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Created 12/26/06 5 <br /> same region of Utah. The use of propane was thought preferable to AgI because of the high <br /> frequency of time SLW was found to occur over the Wasatch Plateau at temperatures too warm <br /> for ice formation by AgI (10, 12 and 25). Randomized experiments are considered the "gold <br /> standard" of experimental evaluations and are necessary to supply the "proof' referred to in the <br /> NAS report (1). Statistical analyses of weather modification experiments, where natural <br /> variability can be 100 times the expected seeding signal, are greatly improved by the use of <br /> covariates (e.g. 26, 27 and 28). The best covariates are control precipitation measurements <br /> (upwind or crosswind of a target area), and these were applied in the Utah study (29). The results <br /> of this 1-season experiment were statistically significant and indicated that seeded periods <br /> produced about 20% more precipitation than unseeded periods (29). <br /> The results from statistical evaluations of the BRE matched very well with what was <br /> learned in the physical studies. Two seasons of a randomized seeding experiment produced <br /> results that showed significant differences between seeded and unseeded populations of events <br /> (4). The main findings were: a) Seeding increased snowfall in the intended target and sometimes <br /> downwind, when the ridge top (-2595 m) temperature was less than -90 C; b) The seeding <br /> increase was found for the entire 100 days which met this criterion over two seasons, as well as <br /> when each season was analyzed separately; c) Positive seeding effects were suggested in the <br /> target and in the valley downwind of the target, also mainly for the colder cases; d) A seeding <br /> effect of about+15% was also found just a few kilometers from the seeding sites, and; e) Double <br /> ratios of target and control gage precipitation suggested seasonal increases of —15% on seeded <br /> days, but increases as great as +50% were indicated when only the colder days were included in <br /> the analysis (a finding in close agreement with the microphysical observations). <br /> Other statistical evaluations of wintertime cloud seeding have produced similar results, <br /> but none are nearly as well documented by physical observations as was the BRE. Another <br /> randomized experiment was conducted by the Pacific Gas and Electric (PG&E) Company in a <br /> region near Lake Almanor in the northern Sierra Nevada (30). A statistically significant result <br /> came from a cold-westerly storm stratification where a 32% increase in precipitation was <br /> indicated for seeded cases. Trace chemical evaluations of snowfall in the Lake Almanor project <br /> area (31, 32) have since helped substantiate the statistical indications. A common finding from <br /> the projects referenced here is that the most pronounced seeding effect occurred in relatively cold <br /> and shallow orographic clouds. Evidence indicated that precipitation can be increased by 50% or <br /> more in these storm periods, which can result in seasonal increases of snowfall by the —15% <br /> augmentation that is quoted in capability statements of.the World Meteorological Organization <br /> and the American Meteorological Society(33). <br /> New Evaluation Technique: Trace Chemistry Analysis <br /> Additional research on wintertime cloud seeding over the past 20 years has produced <br /> some very promising techniques for evaluating seeding effects over basin-sized areas. As shown <br /> in the BRE (4) the trace chemical analysis of snowfall can be used to verify targeting. The <br /> technique has now been used on numerous projects to determine if seeding material has reached <br /> The current NAIWMC membership includes state agencies in <br /> North Dakota,Kansas,Oklahoma,Texas,Colorado,Wyoming,Utah,Nevada and California <br />