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4. Seeding the cloud with a combination of exothermic and endothermic hygroscopic particles at different stages of cloud development to: a) invigorate the cloud's updraft, thereby initiating warm convective clouds and/or stimulating their growth, and then b) increase the coalescence rate of conversion of cloud water to precipitation particles with respect to the rate of loss of cloud water to other sinks and, at the same time, invigorate cloud downdrafts to help the hydrometeors reach the ground. The first three approaches are designed to produce microphysical effects that will improve the efficiency of the rain evolution mechanisms and decrease the time required to initiate the precipitation process; the fourth approach is designed to produce a combination of microphysical and dynamic effects to maximize the rainfall falling at a specific location on the ground. The reader is referred to the WMO Report of the Meeting of Experts on Warm Cloud Modification, Kuala Lumpur, March 18-24, 1981 (WMO, 1981b), for a more complete discussion of this subject. 3.3 Summary of Results of Relevance to Thailand Since about 1955, experiments and projects designed to increase precipitation through hygroscopic particle seeding have been conducted, involving both ground and airborne seeding methods. Only the most relevant scientific studies are mentioned here; the reader is referred to Cotton (1982) for a more complete discussion of this subject. Biswas and Dennis (1971) reported that the seeding below one end of a line of stratocumulus clouds with 350 pounds of sodium chloride resulted in a shower, and that no rain fell from any other clouds within 50 miles of the :seeded cloud. In subsequent calculations (Biswas and Dennis, 1972), they postulated that a ehain reaction process stimulated by the salt seeding was likely involved in the evolution of precipitation. A long series of hygroscopic particle seeding experiments on warm convective clouds has been carried out in India (see, for example, Biswas et al., 1967; Kapoor et aI., 1976; and Murty, 1989). Murty (1989) reported the results of an ll-year randomized crossover experiment in which warm convective clouds were seeded at cloud base with sodium chloride particles 10 micrometers in size. The seeding on da.ys when the experimental area was covered by clouds (area seeding days) was about 10 to 3:0 kilograms per kilometer, and the seeding on days when the experimental area had only a few clouds (target-control days) was 700 to 1000 kilograms per cloud. For 80 pairs of area seeding days, he reported an increase in rainfall of24 percent at a 4-percent significance level. For 62 pairs oftarget-control days, he reported a decrease in rainfall of 35 percent, which was not statistically significant. Experiments to increase precipitation from warm cloud base convective clouds using hygroscopic flares are being conducted in South Mrica (Mather and Terblanche, 1992). In these experiments, hygroscopic flares are burned in the sub cloud layer which introduce about 1011 particles per flare greater than 1 micrometer in diameter. The particles are a mixture of sodium chloride, potassium chloride, lithium carbonate, and magnesium oxide. Mather and Terblanche (1992) report that cloud physics measurements taken during seeding trials with the hygroscopic flares support the hypothesis that the seeding is initiating, or at least enhancing, the coalescence process. Statistical analysis of the first year of a multi-year hygroscopic flare seeding experiment in South Mrica (Steffens, 1992) indicates that rainfall from seeded clouds is greater than that in non-seeded clouds, but has no statistical significance. 23