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<br />transformed into precipitation size particles capable of reaching <br />the ground. They either blowout the top and sides of the <br />cumulus clouds or blow over the mountain crest to the lee <br />side, and evaporate. Introduction of artificial ice-forming <br />nuclei into these clouds could qecrease these losses, thus <br />enhancing precipitation. Furthermore, it was found that under <br />some conditions an increase in updrafts, and in cloud development, <br />occurred with artificial nucleation. This could lead to more <br />total condensation, and therefore to more precipitation. <br />Details appear in Section 4: "Sci$ntific Basis for Seeding". <br /> <br />1.5 The Conduct of Cloud Seeding Operations <br /> <br />The manner in which artificial nucleation can be employed <br />to increase the efficiency of the natural precipitation or <br />to induce cloud development is covered in the Section 5: <br />"Seeding Modes and Instrumentatio*". Artificial ice forming <br />nuclei can be produced by a var~ety of means, but the most <br />commonly used is a seeding device or "generator" that emits <br />about 1014 smoke particles per ~rftm of AgI (silver iodide) <br />serving as effective ice forming nuclei at a temperature of <br />-150C. The generator first vaporizes the silver iodide, then <br />the vapor is rapidly cooled in the immediate environment, <br />subliming to the form of submicrometer size smoke particles. <br />A few minute's supply of this smoke is enough to provide an <br />average of 10 nuclei per liter in a small cumulus cloud. <br />Ice forming nuclei can be generated the same way by using <br />lead iodide and other compounds :including organics, but none <br />are as effective as AgI. <br /> <br />A second often used method is to drop dry ice pellets <br />into a cloud. In the wake of a pellet the air is cooled below <br />the critical temperature of -40oC where all liquid water particles <br /> <br />6 <br />