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<br />Unstable clouds with the same moisture, temperature, and trajectory <br />characteristics showed increases in all three areas. Unstable clouds <br />with low cloud water, cold cloud tops, and a blowover trajectory showed <br />decreases'in all three areas. Under these conditions, the precipita- <br />tion particles are probably lifted high in the clouds and carried down- <br />wind where they evaporate in dry air. <br /> <br />IV. Future Activities <br /> <br />Although the four variables used thus far in the search for generalized <br />seedability criteria identified definite windows, much work remains to <br />be done. A number of computed variables that could have been used in <br />this study were not sufficiently investigated. A review of these vari- <br />ables, and promising new variables, will insure that the best predictors <br />of seeding effects are identified. <br /> <br />Window bounds need to be refined so that positive and negative seeding <br />effects can be identified more clearly. This will involve trade-offs <br />among the maximum size of the window, the magnitude of the seeding <br />effect, and the certainty of the effect. Modification to windows may <br />be required to meet operational constraints. <br /> <br />After seeding windows have been clearly defined, the expected effect <br />must be translated into an estimate of total precipitation for a given <br />area. The percentage increase in precipitation associated with a given <br />window must be weighted against the frequency of clouds within that <br />window and the contribution they make to total seasonal precipitation. <br />Even small seeding-induced increases for individual events can provide <br />relatively large increases in total precipitation for the winter season <br />if those clouds occur very frequently. Judgments about the usefulness <br />of a given window may be made more easily from a climatological view- <br />point. Its practical value is also dependent on how accurately it can <br />be forecast. <br /> <br />12 <br /> <br />GPO 840-624 <br />