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been mixed. Use of such "lower slope" locations would usually require remote-controlled AgI generators. Review of the evidence shows that the frequency of targeting success is significantly greater from higher elevations. Therefore, there seems little point in siting expensive remote- controlled AgI generators on lower slopes. Much of the following discussion has been paraphrased or taken verbatim from Super (I 999a), in which he presented the results of a feasibility study for seeding Colorado's northern mountain ranges. The same is true for portions of Appendix B, as little new information as been published since the 1999 study was completed. 6a. Seeding from Valley Locations A considerable body of evidence, including some presented in Appendix B, indicates valley- released AgI plumes are often trapped by stable air, especially when valley-based inversions are present. Sometimes such AgI plumes are transported parallel to mountain barriers rather than over them (e.g., Parish 1982). A number of scientists have recognized for several years that attempting to seed winter orographic clouds from valley locations is fraught with difficulties. When one considers that the SL W zones over mountains are typically thousands of feet above valley AgI generators, and that the atmosphere is often stable for some distance above valley floors, the potential problem is obvious. Furthermore, part or the entire SL W zone may be too warm for AgI to produce adequate seeded ice crystal concentrations. Therefore, even when AgI is vertically transported to the SL W zone, seeding may still be ineffective. Smith and Heffernan (1967) expressed the problem well when they stated, " --- persons who release silver iodide from the ground would be wise to find out where it goes. " Yet this seemingly obvious advice has seldom been followed, in part because tracking plumes over rugged terrain can be difficult and expensive. As a body of evidence built up, the review articles by Rangno (1986) and Reynolds (1988) recognized that achieving adequate transport and dispersion was possibly the most difficult problem facing winter orographic cloud seeding. The difficulties of targeting SL W regions in winter orographic clouds were more recently noted in the review article by Bruintjes (I 999). But the special difficulties with valley AgI releases were recognized well before these publications. Some of the earliest airborne AgI plume tracking was done over the Park Range of northern Colorado as reported by Langer et al. (1967) during testing of a new acoustical ice nucleus counter. A limited data set resulted which the authors stated was collected, "--- under conditions approximating those conducive to cloud seeding." It was found that much of the Agl released from the 8,200 ft top of Emerald Mountain was trapped in the Yarnpa Valley between the release point and the intended Park Range target area to the east. None of the tests showed reasonable Agl concentrations more than a few thousand feet above the ground. A much larger set of observations from five winters was used by Rhea et al. (1969) to demonstrate that valley-based trapping inversions typically occurred at least halfway up the Park Range during snowfall. Such inversions would be expected to severely limit the vertical dispersion of valley-released AgI. Similar findings were reported by Super et al. (I 970) who presented the results of seven months of observations of atmospheric stability during snowfall over the Bridger Range of Montana, based on surface temperature measurements along the windward slope. Their stability data also indicated that AgI generators needed to be at least midway up the windward slope to be above the frequent stable layer extending above the valley floor. As might be expected with a stable atmosphere, 17