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Background Since the weather is constantly changing, measuring the effects of cloud seeding is difficult. Even the long-term average is not conslant because there are normally "dry" and "wet" decades - it is normal fOf the weather to be abnormal! The hypothesis of the weather modification experiment is thaI cloud seeding will produce a change in nalural precipitation of 10 10 25 percent. Natural variations in precipitation, however, range from 50 to 300 percent of the long-term average. Therefore, the difficulty in assessing the effectiveness 01 cloud seeding lies in separating it from lhese widely ranging nalural varia- tions. Depending upon the size of the seeding ellee! and gi'Jen OUf current difficulty in measuring hail, statis~ tical studies of these variations have indicated 30 or more years of seeding would be necessary to be confident of a 25 percent suppres- sion effect. How then should one evaluate cloud seeding? The past decade of seeding has indicated the need Summer rainfall 1979.1983 1974.1978 - -.......... ."20'1._""""" 0,,20'10__ . . . . ~ ; . , '" Weather patterns vary widely, as illustrated in comparing averages for these five-year periods, making assessment of seeding effects difficult. for emphasis on measurements, physical observations and experi. ments in controlled conditions to track down what is, or is not, hap- pening inside the clouds following seeding. The precipitation process chain was developed to tie the observa- tions together in an orderly fashion so thtlt knowledge gained in one "link" could be used to study adja. cent "links." In past evaluations, researchers tried to bypass under- standing the complex processes in. volved and use statistical inferences to jump from one end of the chain 10 the other. Now the researchers have found thaI, in addition to understanding each "link," they must know how the links are related and how changes produced by seeding in one "link" affect the next one. , . g. l'll0l10 " . , E ~ . -~ ~~ e~ " I ~ Cloud seeding ~ III. IIl.JI l.d.u.hlll..1I ...luJ .Il .... ,... 19BO Annual haiJ insurance losses. as a percentage of hail insurance purchased within the central Alberta experimental region, further indicate the wide variability of weather patterns before. as well as since. cloud seeding began.