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concentrations at aircraft sampling altitudes were very limited, indicating the seeding material was confined to a layer no more than 2000 ft thick while being transported over the Plateau. Further model evidence of the gravity wave mechanism sometimes transporting valley-released AgI to plateau top elevations was presented by Heimbach et al. (1998). Their investigation stratified rawinsonde observations into five stability classes, although several soundings did not fit the criteria for those classes. Not unexpectedly, the most unstable sounding class produced the best targeting. It also had the coldest temperatures, resulting in more effective AgI ice nuclei. That class had 26% of the 46 soundings which meet the criteria but only 17 % ofthe total of72 soundings. However, as discussed in Appendix C, a number of soundings were not made during actual storm conditions. Only weak vertical transport was indicated for another stability class and virtually none for the other three classes. Cases studies with comprehensive observations were found which represented the various classes, and generally good agreement existed between these measurements and model results. 6b. Silver-in-snow Concentrations One approach for testing whether AgI seeding material was transported over a target area is to monitor the concentration of silver-in-snow to determine whether it exceeds natural background levels (Warburton and Young, 1968). Enhanced silver concentrations from AgI seeding have been found for some programs including Super and Heimbach (1983), Heimbach and Super (1988), Chai et al. (1993), Warburton et al. (1995a), Warburton et al. (1995b) and McGurty (1999). These programs all used high altitude AgI generators. Reynolds et al. (1989) reported upon ground-based seeding experiments with AgI generator sites ranging from the foothills to sites on the west slope of the Sierra Nevada of California. A network of24 generators consisted of3 long-term operational networks of remote-controlled generators and 1 specially-installed network of manually-operated generators. All generators were operated in a coordinated fashion during a 2-mo period. A numerical targeting model was used to compute nucleation and fallout locations for each generator on a given day. It appeared to provide reasonable estimates of AgI plume transport and dispersion when compared with aircraft observations. Silver iodide plumes released from foothill generators frequently had trajectories parallel to the mountain barrier rather than over it. Targeting was more successful downwind of higher elevation generators, above 6600 ft. Extensive snow chemistry analysis revealed targeting effectiveness at 14 sampling sites within the intended target, from north to west to south of Lake Tahoe, for the combined generator network. Finding increased silver-in-snow does not prove that AgI seeding produced snowfall because natural snowfall will scavenge AgI and bring it to the surface. However, failure to find silver greater than natural background levels indicates the Agl plume did not pass over the sampling location in adequate concentrations during the sampled snowfall period. Therefore, lack of enhanced silver is interpreted to mean seeding was ineffective. A large number (1,681) of individual target snow samples were collected from the 14 sampling sites for chemical analysis of silver concentration. Less than 15 percent of the samples indicated any silver greater than background. In summarizing the silver-in-snow sampling program, Reynolds et al. (ibid.) concluded that, "These are disturbing results, even if one considers only scavenging, in that the AgI must not have passed over large regions of the target during precipitation events. Much of the AgI may be transported westward or northwestward at low levels, effectively not passing over the barrier." These results are indeed disturbing, especially since most of the generators were used for three long-term operational seeding projects, and some 20