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<br />increases in precipitation over areas the size of typical operational winter program target areas on a <br />seasonal basis. It is recommended that the funding for this research program be obtained from <br />federal sources and consequently the costs of conducting such a research program are not included <br />in the cost estimatcs contained in Section IS, <br /> <br />In a similar vein. thcrc havc not becn any rescareh programs that have demonstrated that <br />dropping dry ice particles from aircral1 can produce increases in precipitation over a sizable fixed <br />target area over a substantial pcriod of time. As a consequence. dry ice is also not reeommcnded <br />for use on potential near term opcrational programs in the Colorado River Basin. <br /> <br />Silver iodide is thc seeding agent currently in use in the on-going operational cloud seeding <br />programs in the Intcrmountain \vest. Silver iodide is the agent recommended for use on any new <br />operational Colorado Rivcr programs that may be implemented, <br /> <br />3.2 Sccdin~ Modes <br /> <br />Secding modc refers to the method(s) used to releasc seeding agents. Silver Iodide (Agl) <br />seeding material can be released from both ground and aircraft platforms. The ASCE Standard <br />42-04 describes how silver iodide nuclei may be crcated as follows: <br /> <br />In many cases. Agl is released by a xenerator that \'{/I'orizes an acefOlle.,\'iI\'er <br />iodide solution ,'of1laininx 1.2% AXI and produce,5 aerosols with partides of 0.1 to 0.01 <br />pm diameter. Agl is insoluble in acetone: commonly IIsed solubilizing agents include <br />ammonium iodide (NI141), and any of the alkali iodides. Additional oxidizers and <br />additires {'ommonly include ammonium perchlorate (NII4CI04), sodium fJachlomte <br />(1\'aCI04). and paradichlorobenzene rC"J14Cly. nIe relatiw (m/OlInls oImch additil'es <br />and oxidizers modulate the yield. f1ucleation mechanism. and ice crpwl production rates. <br />The generation of Agl aerosols can al.m be accomplished bJ' burning speciali=ed <br />p.rroteclmics. In recent years, llllwlflces in nllcleation physics haw resulted in a numher <br />0.( more effecti\'(' pyrotechnic formulatiom which produce nudei that, in addition to <br />lun'ing ice nucleation thresho/cl5 near -re. are also somewhat hygroscopic. The <br />resulting nuclei are not only eJfecti\'e CIS IN. but they also aUract water molecules. This <br />results in particles that in high re!atiw humidities (near saturation) qllic:klyfi.mn droplets <br />of their own, which Ilumjreeze shortly ajier becoming supercooled. This condensalion- <br />ji'eezing nucleation proce.n genera/lyjimctions jilsler llulfI that achieved tHing simple <br />Agl. Laboratory testing has shown that Ag/ by ilseljjimctions primarily b.v the contact <br />nllcleation process. u.hich is more (It-pendeftt upon cloud droplet concentration. and <br />co"sequen/~l'. a much slower process (De.\IouI99/). <br /> <br />Ground based siTver iodide gcnerators can either be manually operated by local residents <br />or remotely operated from higher elevation. unmanned locations. There are advantages and <br />disadvantages of each type. Silver iodide can also be released from aircraft using either liquid <br />fueled gcnerators or pyrotechnics. Again. there are advantages and disadvantages associated with <br />aircraft seeding. In gencral. remotely controlled ground equipment or aircraft seeding may be <br />more elTective in some situations than lower elevation ground gcnerators. but they will be more <br />costly. <br />