In association with Grossversuch IV, a program designed to test the Soviet hail suppression method by seeding clouds with AgI from Oblako rockets, a complementary program was conducted by l'Observatoire du Puy-de-Dôme and the Desert Research Institute to study the diffusion of the seeding material (AgI) in the clouds, based on the analysis of silver in precipitation. This program covered the summers of 1977 and 1978, and this paper describes the results of measurements of natural background silver concentrations in unseeded precipitation. It also describes a new automatic precipitation collector, five of which were first tested in the field in 1977. A more extensive network of 15 collectors was deployed during two months of the 1978 summer.
Based on the analysis of 118 unseeded precipitation samples collected in 1977, the natural background concentration of silver was estimated as 0.9 × 10−11 g mL−1(σ = 0.6 × 10−11 g mL−1). Although the standard deviations overlap, the 1978 season results appear to indicate a lower background of 0.5 × 10−11 g mL−1 (σ = 0.3 × 10−11 g mL−1), based on the analysis of 414 rain samples. The average value for the two seasons was 0.6 × 10−11 g mL−1 with a standard deviation of 0.5 × 10−11 g mL−1. These background concentrations were found to be independent of both the length of sampling period and the precipitation intensity, averaged over the sampling periods of the collectors.
The background is sufficiently low to permit the detection of the presence of silver iodide emitted from the Soviet rockets in the precipitation. The preliminary results from one case study are presented to support this conclusion.