Silver Content of Precipitation from Seeded and Nonseeded Florida Cumuli

Abstract

A subprogram of NOAA's 1973 Florida Area Cumulus Experiment (FACE) was undertaken to determine the silver content of precipitation associated with convective clouds massively seeded with silver iodide nucleant over southern Florida. An atomic absorption analysis of 127 rainwater samples collected just below cloud base by a polypropylene-lined scoop mounted on the fuselage of the NOAA DC-6 aircraft indicated that the mean concentration of silver obtained on seed days (69 samples) was no greater (and, in fact, appreciably less) than that obtained on no-seed days (58 samples). In both sets of samples, the median concentration of silver was more than two orders of magnitude lower than the U. S. Public Health safety limit of 5 ? 10?8 g ml?1. Of the 69 aircraft samples collected on seed days, only two contained a concentration of silver in excess of 1 ? 10?9 g ml?1. Of the 58 aircraft samples collected on no-seed days, eight contained silver in concentrations exceeding 1 ? 10?9 g ml?1. The samples collected from the aircraft showed higher mean concentrations of silver than did those collected on the ground. An atomic absorption analysis of 79 rainwater samples collected at both fixed and mobile sites on the surface showed that the mean concentration of silver on seed days was three orders of magnitude less than 5 ? 10?8 g ml?1; the maximum concentration of silver found in any sample did not exceed 1 ? 10?9 g ml?1. Statistical results from the nonparametric Mann-Whitney Wilcoxon test suggest that the surface seeded (34 samples) and surface nonseeded (45 samples) data come from the same population (i.e., no significant differences between the two data sets). There is some evidence (from a separate set of surface rainwater samples collected upwind of the target area) to suggest a persistently higher (by about a factor of 2 or 3) mean concentration of silver during the course of the experiment than either before or after the experiment.