An Interpretation of the Mechanisms of Ice-Crystal Formation Operative in the Lake Almanor Cloud-Seeding Program

Abstract

In a 1984?85 winter cloud-seeding program at Lake Almanor, California, indium sesquioxide (In2O3) aerosol particle generators were collocated with silver iodide (AgI) aerosol particle generators as a source of inert tracer aerosol. The In2O3 aerosol served as an indicator of the amount of AgI aerosol scavenged. Based on the aerosol emission rates, if AgI aerosol was only captured by scavenging processes, and played no part in forming ice crystals and snowfall, the silver to indium ratio (Ag:In) in the analyzed snow would be 0.8. Analysis of snow samples from the target area produced frequent Ag:In ratio values in excess of 1.1. In the snowfall at the closest sampling sites to the aerosol generator the high ratios of Ag:In cannot be explained by the contact-freezing ice formation mechanism. A mechanism with a much faster rate than possible by contact freezing is necessary to produce the high Ag:In ratios that were observed. Part of the AgI seeding aerosol functioned rapidly to produce ice crystals by a forced condensation-freezing mechanism immediately after generation, and those ice crystals contributed to the snowfall at those sites closest to the generator.