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5.0 ASSESSMENT OF SEEDING EFFECTS 5.1 Backl!:round Evaluating the results of a cloud seeding program is unfortunately rather difficult. The seemingly simple problem of determining the effects of cloud seeding has received considerable attention over the years. The primary reason for the difficulty stems from the large natural variability in the amounts of precipitation that occur in any given area. Since cloud seeding is only feasible when there are clouds and usually only when there are clouds that are near to or are producing precipitation naturally, the question then becomes, "How much did the seeding increase the precipitation that was observed over that which would have occurred naturally?". The ability to detect a seeding effect becomes a function of the size of the seeding increase compared to the natural variability in the precipitation pattern. Larger seeding effects can be detected more easily and with smaller numbers of seeded cases than are required to detect small increases. Historically, the most significant seeding results have been observed in wintertime seeding programs in mountainous areas. However, the apparent differences due to seeding are relatively small, being on the order of a 5-15 percent seasonal increase. In part, the relatively small percent increase accounts for the significant number of cases required to establish these results (often five years or more). In spite of the difficulties involved, there are techniques available to evaluate the effects of operational seeding programs. These techniques are not as rigorous or scientifically acceptable as is the randomization technique used in research, where rougWy one half the sample of storm periods is randomly not seeded. They do, however, offer the potential of at least establishing an indication of the effects of seeding on operational programs. 5-1