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I I I I I I I I I I I I I I I I I I I large degree control the seedability. If the stable region i.s too intense and occurs at a relatively warm temperature (warmer than about _60C) the AgI pyrotechnics do not produce suffiCient glaciation to generate- buoyancy and updrafts beyond the stable layer (similar effect occurs if the liquid water content is low). In summary, the physical properties of the clouds studied fit into the pattern of those measured elsewhere under similar meteorological conditions. Dynamic seedability appears to be pri- marily controlled through the existence and intensity of mid-level stable layers. 6.4 Relation to Previous Studies Simpson, et al (1978) have conducted seedability studies of the region using the Dulles soundings and a one dimension steady-state cloud model. The cloud model is utilized to estimate dynamic seedability in terms of cloud top increases resulting from simulated seeding calculations (model calculated height changes have been correlated to rainfall increases for clouds in Florida). Five years of sounding data from Dulles have been processed and categorized. Simpson's studies suggest a relatively frequent occurrence of dynamic seedability (about 50 days per season or 43 per cent of the soundings analyzed). The analysis for the 1978 season suggest potential dynamic seedability on a total of 38 days. Thus considering year to year variations, one can estimate 30-50 days should be suitable for seeding. Simpson suggests that the sea breeze might be effective in the generation of convective cells. Our studies show no evidence of this occurring in 1978. Simpson also mentions the divergence pattern associated with the bay region. Our obsE~rva- tions (which correlate with many farmer's impressions) support 118 ",