Assessment of Summer 1979 Weather Modification Effort Southeastern Illinois

An operational cloud seeding project took place in southeastern Illinois during a 40-day period in the summerof 1979. A historical target-control evaluation design, using NWS raingage data, indicated an estimated rainfall increase of 39.5% in the target. Three statistical evaluation techniques - principal component regression, multiple regressions and double ratio, were employed and compared. The (randomization) significance levels were .10, .26, and .26, respectively. The possibility of the east control being contaminated was studied and found rather unlikely. Investigation of the 1979 isohyetal pattern, based on a 92-raingage network data revealed that within the target there were wide extremes, from very low to very heavy, and a localized high existed in the target on seeded rain occasions which was not present on no-seed rain occasions.