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55 precipitation that will fall to the target area. Notice that, although this conclusion is modified somewhat by the vertical transport due.. to convection, most of the region over the mountain is unseedable even in the presence of greatly enhanced vertical transport. The line labeled "1 m/sec" on Fig. 3.8 indicates the fall expected for an ice crystal falling 1 meter/second in the absence of any updraft, and the liquid water over the mountain lies mostly to the right of this line. Thus the best target for seeding is the convective zone ahead of the mountains. This zone is located above the convergence zone which is over the seeding generators. ,In terms of ground-based seeding, the generators are apparently placed in as good a location as possible for the unstable storm type. The trajectories of Fig. 3.5c and 3.8 suggest that seeding should be effective in this region. In both unstable and neutral storm stages, the liquid water region over the mountains is seedable, but was not useful to the San Juan Project because the precipitation formed in this region was swept beyond the continental divide and could not reach the target area. Seeding in this region probably accounts for the high downwind increases observed for unstable cases (Elliott, "1975) . Our picture of the most seedable storm situation is one of a moderately unstable cloud, with a surface convergence zone and an associated convective zone ahead of the target area. The cloud should be relatively shallow, to prevent fall of naturally formed ice crystals from high levels into the seedable lower levels. The storm should also have warm cloud tops and cold cloud bases. To gain time for the AgI to act, the winds should be moderate. d. Ihe dissipation stage Since a weak cooverg~nce zone and cumulus clouds containing liquid water are present, it may be possible to extend the precipitation period of the storm by I I I I I I I I I I I II I I I I I I I