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increase in IPe for the seeded distribution. with most of the seeded minus nonseeded differences in the range of 10 to 20 crystals per liter. These differences are very similar to TAR :!D-C probe results from previously published propane seeding case study eXpl.'Timents which had lower propane rates released from the liAS. The probability that the seeded and nonseeded dilTerences ,",,'e:-re caused by seeding. as suspecte:-d. cannot be staled because of the:- limited sample size and lack of crosswind control observations. All naturallPCs. average:-d over the:- 40 minute EU periods. were:- between JO to 90 per lite:-r whatever the:- SWE observed by nearby gauge GTR. This is in good agreement with a pre....iously published much larger data set which showed that all but a small ponion of natural TAR IPCs were between 10 to:!OO per liter when precipitation was 0.01 inch per hour or more. These findings demonstrate that nalurallPCs are seldom low enough 10 prevent ma~king of seeding effects in case sludies. Another important implication is that natural snowfall of detectable (and meaningful) rates re:-quires a minimum IPC of at least 10 per literan the plateau top. A minimum of20 per liter is probably a more realistic threshold. Seeding attempts which cannOI achieve higher IPCs are unlikely to produce useful snowfall. especially since seeded crystals lend to have more limited growth times and. therefore. smaller masses than natural crystals. Summar~' or Randomilcd Elperimcnt To summarize:-. the essential features of the Utah randomized experiment were: . Used propane releases to treal SLW cloud.. which were often 100 waml to be seeded with Agl. . Conducted using a seeding sile already demans\nued to pro....ide rOUline targellng. . Used a fully automaled operatlon with seeding or placebo treatment triggen.>d by SL \\-. detection. . Experimental Units (EUs) blocke:'d into pairs with one EU randomly seeded. . Target precipitation gauges operated along the e:-xpeCled seeding plume trajectory. . Control gauge opt.-r3te-d crosswind to monitor natural precipitation variations. . Supponmg wind and other observations collected for panitioning EUs into similar subpopulations. . Applied three sophislicated statislical testmg procedures. all using the control gauge as a eO\'ariate to substantially reduce the variance- caused by natural snowfall fluctuations. Summar~' or Statistical Conclusions Resulls of slatistlcal tesling oflhe available EU sample population were considered signilicant and "slrongly suggestive" if the probabiliry of indicated seeding-caused differences between seede:-d and nonseeded EUs nut being by chance was at least 95 out of 100 (a-level '" 0.05). Results were considered "somewhat suggestive" if the same probability was from 90 to 95 OUl of 100 (a-level = 0.06 to 0.(0). Results to follow discuss the four gauges consisting of (11 GSC midway between the seeding location and (2) GTR. the primary intended target gauge on the west edge of the plateau top. (J) GSO located I km due south ofGTR also on the plateau lOp. and (4) GDN.lhe only gauge downwind ofGTR. about 6.5 km plume lrawl distance from the seeding sile:'. Because resulls vary among the statislicallests. each having different strengths and weaknesses. the predominant findings of two or more tests will be notoo below. The results of slalistical testing of the Ulah randomized (,:'Iperiment can be briefly summarized as follows: . Tests of the entire sample of98 EUs without any panitioning we:-re strongly suggeslive ofa real seeding dTectllf increa~ed sno.... fall at gauges GSC. GTR and GSO. . Results for the 98 EUs were inconclusive at the downwind gauge GDr" meamng no valid conc\u...ion can be made whether se:-eding was elTective there or not. . Results for the:' 69 EL's panitionL>d by south.....est quadranl seeding site winds. known ta transpon seeding plumes by the primary larget. were also strongly suggestiw of snowfall increases allhe GTR and GSQ gauges on the:- plateau top. Results were somewhat suggestive at the GSC gauge c10sesl to Ihe propane dispensers. and also at the downwind gauge GD!'\ furthest from the seeding localion. . Comparisons of the wind direclion- panitioned 69 EL's with all available 98 EUs revealed "