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, , '. Wi".. 4 ;The conclusion just stated is based on the assumption that rainfall in- creases can be produced by seeding cumulus clouds. Seeding experiments de- signed to test that assumption have been conducted over the Great Plains and the Mississippi Basin for over 30 years (Coons and Gunn, 1951; Braham et al., 1957; Panel on Weather and Climate Modification, 1966, 1973; WMAB, 1978a, b). The experiments have increased in complexity, with each round of experiments featuring improved instrumentation, data handling, and cloud modeling work as compared to its predecessors. However, the physical hypotheses tested by the experiments have not changed much. Coons and Gunn (1951) considered speeding the coalescence of cloud drop- "" .' lets into raindrops by waterspray or hygroscopic agents and the initiation of the Bergeron Process in supercooled clouds by seeding them with dry ice or silver iodide. The Bergeron Process involves the growth of ice particles into precipitation embryos because of the difference in equilibrium vapor pres- sure over ice and over supercooled water surfaces, which causes simultaneous evaporation from supercooled droplets and deposition of water vapor on the ice particles. This paper describes the results of recent experiments designed to test some of the same physical hypotheses. HIPLEX The most recently completed program on the modification of cumulus clouds in the Central United States is HIPLEX. HIPLEX was undertaken by the Bureau of Reclamation in 1973 in response to a directive from the Office of Management and Budget. The general objective of HIPLEX was to reduce scientific uncertainties associated with attempts to increase rainfall by cloud seeding in the High Plains region. HIPLEX was a cooperative venture between the Federal Government and State governments. The Canadian Government, universities, and private firms also participated. HIPLEX in- cluded economic and environmental studies in addition to work on methods for modifying cumulus clouds. Fieldwork was conducted in Texas, Kansas and Montana. HIPLEX made extensive use of newly available equipment and techniques. Notable among them were PMS (particle measuring systems) probes on aircraft for measur- ing cloud and precipitation particles, computers on aircraft for real-time analysis of cloud situations, recording of radar data on tape, mesoscale net- works of automated weather stations reporting via satellite relay stations, and advanced numerical cloud models for simulating seeding experiments in computers. HIPLEX-1, a randomized experiment conducted in the Miles City, Mon- tana area during the summers of 1979 and 1980, is of particular interest. It was designed to test the applicability of a specific physical hypothesis to iso- lated cumulus congestus clouds in the Miles City region. The hypothesis in- volved the formation of ice crystals by seeding with dry ice, growth of some