An Assessment of NHRE Hail Suppression Seeding Technology Based on Silver Analysis

Abstract

Detailed analyses of three storms seeded in the National Hail Research Experiment (NHRE) during 1974 investigated the relationships between the occurrence of silver (from AgI) in precipitation and both the seeding activity and the storm histories as documented by radar, aircraft, the precipitation network and radiosondes. Although the results show that the AgI was broadly dispersed when weak, poorly organized storms were seeded, it appears that the seeding agent was confined to only limited regions of the more vigorous storms which had well-defined internal circulation patterns. In several cases, it has been possible to use radar data to identify a specific cell within a storm which produced ?seeded precipitation,? and thereby roughly define the pathways taken by the AgI aerosol after release from the aircraft. Analyses have been performed on precipitation from 18 storms seeded by NHRE in the 1973 and 1974 seasons in order to determine the proportion of samples which contained ?seeding? silver. In 1973, 50% of the samples contained silver significantly above the background concentration of 10?11 g ml?1. In 1974, the figure was 70%. However, by comparing the observed concentrations with those expected from theoretical predictions, the proportions of samples containing enough silver to represent a significant seeding effect became very low (probably <10%). If the silver content of the precipitation is taken as an index of targetting effectiveness for modification of hail, then its ?spotty? distributions, both temporally and spatially, suggest that there may be serious difficulties in assessing hail mass modification based on a statistical comparison of seed versus no-seed days.