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and the downwind valley, there are excesses, as well as in all of the western groups. Except for those in the dO~lwind valley (San Luis Valley), no statisti- cal significance is attached to them, In the San Luis Valley the difference between the seeded and not seeded precipitation ranks is due mainly to a large reduction in the number of zero caseB with seeding. The presence of excesses downwind of the crest is not unexpected on the basis of physical principles. There appears to be a shifting of the area of Figure 5 excesses downwind in Figure 6 and at the same time a net reduction in net precipitation over the entire barrier. Precipitation is naturally heavier in the deep cold top clouds; hence, the lower percentages associated with larger differences in Figure 6 relative to Figure 5. The influence that the 700 mb wind component normal to the barrier exerts under cold top cases is illustrated l)y Figures 7 and 8. These cold cloud top cases include only those for which the cloud top temperature is -27 C or less. There iB a distinct shift downwind in the pattern of deficits with the strong- er relative to the weaker winds. This affect is also expected on the basis of physical principles and has already been referred to as the blow-over effect. In this analysis there is essentially no statistical significance attached to these two figures. A three-dimensional analysis of flow patterns was made using the rawin- sondes and telemetered wind data from vari.ous sites at valley and high eleva- tions. This, in conjunction with nuclei concentra- tion data from a number of coun- ters, distributed over the target and upwind areas, was employed to form an estimate of transport and diffusion of the nucleant. As a result of this analysis, with reference to past findings in other areas, three modes of plume behavior have been identi- fied, each of , which is illus- trated in Figure 9. The pooling mode, it turns out, was an unexpectedly prevalent mode of seeding. Inthis mode nucleant is transported slowly in blocking type flow to the west around the western end of the San Juan massif. In this situation, the air within which all but the highest level generators lie is stable. Above this stable layer lies a northeastward flowing air mass which is usually neutral or un- stable. The highest concentrations of nuelei (plume centerline) remain on the ",...--- ' I :--- f "", ''''\ _--/ - DURANGO COLORADO -- ---.... ----- HEW MEXICO mti.u. FOUR YEAR SEED(- HO-~EEn DIFFERENCES (111116 'HRS) FOR COI,D CLOUD/T~~ CASES T i -Z7 C) AND LIGllT NORIW. WINDS (VN < b,., ", sl. , ,. I .. . .. . ....",.. .... -. 15