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The design and conduct of HIPLEX-1, a randomized seeding experiment carried out on small cumulus congestus clouds in eastern Montana, are outlined. The seeding agent was dry ice, introduced in an effort to produce microphysical effects, especially the earlier formation of precipitation in the seeded clouds. The earlier formation was expected to increase both the probability and the amount of precipitation from those small clouds with short lifetimes. The experimental unit selection procedure, treatment and randomization procedures, the physical hypothesis, measurement procedures and the response variables defined for the experiment are discussed. Procedures used to calculate the response variables from aircraft and radar measurements are summarized and the values of those variables for the 20 HIPLEX-1 test cases from 1979 and 1980 are tabulated.

The Sierra Cooperative Pilot Project (SCPP) is an investigation of cloud seeding as a means of increasing winter precipitation on the Sierra Nevada. It is a concerted effort in the development of a physically sound cloud-seeding technology. It involves the use of remote-sensing devices, in situ observations, and the application of a numerical targeting model in randomized seeding experiments. The results have led SCPP scientists to believe that shallow but widespread orographic clouds provide the best opportunity for glaciogenic seeding in the central Sierra Nevada.

Results of statistical analyses for HIPLEX-1, a randomized cloud seeding experiment, are presented. The analyses are based principally on multi-response permutation procedures (MRPP) as specified before the HIPLEX-1 experiment was initiated. Even though the sample sizes are very small, due in part to the premature termination of this experiment, the three primary response variables measured in the first five minutes following treatment indicate pronounced differences in the development of ice crystals between nonseeded and seeded events. However, the response variables measured more than five minutes after treatment generally do not indicate obvious differences in the subsequent development of precipitation between nonseeded and seeded events. This lack of difference is a possible consequence of 1) lack of a seeding effect, 2) inadequacies in the physical hypothesis, or 3) the small sample sizes. Consequently, only the initial steps in the HIPLEX-1 physical hypothesis could be confirmed in this e...