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<br />Hygroscopic Seeding Concepts & Comparisons with AgI <br /> <br />To develop precipitation particles in clouds, particles must grow by <br />coalescence with cloud droplets. Within the time frame of ordinary <br />clouds, growth by coalescence proceeds only when the initial particles <br />are larger than about 50 pm diameter. The basic problem then is to <br />provide, naturally or artifically, these 50 pm diameter particles. Both <br />hygroscopic seeding and ice nuclei seeding can introduce these precipi- <br />tation particle embryos into the cloud. In a region where there is a <br />deficiency of naturally-caused embryos, the seeding can have direct <br />effects on precipitation, and indirect effects on subsequent cloud develop- <br />ment and precipitation. Figures 6 to 10 present the main points of <br />hygroscopic seeding and the comparison with AgI. AgI introduced into <br />a cloud can initiate ice crystals, which then grow to the 50 pm size in <br />the order of 1 to 3 minutes. Thus in an upcurrent the 50 pm diameter <br />particles are created at about the -8e level. One can dispense 50 pm <br />water droplets into a cloud base whatever the temperature. If the base <br />is at +14C, a typical value in Oklahoma in summer, then such particles <br />can draw on cloud droplets for about 11, 000 feet of cloud depth before <br />reaching the height where silver iodide could be effective at providing <br />an alternate source of precipitation particle embryos. This lower <br />depth of the cloud may have moderate upcurrents, which can prolong <br />the time the growing hydrometeor may be in a favorable growth environment. <br />On the other hand, the particles started higher by silver iodide may be in a <br />very strong upcurrent which gives them only a short growth period before <br />blowing up through the -40C level (above which growth decre ases markedly <br />or even ceases because no liquid remains in the cloud, just ice). <br /> <br />One could dispense 50 pm water droplets from an aircraft near cloud base. <br />However, it turns out that the amount of water required to generate a <br />sizeable shower is large. If one puts out 20 pm diameter hygroscopic <br />particles, each particle can quickly grow to 50 pm by the condensation of <br />water vapor from the atmosphere - - and one need only put out about 1/ 15th <br />as much material. Since the cost of operating aircraft to dispense seeding <br />material is high, there is a sizeable economy in reducing the required <br />payload even if the payload material costs more. A liquid fertilizer containing <br />440/0 ammonium nitrate, 350/0 urea, and 21% water is readily available, <br />inexpensive ( $50/ton), is very hygroscopic, and so is convenient to use for <br />seeding. <br /> <br />17 <br />