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<br />1. The investigation was carried out in successive stages, with a) an <br />initial exploratory stage to develop an understanding of the <br />clouds of the area, upon which the design of a physically meaning- <br />ful seeding experiment could be based, b) preliminary calibration <br />seeding trials to develop a methodology for executing the seeding <br />operation as required by the physical hypothesis, and c) an <br />exploratory randomized seeding experiment (HIPLEX-l) on indivi- <br />dual, small clouds to test the seeding hypothesis. It was planned <br />that follow-on experiments would be conducted that would move up <br />the scale to larger cloud systems as increased understanding per- <br />mitted, leading ultimately to an area seeding experiment; however, <br />this is not yet possible for both scientific and support reasons. <br /> <br />2. A detailed multi-step physical hypothesis was formulated with <br />emphasis on expected differences between seeded and non-seeded <br />clouds. For each step of the hypothesis, response variables <br />capable of being measured were defined, so that the expected dif- <br />ferences could be monitored. For the response variables asso- <br />ciated with each step, indications were provided of how, when and <br />where the seed/non-seed differences were to be measured. This <br />aspect is particularly important because it provides for deter- <br />mining not only whether the seeding produces increased rainfall, <br />but also whether the physical mechanism is correctly understood. <br /> <br />3. Physical measurements of the characteristics of candidate clouds <br />were made and analyzed in real-time to determine their suitability <br />as experimental units. <br /> <br />4. Provisions were made for a statistical evaluation and a complemen- <br />tary physical evaluation of the experiment. The statistical eva- <br />luation of the physically based experiment focused on establishing <br />proof changes due to seeding and on verifying the hypothesized <br />physical sequence of events leading to the changes (Mielke et al., <br />1984). The physical evaluation focused on case studies to <br />establish the cause-and-effect relationships, or lack thereof, of <br />any seeding-produced modifications to the cloud and its physical <br />mechanisms (Cooper and Lawson, 1984). <br /> <br />Because of the experimental approach used in HIPLEX-l, it was possible to <br />determine where, how and why seeded clouds behaved differently from natural <br />clouds and, more important, behaved differently than hypothesized. It was <br />possible to provide a better understanding of basic cloud processes and <br />their response to seeding in spite of the limited data set that was <br />available. <br /> <br />7. Future <br /> <br />The SCPP, CRADP and CREST programs are based on the experimental strategy intro- <br />duced by HIPLEX-l. Indeed, we have probably seen the last of the so..called <br />"black-box" experiments which try to relate cloud seeding to precipitation on <br />the ground while ignoring the complex chain of intermediate processes. Our ina- <br />bility to understand the physical reasons underlying the FACE-1 and FACE-2 <br /> <br />11 <br /> <br />---1__ <br />