the effect is due to natural variability. Other effects in the target or ?downwind? must likewise be regarded as random. Smaller time intervals have also been tested, but the results are inconclusive.