This research continues the exploration of improved cloud seeding technology through the use of airborne seeding, cloud physics measurements from a research aircraft and ground based measurements for the purpose of understanding transport and diffusion of seeding material, developing seedability criteria, and documenting in-cloud responses to seeding. Development of precipitation measuring and memory devices was continued with the aim of obtaining a network of gages well suited for measurement of winter snowfall in mountainous regions. Results of the research are highlighted by the development of criteria for seeding winter orographic storms. It was found that the cloud top temperature and vertical motion apparently are the primary factors governing seedability. Indices of vertical motion are also described, so the seedability criteria can be measured readily in operational type projects. It was found that silver iodide released from aircraft upwind of the target area did not diffuse very well by the time the plumes arrived over the target area about an hour later, so that overseeding occurred inside the plumes, and outside the plumes the clouds were underseeded. Precipitation measurements in a target area and other locations as possible controls indicate that correlations of around 0.8 or 0.85 probably could be obtained with suitably placed gages, with the consequence that the duration of an experiment to verify a set of seeding criteria would be reduced by a factor of 3 or 4 compared to what would be required in the absence of controls.
Hill, Geoffrey E., "Research on Increased Winter Orographic Precipitation by Cloud Seeding (FY 1979) Development of Cloud Seedability Criteria" (1979). Reports. Paper 424.