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The orographic effect upon the modi- fication potential is demonstrated by the observed increases in snowfall when seeding those flows nearly normal to the mountain barrier. Also, positive seeding effects are obse rved to increase irregularly with the speed of the horizontal flow except at the highest wind speeds where snowfall decreases are generally observed again. 5. Discussion The following observations are consistent with current cloud physics theory. The largest snowfall increases are realized at cloud top tempera!: ures where the greatest deficiency of effective ice nuclei is observed. No snowfall increases are obse rved where measurements indicate effective ice nuclei are normally plentiful. The largest snowfall increases are observed when the wind flow and topographic features combine to produce an orographic stream having strong upward motions. The largest snowfall increases are observed for events having the largest moisture supply. Snowfall decreases are observed for the coldest and driest events, and the lowest wind speeds. These are the events that would have low supply rates of cloud water and normally excessive concentrations of effective ice nuclei. Snowfall increases are observed at somewhat colder cloud top temperatures in the area having stronger upward speeds on the average. Snowfall increases are observed when a moderate baroclinic zone is present in the 700 mb to 500 mb layer. Two observations appear at first glance to be somewhat contradictory to the presented theory; the decreases in snowfall observed at the higher wind speeds and for the very strong baroclinic conditions. At this point one can only speculate on these findings. ~ very strong baroclinic events may be accompanied by higher than usual cloud tops as the strong baroclinicity probably reflects a stronger than normal synoptic disturbance. Under these conditions cloud tops may extend a few thousand feet above the 500 mb level and consequently, cloud top temperatures would be con- siderably colder than indicated by the 500 mb tem- peratures. Thus, effective ice nuclei might be naturally plentiful in these cloud systems. Since the extremely high wind speeds would normally accompany these intense baroclinic disturbances one could invoke' this same argument to explain the decreases at high wind speeds. If one interprets the seeding effect at these high wind speeds as not really a decrease, but rather that it represents no effect another argument is possible. It c~n then be argued that the artificial nuclei do not have time to reach the cloud system, activate and grow to sufficient size to settle at the mountain summit, but rather are carried over the barrier. This infers a greater target-generator spacing should be employed during the high wind speed events than is available in the pre~ent design. If ~hese decreases in snowfall are accepted, however, it 1S not clear why overseeding should occur with these high wind speed events unless the natural snow crystals are already critically small possibly due to small residence times within the c.loud system. a. Cloud physics model The cloud physics ~odel presented appears to be consistent with the observed climatology of natural snowfall at Wolf Creek Pass and Climax Colorado. The daily snowfall:at these ' locations reaches a maximum in the cloud top tem- perature range where effective natural ice nuclei and supply rates of cloud water combine in an optimum mode. The modification potential delineated by the model extends into colder cloud top temperatures for the higher upward speeds present in the Wolf Creek Pass area, and snowfall increases, were observed when seeding events within this intermediate tempera- ture range. j ~.- The modification potential delineated by the cloud physics model is generally verified by the results of the Climax and Wolf Creek Pass experi- ments. No increases of snowfall were observed when seeding events which the model indicated had suf- ficient natural ice nuclei. Large showfall increases were observed in the cloud top temperature range where the model indicated a deficiency of effective natural ice nuclei. The agreement' of the three independent samples with the model derived from current cloud physics knowledge validates the approach. It is envisioned that a similar model could be used on a real time basis to control opera- tional cloud seeding projects. The.main obstacle to such an approach would be the development of numeri- cal models that would yield acceptable vertical motion fields for the specific mountain barrier to be treated. Instrumentation will be needed which measures ice nuclei concentrations upwind and iC,e nuclei and ice crystal concentrations in the cloud system. This instrumentation should be amenable to rapid computer input. Serial rawindsondes, together with mou~tain top meteorological stations could supply the requ1red meteorological input to the model. The desired treatment for the cloud system could then be computed and instigated. The model might include not only the maximizing of the precipitation process but optimizing the distribution of the snowfall on the mountain barrier. The choice of mountain areas to be included in operational cloud seeding programs will probably depend significantly on their inherent modifi- cation potential. In order to intelligently define the modification potential for a specific area the follow- ing data is needed: Distributions of natural precipitation with cloud height and cloud top temperature Determine the relative importance of the diffusion and accretion processes 26