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<br />1. Introduction <br /> <br />Ice phase seeding for microphysical effects, the so-called static mode <br />seeding hypothesis, has been the physical basis for most precipitation <br />enhancement programs on convective clouds. Evaluations of the results of <br />these programs, mainly statistical in nature, report precipitation <br />increases in a few cases, precipitation decreases in a few cases, and no <br />significant change in precipitation in most cases (Dennis, 1980; 1984). A <br />few of these programs had a physical component that attempted to gain an <br />understanding of natural precipitation processes and their response to gla- <br />ciogenic seeding. This paper focuses on the observations and results of <br />these physical studies in an assessment of the state of knowledge of the <br />static mode seeding hypothesis for convective clouds. <br /> <br />The central thesis of this review is that the results of past experimental <br />work are diverse but valid and that credibility of the science depends on <br />understanding the physical reasons for the diverse results. Consequently, <br />areas of uncertainty and apparent conflicts in evidence are highlighted in <br />an effort to identify what issues need to be resolved to further progress <br />in precipitation enhancement research and applications. The review begins <br />with a discussion of the evolution of the physical hypothesis. Using the <br />physical hypothesis as a framework for further discussion, research fin- <br />dings related to the selection of suitable clouds, seeding agents and their <br />delivery to the clouds, and the chain of physical events following seeding <br />are presented. Some of the key issues raised in this review paper are <br />discussed in greater detail in other papers in this volume. <br /> <br />1 <br />