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<br />.. <br /> <br />natural clouds is substantially greater than that found in cloud chambers. It is also possible (~ut believed <br />unlikely) that some form of ice multiplication routinely creates an order of magnitude or more additional <br />ice particles without further nucleation by AgI. Unless such mechanisms are involved, the calculations <br />strongly suggest that the current network of Utah generators cannot produce snowfall enhancements <br />approaching 10 percent with the type of seeding solutions used and an AgI release rate of 6 g h-I . <br /> <br />The real-time sampling of AgI and SF 6 in one canyon several miles above two seeding generators <br />produced puzzling results. AgI was detected in every instance when generators were operated near the <br />bottom of the canyon, yet snow samples immediately above the head of this canyon rarely yielded <br />enhanced Ag concentrations. This result might be partially due to low AgI concentrations. When the <br />mean IN concentration from the acoustical counter (operated at -20 oc) was reduced to account for <br />decreased effectiveness at a more typical liquid water temperature of 10 oC, the IN concentration dropped <br />to near 1 IN per liter. At this low concentration, seeding would probably not significantly increase <br />snowfall, nor would enhanced Ag be detected in the snow downwind. One special experiment using a fast <br />- response SF 6 detector in the canyon supported this low concentration explanation. At the altitudes and <br />temperatures where the gas was detected the corresponding AgI concentrations (estimated from SF 6 <br />concentration) would have been very low. Nucleation at concentrations of about lOIN I-I would likely <br />have occurred only at higher (colder) above crest line altitudes. If these canyon conditions were typical of <br />most storms, the lack of Ag in snow just above the canyon head is not surprising. <br /> <br />". <br /> <br />Most winter cloud seeding projects have only assumed, but not tested, the adequacy of their targeting. <br />Physical evidence continues to accumulate that routine targeting of adequate In concentrations to SA W <br />regions may be the exception and not the rule. It is strongly recommended that projects which have not <br />done so, carefully scrutinize their targeting. We cannot claim to have a credible technology unless we <br />demonstrate that our seeding methods actually treat the clouds. <br /> <br />8.5. Super, A. B., and A. W. Huggins, 1992b: Investigations of the targeting of ground-released silver <br />iodide in Utah. Part II: Aircraft observations. J. Weather Modification, 24, 35-48. <br /> <br />ABSTRACT <br /> <br />.- <br /> <br />As part of a cooperative research program between the Utah Division of Water Resources, the National <br />. Oceanic and Atmospheric Administration, and the Bureau of Reclamation, a series of aircraft missions <br />was flown to track silver iodide plumes in the Utah operational cloud seeding program. Both valley floor <br />and canyon mouth generator sites were tested using releases of sulfur hexaflouride tracer gas and silver <br />iodide. Optically-tracked Airsondes provided supporting wind and stability data. Five missions were <br />flown under atmospheric conditions that either simulated, or were the beginning of, the prefrontal phase of <br />typical Utah winter storms. The silver iodide and tracer gas were confined to the lower atmosphere during <br />four flights and were not transported over the intended mountain barriers. The plumes did cr9SS the <br />Wasatch Plateau during part of the fifth sampling mission. Ice nucleus concentrations were estimated <br />from the tracer gas measurements of the fifth mission for typical supercooled liquid water temperatures. <br />These estimates indicated that limited ice crystal concentrations would be formed with the generators and <br />seeding agent currently used in Utah. <br /> <br />43 <br /> <br />