Silver-Iodide Seeding and Precipitation Initiation in Convective Clouds

Abstract

Convective clouds on seeded and not-seeded days were observed by means of radar and a pair of high quality, ground-based cameras. Stereographic analysis allowed calculation of cloud-top heights, bearing, and distance from the radar set. Radiosonde data were used to convert cloud-top altitudes to summit temperatures. During the periods 1957 to 1960 (Program I) and 1961 to 1964 (Program II) the total number of clouds observed were 1249 and 522, respectively. The analyses indicate that on days when silver-iodide seeding was carried out from an airplane, a higher fraction of clouds developed precipitation echoes. When all the clouds in Program I with temperatures between ?18 and ?42C are considered, the effects of seeding were found to be significant at the 0.07 level according to the Mann-Whitney U test. When the data for both programs are combined, the same test yields a significance level of about 0.03 for clouds with summit temperatures between ?18 and ?42C. The results lead to the conclusion that airborne silver-iodide seeding may influence the precipitation-initiation process in convective clouds.