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4. RESULTS OF PHYSICAL CLOUD SEEDING EXPERIMENTS 4.1 Background Information There is no doubt that AgI released into sufficiently cold SL W cloud will produce multitudes of embryonic ice particles. The same result is achieved when liquid propane is expanded into even slightly supercooled liquid cloud. The challenge is to create the seeded ice particles at such locations that their subsequent trajectories will be within SL W cloud for a sufficient time (distance) to permit growth to precipitation sizes such that the particles will fall to the mountain surface before sublimating in the lee subsidence zone. Ideally, the tiny seeded crystals should be formed very soon after SL W condensate is produced as air is forced up a mountain barrier, is carried upward by embedded convection, is transported upward by gravity waves, or ascends'by some combination thereof. Releasing AgI even at cloud base will not result in immediate nucleation unless the temperature is colder than -6 oC. . Numerous attempts were made to document the effectiveness of AgI and propane seeding in creating ice particles and snowfall during the Plateau experiments. Most emphasis was placed on such documentation during the limited programs of the 1994-95. and 1995-96 winters, as summarized by Super and Holroyd (1997) and Holroyd and Super (1998): "'. With two exceptions previously noted (March 2 and March 6, 1991) and discussed by Super (1995a), all AgI and propane seeding experiments which demonstrated IPC enhancements used high altitude release sites well up the windward slope of the Plateau. Unless specifically mentioned otherwise, it can be assumed that high altitude seeding sites were used in the cases to be discussed. 4.2 Case Study Analyses , The first article in this series to document seeding-caused ice particles was by Super and Holroyd (1994). A several-fold enhancement in IPC was shown at aircraft levels for the February 17, 1991, experiment. The co-located AgI and SF6 were both detected with aircraft instruments. Measurement ofSF6 with a fast- response detector allowed for precise delineation of the seeded zones. Seeding-caused IPC was near 70 L-1 at cloud temperatures of -13.0 to -15.5 oC over the Plateau top's west edge. Holroyd et al. (1995) presented a detailed analysis, with numerical modeling support. of the February 21, 1994, experiment. It was shown that high concentrations of ice particles were associated with measured and predicted plume locations sampled on the Plateau top with a 4-wheel drive vehicle, and above the Plateau with the NOAA aircraft. Ice particle concentrations and precipitation rates were enhanced by a factor of about 10 along the Plateau top's west edge highway, and by about a factor of 40 according to aircraft sampling above the west edge. Most growth was upwind of these sampling tracks, above the windward slope where SL W was concentrated. Aggregation of high com;entrations of ice particles appeared to be the primary snowfall mechanism. Plateau top observations suggested that only limited precipitation reached the surface, perhaps 0.5 mm accumulation over a few hours at some gages. But no -- gages were available near the windward edge where most seeded snow likely fell. Super (l995b) presented detailed analyses that demonstrated an obvious increase in IPC and snowfall associated with the propane seeding experiment of March 5, 1995. Less obvious but still fairly convincing evidence of IPC and snowfall enhancement was presented from an experiment on March II, 1995. Light natural snowfall "contaminated" the impacts of propane seeding during the latter experiment. 17