ANALYSIS OF A POSSIBLE CROP RESPONsE TO HAIL SUPPRESSION SEEDING: THE NELSPRUIT HAIL SUPPRESSION pROJECT.

Abstract

A commercial hail suppression program has been in operation at Nelspruit, South Africa (25 degree S, 31 degree E) since December 1971. The seeding technique employed has been the ″on-top″ method, which consists of dropping silver iodide pyrotechnics into the tops of cloud turrets growing on the flanks of mature storms. Daily crop insurance records of hail damage to tobacco are compared for seeded and nonseeded days. A ″hail severity ratio″ , defined as the ratio of the crop lost to the total crop planted in the areas hit by hail, is derived from the hail-damage assessments and appears to be the parameter most sensitive to the claimed treatment effect. A comparison of seeded days during the initial phases of the program, when propellor aircraft were used, with more recent days when one or two jets were used for the seeding, indicates significantly lower severity ratios for jet-seed days.