Responses of warm-based, midwestern cumulus congestus to dynamic seeding trials

Findings are reported from an analysis of AgI seeding effects on individual cumulus congestus clouds in the 1989 Illinois Exploratory Cloud Seeding Experiment. The experiment was designed around a dynamic seeding hypothesis. Randomized treatments of individual clouds were based on "floating' experimental units, initially centered on the first treated cloud. The analysis was based on 12 experimental units having a total of 67 treated echo cores - 32 treated with sand and 35 with AgI. Prior to any analysis for seeding effects, a check of many of the physical conditions at the time of treatment that would govern future cloud growth showed a bias for the sand-treated clouds to be expected to ultimately grow larger than the AgI-treated clouds. Thus, even though randomization produced numerical balance, direct comparison between the posttreatment behavior of the entire sample of sand- and AgI-treated echoes could not be expected to provide a true impression of possible seeding effects. -from Authors