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Placement of Seeding Material with Respect to the Cloud. - In ARIDROP, the seeding material was placed directly into the buoyant updraft in the core of the clouds. In the Catalina experiment, "the airplane was flown back and forth along a line 15-20 mi long per- pendicular to the wind at flight level and about 30 min upwind from the foothills of the mountains." (Battan, 1966). In Experiment I (1957-1960) the seeding was carried out at the _60 C level (18,000 to 20,000 feet), well above the level at which most convective cloudbases subsequently formed; in Experiment II (1961. 1962, 1964) the seeding was done at altitudes 1.000 to 2.000 feet below cloud- base level but still at the same upwind distance and not. as Neyman, Osborn, et al. (1972) say, "at" or "from the cloud base." The exper- imental design postulated that the seeding material became evenly dispersed through a layer of air 1 kilometer deep centered at the seeding level; how well this postulate was fulfilled is very ques- tionable. It is very likely that many if not most of the clouds or cloud-building updrafts penetrated portions of the layer apart from the smoke plume laid down by the seeding aircraft. while the remainder got much heavier doses than the design called for. The difference in procedures is crucial, both for the dynaaic and physical processes involved and for the effective persistence of the silver iodide itself. Since seeding into different parts of the cloud is involved, the two-dimensional model of Murray and Anderson (1965), Murray and Koening (1971), furnishes the most appropriate framework. In this model, the dynamic activity is measured by the 5