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IV. SUPPRESSION OF WEATHER HAZARDS 615 hail suppression will be available in the near fu- ture. It is unclear why the apparent results from the Soviet Union and in some other operational programs have not been duplicated in the ran- domized experiments in Switzerland and the United States referred to earlier. Possible rea- sons for the discrepancies include the small sizes of the experimental areas used, which did not provide very large samples of test cases, dif- ferences in procedures for declaring hail threats and conducting the seeding operations, and vari- ations in the characteristics of storms in differ- ent parts of the world. A final possibility, which has been rejected vigorously by Soviet scientists at international meetings, is that the target-con- trol analyses performed in the Soviet Union paint an overly optimistic picture of the results actually being obtained in that country. D. LIGHTNING SUPPRESSION Attempts to suppress lightning have generally taken one of two forms. The first is to seed clouds in an attempt to modify the microphysi- cal properties in such a way as to reduce the amount of electric charge separation taking place and thereby reduce the frequency of light- ning strikes. The second is to seed the clouds with radar chaff in the hope that the chaff nee- dles will provide enough corona discharge to neutralize the strong electric fields giving rise to lightning discharges. Experiments using the former concept have been carried out in several countries. For sev- eral years, the U.S. Forest Service conducted Project Skyfire in western Montana to test the effects of silver iodide seeding from ground gen- erators on the frequency and characteristics of lightning discharges. The statistical analyses of this randomized program provided some evi- dence of a decrease in the frequency of the types of cloud-to-ground discharges considered most likely to ignite forest fires. However, little fol- low-up action has been taken. Scientists in the Soviet Union have also studied lightning sup- pression, and a number of projects conducted for other purposes, for example, hail suppres- sion, have been analyzed in terms of the fre- quency of lightning discharges. This work has not led to any conclusive or widely accepted results so far. The concept of chaff seeding to preempt natu- ral lightning strikes has been explored in theo- retical calculations and in a few limited field ex- periments. The idea appears to have some merit, but insufficient field trials have been carried out to permit any statement to be made regarding its potential effectiveness. A complete model of the effects of cloud seed- ing on lightning discharges requires analysis with a numerical model capable of simulating the various charge separation processes active in thunderstorms. As scientists still are not sure of the nature of the several mechanisms hypoth- esized to be at work, nor of their relative impor- tance, the seeding conducted so far must be con- sidered as empirical experiments. E. SUPPRESSION OF THUNDERSTORM WINDS It was hypothesized early in the history of weather modification that both the weakening of convective storms by premature release of pre- cipitation through cloud seeding and the shading of the ground by artificially generating large cir- rus anvils would reduce the severity of thunder- storm winds. It was thus hypothesized that seeding thunderstorms in the vicinity of crops susceptible to wind damage would be beneficial. Among crops most susceptible to wind damage are bananas. Therefore, a number of operational programs were conducted in Central and South America during the 1950s and 1960s to reduce wind damage to banana plantations. Data pre- sented by the operators painted an optimistic picture of the results, but the programs have been discontinued in most of the countries af- fected. F. SUPPRESSION OF TORNADOES There have not been any cloud seeding exper- iments intended to modify the behavior of torna- does. Before such an experiment could be un- dertaken, it would be necessary to derive numerical models capable of predicting tornado occurrence and explaining their energy sources and sinks. At the present time there are no ac- tive tornado suppression programs anywhere in the world. G. MODIFICATION OF HURRICANES: PROJECT STORMFURY In 1947, as a part of Project Cirrus, dry ice was dropped into convective clouds associated with a tropical hurricane off the east coast of the United States. In reporting the event, Langmuir stated that it would some day be possible to af-