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<br />occurred, rates were light as is typical of natural snowfalls. The heaviest hourly accumulation <br />observed during the seeding experiments was 1 nun liquid equivalent. - <br /> <br />. <br />. <br /> <br />n. Propane releases at rates of 3-7 gal h-1 were clearly demonstrated as capable of producing 10 to <br />20 ice crystal per liter even during slightly supercooled conditions. This approach offers a <br />practical adjunct or alternative to AgI seeding during the mildly supercooled episodes typical of <br />Utah winter orographic storms. Moreover, propane seeding was totally automated, using an icing <br />rate meter to detect SL W cloud and a data logger to "decide" to release the liquid propane only <br />during seedable conditions. <br /> <br />7.4 Summary of Findings and Recommendations <br /> <br />In summary, SL W, the necessary "raw material" needed for cloud seeding to be effective, is frequently <br />present in Utah's orographic clouds. Amounts of SL W are adequate to provide the potential for artificial <br />nucleation by cloud seeding to enhance the mountain snowfall at higher elevations. However, numerous <br />physically-based investigations reported herein strongly suggest that the current operational seeding <br />program is unlikely to convert much of the available SL W to additional snowfall. <br /> <br />~ <br />~ <br /> <br />The main problem with the Utah operational seeding program is that observed concentrations of effective <br />AgI in nuclei are too low for significant snowfall enhancement from the mildly supercooled clouds. Much <br />of the time, the entire SL W layer is too warm for effective seeding with AgI, which begins to nucleate ice <br />crystals near -6 oC, but which is ineffective ,in the concentrations observed until the SLW cloud is colder <br />than about -9 oC. Under such circumstances, a different seeding agent is needed, such as liquid propane. <br />When colder SL W cloud does exist, the ''window of opportunity" is quite narrow before natural processes <br />produce sufficient ice particles for effective snowfall. <br /> <br />A number of steps aimed at increasing the operational program's effectiveness can be taken. Admittedly, <br />most of them would increase the program's cost. However, in terms of snow enhancement, these <br />improvements may make economic sense. It is beyond the scope of this work to provide such economic <br />evaluations. <br /> <br />7.4.1 Recommendations to Improve AgI Seeding Effectiveness <br /> <br />If the Utah operational program is to continue to rely on AgI seeding, some of the following possible steps <br />ought to be considered because they should improve program effectiveness. <br /> <br />a.. Convert to a seeding solution capable of producing an AgI aerosol that nucleates by the <br />condensation-freezing mechanism rather than relying on the contact-freezing mechanism. <br /> <br />b. Significantly increase the density of seeding generators and the ice nuclei output per generator. <br /> <br />c. Place generators in canyon mouth rather than valley floor locations. <br /> <br />. . <br /> <br />d. Use high altitude generators located at least halfway up the windward slope of the targeted <br />mountain barriers. <br /> <br />32 <br />