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ARCHIE M. KAHAN i ~ A further analysis of the same data by Charles Chappell, also of Colorado State University, brought out the importance of wind velocity during seeding. Precipitation increases were shown to be greatest when the wind velocity ranged between 22 and 28 meters per second. The decreases observed were associated with winds of 10 meters per second or less. The cold cases tended to be the ones having light winds. The power of relatively simple mathematical models of cloud behavior in guiding field experi- mentation has been demonstrated quite recently. A one-dimensional model developed by Professor L. G. Davis and Mr. Alan Weinstein at Pennsylvania State University has been applied by them in the field at Flagstaff, Arizona, in connection with the Bureau of Reclamation sponsored program conducted by Meteorology Research, Incorporated. With the help of the mathematically derived prediction of how clouds should develop if seeded or if left unseeded, it has been possible to conduct more meaningful experimentation. Increases in cloud height and precipitation amount due to seeding have been clearly demonstra- ted. A similar mOdel employed by Dr. Joanne Simpson of ESSA in guiding the seeding of cumulus clouds in the vicinity of Miami, Florida, resulted in preCipitation from individually seeded cumulus clouds than unseeded cases. Fourteen randomly selected seeded clouds produced an average of 237 acre-feet per cloud as measured by a calibrated radar. Five unseeded cases averaged 97 acre-feet. This , represents an increase of 144 percent due to seeding. These models also give warning of what clouds should go unseeded if you would avoid precipitation decreases. Some past seeding efforts would have benefited had present models been available. A third area of important recent progress worthy of special mention has to do with an increas- ing ability to pin down what happens to silver iodide once it has been released by the generating device. Detection of plumes of seeding nuclei is now possible in three dimensions, thanks to a continuous nuc- lei counter developed at the National Center for Atmospheric Research and manufactured by E. Bollay Associates. A number of these are in use in aircraft and at ground installations throughout Project Skywater. Using one of these counters, the University of Wyoming has been able to show the relation- ship between plume width and the stability of the atmosphere, In Figure 2, one can see how narrow a seeding plume (80 to 90) is observed when lapse rates attain values a,ssociated with unstable condi- tions. The broad plumes (30 degrees for example) that were frequently formerly assumed to exist gen- erally appear to be confined to the more stable conditions. This relation provides quidance in the design of seeding experiments and operations alike. Another facet of progress is the detection of seeding material in samples of precipitation. A method developed by Professor J .A. Warburton at the University of Nevada employes neutron activ- ation analysis to determine the concentration of silver present in carefully taken snow and rain sam- ples. Silver concentrations in samples taken from several unseeded storms were about 20 x 10-12 grams of silver per milliliter of precipitation. In seeded storms, silver concentrations ranged from 20 to 200 x 10-12 grams of silver per milliliter. This is a concentration of from 0.02 to 0.2 parts per billion. This sensitive method permits determination of where silver iodide was present while precipitation was in progress. While the presence of silver in precipitation does not prove that silver iodide played a role in producing the precipitation, it does provide a means of differentiating between parts of a storm that were seeded and parts that were not. Preliminary indications from an experiment in the Park Range Mountains of Colorado suggest that there is association between the larger silver concentrations and the larger precipitation amounts. One of the frequently voiced concems about cloud seeding has to do with downwind effects, Many people are prone to draw on the analogy of upstream appropriation of streamflow to conclude that upstream cloud seeding must produce downwind decreases to compensate for upstream increases. A study by Keith Brown and Robert Elliott of Aerometric Research, Incorporated has examined this question. The results of looking at precipitation records for stations downwind from seeding projects indicated centers of precipitation increase extending approximately 100 miles downwind. An analysis by E. E, Adderley of rainfall experience downwind from cloud seeding projects in Australia shows similar extensive areas of increase. Professor Braham's results in Project Whitetop in Missouri show radar echo increases, then decreases, then increases downwind. While none of these studies can be said to answer the question of downwind effects for all circumstances, they certainly suggest that the frequent assumption of extensive downwind decreases is not well supported by exper- imental data collected to date. -35-