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Printed January 30, 1990 4. Augmentation of Rainfall from Convective Clouds Convective clouds are prominent in the tropics at all times of the year and are important rainfall producers in midlatitudes during the summer. Precipitation forms in convective clouds through coalescence of liquid droplets as well as around ice particles, the relative importance of the two processes depending upon the cloud characteristics. Coalescence is favored in clouds with high liquid water concentrations (LWC), which means convective clouds with warm bases, and broad droplet spectra. In many countries, especially in the tropics, convective clouds are seeded with hygroscopic agents to promote the coalescence of cloud droplets into raindrops. Hygroscopic seeding can be practiced on warm clouds, that is, clouds that do not extend above the freezing level. Clouds that extend above the freezing level can also be seeded with glaciogenic agents. Glaciogenic seeding of a supercooled cumulus cloud may initiate solid precipitation in the form of hail, graupel, or snow, or may lead to an overseeded condition, depending on conditions inside the cloud. Glaciogenic seeding also changes the cloud dynamics, the most obvious dynamic effects being produced by latent heat release when supercooled water is frozen. Under certain conditions the heat release can raise the temperature inside a rising cloud tower by 10C or more, thereby increasing buoyancy and causing the cloud to grow larger. Dynamic effects are tremendously important, because an increase of only a few hundred meters in the height of a convective. cloud is required to double the rainfall from it. Therefore, some experimenters have concentrated on the production of dynamic effects by heavy silver iodide seeding, even at the risk of temporarily overseeding the target clouds. Others have followed a more conservative approach, trying to improve, or at least maintain, precipitation efficiency. Increases in rainfall from individual convective clouds have been reported by experimenters using both approaches (Simpson and Dennis, 1974). However, because of the possibility of interactions among neighboring clouds or even among clouds some tens of kilometers apart, these increases can not be accepted as proof of increases in areal rainfall. According to the AMS (1984), Attempts to increase precipitation from warm season convective clouds have indicated local increases under certain circumstances and, under other circumstances, local decreases. There are indications that precipitation from selected convective clouds, particularly in semitropical regions, can be increased by seeding to enlarge the cloud systems. However, convincing evidence that such seeding can increase rainfall over a sizable area is not yet available. Among the best known projects involving convective cloud seeding to increase areal rainfall through microphysical effects are those conducted in Israel from 1961 to 1967 (Israel I) and from 1969 to 1975 (Israel II). Silver iodide was the seeding agent on both projects, which treated wintertime, post-frontal, convective clouds. It was argued that rain did not form by coalescence of liquid drops in the target clouds and that ice multiplication did not occur because the clouds were continental in nature (no large drops above say 30-jlm diameter). Therefore the clouds with top temperatures between -5 and -250C were considered promising targets for silver iodide seeding for microphysical effects. On Israel I, clouds were seeded with silver iodide released from an aircraft flying betow cloud base upwind of the target areas (Gagin and Neumann, 1974). The seeding rate was: 500 g. h'l. 7 l____._