Field Observations of the Persistence of AgI-NH4I-Acetone Ice Nuclei in Daylight

Abstract

A field method of estimating the persistence of a commonly used silver iodide seeding agent is described. The method involved measurement of the AgI plume structure at two downwind distances from the ground generator(s). Distances between the nine available pairs of downwind measurement planes ranged from approximately 10 to 100 km. An NCAR acoustical ice nucleus counter in a light twin aircraft was used to sample the AgI plumes. A series of passes was made through the entire vertical and horizontal extent of the plume at each downwind distance. These measurements, together with pilot balloon observations, permitted calculation of the flux of AgI through each vertical cross-sectional plane. The difference in flux measurements yielded an estimate of the persistence of the seeding agent over the period of transport between the two vertical planes. This method was applied at three separate locations, during different seasons, and with various degrees of cloudiness. Resulting estimates of deactivation rates of the ice nucleating ability of AgI ranged from no loss to 70% loss per hour. The implications for possible cloud seeding effects beyond the intended target area are discussed.