Arizona Weather Modification Research Program

The paper presented here is an interim progress review for the period ending with the fourth summer season at Flagstaff. In 1967, randomized seeding of isolated cumulus clouds was the main experiment. A simple computer model served successfully for test cloud selection, and evaluation of the results. This experiment demonstrated statistically significant cloud-top height increases averaging 6,000 ft and approximate doubling of precipitation. Cloud interiors were probed with an instrumented aircraft. In contrast to events in unseeded clouds, the observations in seeded cumuli showed rapid spread of glaciation throughout the clouds. Subsequently, additional buoyancy and organization of the upcurrent, and early shutting off of precipitation growth were noticed. The study in 1967 also involved exploratory trials with the seeding of larger storm systems, in order to organize and stimulate them. In 1968, a program called COOP II brought together the capabilities of several groups to continue the studies with special emphasis on the storm systems and on the optimizing of seeding techniques.