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1.3 Summertime convective cloud seeding Seeding of convective clouds to enhance rainfall or suppress hail have been of great interest in the past 40 years. Numerous operational, some with a research component, and research projects have been conducted worldwide. The results of certain projects indicated positive increases of rainfall due to cloud seeding, others showed a decrease in rainfall w~le the majority indicated inconclusive results. Again, the initial optimism has been replaced by a more cautious approach. Braham (1986) lists a number of factors that have limited research progress, which can be summarized in two points: the large natural variability, and an incomplete understanding of the physical processes involved. (' - Most of the seeding hypothesis developed and tested in the past never tried to quantify the results but rather produced statistically qualitative results of responses to seeding from a randomized seeding experiment conducted for an extended period of time. These experiments usually treated the physical precipitation formation mechanisms within the cloud as a 'black box'. Seeding effects were evaluated by analyzing rainfall measurements at the surface or from radar utilizing a z..R relationship. The motivation and conception of these projects were based on conceptual models developed from past experience defining conditions that are conducive to positive seeding effects. These conceptual models were based on i) visual observations of clouds that did not precipitate, ii) the presence of supercooled water, ill) the similarity with clouds in other regions which responded positively to seeding and iv) data collected with aircraft and radars amongst others (Vali et al., 1988). Another general approach that was used during the late 70's was to establish a concise hypothesis describing each step in the physical chain of events leading to precipitation formation in both seeded and natural clouds. The final link in the chain of events was precipitation at the surface. An example of this approach is evident in the HIPLEX-l experiment. A detailed seeding hypothesis (Smith et al., 1984), together with a well-designed field program, which monitored each step in the physical 11 . . .