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II. CLOUD SEEDING AGENTS AND DELIVERY SYSTEMS 605 and suppresses the tendency for the AgI to dis- sociate. There have been hundreds of mixes tried for pyrotechnics. One commonly used for- mulation is 78% AgI03, 12% aluminum, 4% magnesium, and 6% binder. It is customary to rate pyrotechnic units in terms of nuclei pro- duced per gram of AgI consumed. The AgI03 in one commonly used flare, the EW-20, would yield 20 g of Agl upon reduction, so the EW-20 is described as a 20-g flare. Another interesting pyrotechnic generator is Weathercord, which is explosive Primacord pre- pared with an AgI mix. It was developed to be dropped from aircraft. The most extensive development of pyrotech- nic generators in the world has taken place in the Soviet Union. In addition to developing a vari- ety of pyrotechnic cartridges to be fired into clouds from aircraft, Soviet scientists and engi- neers have designed and built a wide range of rockets and artillery shells for cloud seeding. The rockets vary considerably in size; some of them are over 4 m long and weigh as much as 10 kg (Fig. 3). Some of the earliest devices released their payload in an explosion, but later models provide for a controlled payload burn initiated at a certain range from the launch site or a speci- fied elevation. The Alazan rockets were pro- grammed to explode into fragments after com- pleting their payload burn to minimize danger to populations on the ground. However, this pro- cedure was not completely satisfactory because of some substantial fragments that were ob- .". ....... ('" .': ~..... , .... , . .. ... .Jlf.W ...... FIG. 3. A small spin-stabilized rocket manufactured in the Soviet Union for use on hail suppression proj- ects. Most Soviet rockets are larger and rely on fins for stability. (Courtesy of Atmospherics, Inc.) served to fall, as well as the occasional misfire that resulted in a rocket shell reaching the earth still carrying its explosive charge. A later rocket, the Oblako, deploys a parachute follow- ing completion of the payload burn, which lowers it safely to the earth. Even so, opera- tional restrictions are applied in densely popu- lated areas. Because of the need to reach the upper re- gions of tall cumulonimbus clouds and to cover an appreciable area from a single launch site, rockets with effective ranges of up to 10 km have been developed. These can leave an aero- sol trail as much as 8 km long and extending to 8 or 9 km above ground at the highest point of the rocket trajectory. Some Soviet rockets carried large payloads of as much as 5 kg of reagent of which 50% might be AgI. For reasons of economy some of the devices used lead iodide (PbI2) or some mixture of chemicals rather than pure Agl as the seeding agent. About 1977 Soviet scientists began exper- imenting with much leaner mixtures. For exam- ple, the mixture Silverspare contained only 2% AgI, yet it was reported to have roughly the same t:ffectiveness as the mixtures used previ- ously. D. FACTORS CONTROLLING PARTICLE YIELDS The AgI particles produced by a generator are the result of gas-to-particle conversions as the AgI vapor is quenched on leaving the flame. The number of embryonic particles exceeds the number of particles released to the ambient air because of rapid coagulation due to Brownian motion. Because of coagulation, no generator can produce more than about 108 particles/cm3 of effluent. Therefore, the yield from a generator is controlled by the amount of air passing through it, and the amount of AgI consumed af- fects only the particle size. X-ray work has shown some generator products to be aggregate particles with diameters averaging from 0.15 to about I ILm, but with the individual crystallites averaging 80 nm. This suggests that an aggregate can contain as many as 1000 crystallites. Brownian motion produces very frequent col- lisions for particles of widely different sizes. Therefore, the very small individual crystallites and small aggregates rapidly coagulate with the larger ones so that the final particle sizedistribu- tion is narrowed. Electron microscope observa- tions have indicated that many generator prod- ucts have a log normal distribution with the