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OD3188 The most recent weather modification policy statement of the 10,000 member AMS (American Meteorological Society), published in the March 1992 issue of the Bulletin of the AMS, indicates that scientific proof remains elusive. The policy statement notes that, "There is considerable evidence that, under certain conditions, precipitation from supercooled orographic clouds can be increased with existing techniques. Statistical analyses of precipitation records from some long-term projects indicate that seasonal increases on the order of 10 pct have been realized. The cause and effect relationships have not been fully documented; however, the potential for increases of this magnitude is supported by field measurements and numerical model simulations." The policy statement further notes that, "The processes culminating in increased precipitation have recently been directly observed during seeding experiments conducted over limited spatial and temporal domains:' These brief seeding experiments which affected small areas were similar to, but less comprehensive than, the direct detection experiments discussed in section 4. The newest policy statement of the WMO (World Meteorological Organization), affiliated with the United Nations, was approved in July 1992. Both the AMS and WMO policy statements were prepared by panels of experts, the latter from several countries worldwide. The WMO statement notes that, "In our present state of knowledge, it is considered that the glaciogenic seeding of clouds or cloud systems either formed, or stimulated in development, by air flowing over mountains offers the best prospects for increasing precipitation in an economically viable manner:' The policy statement notes that statistical analyses suggest 10 to 15 pct seasonal precipitation increases in certain project areas. The statement continues, "Physical studies using the new technology highlighted above, give convincing evidence of the production of an effective seeding agent, the tracing of the agent to SL W portions of the cloud, the initiation and development of ice crystals to precipitation size particles, and the fallout of additional precipitation on mountain slopes in favorable situations over limited areas. Numerical simulations of the process corroborate the physical studies. This does not imply the problem of precipitation enhancement in such situations is solved. Much work remains to be done in pursuit of the goals of strengthening the results and producing incontrovertible statistical and physical evidence that the increases occurred over a wide area, over a prolonged period of time and with minimum, or positive, extra area effects." The goal of the CREST is to provide such statistical and physical evidence in the Basin, , I I The policy statements are markedly more positive than previous AMS and WMO statements, largely because of recent direct physical evidence from a variety of new observing systems. This evidence comes from a limited number of brief (1 to 3 h) experiments from a few locations as discussed later. The general seeding hypothesis for winter orographic clouds apparently applies to some cloud conditions, which is certainly encouraging. However, as suggested by the policy statements, convincing physical evidence is needed to demonstrate that area-wide precipitation can be increased in an economically viable manner over the course of several winter seasons. A need to provide such convincing scientific evidence clearly exists before the emerging technology will gain wide acceptance as a practical option for water resources management. 2.2 Overview of Past Investigations Scientists have been aware of the key processes in the general winter orographic cloud seeding hypothesis for decades (e.g., Ludlam, 1955). These processes are known to operate as expected when meteorological conditions are suitable. Silver iodide ice nuclei will definitely create high concentrations of ice crystals if introduced to sufficiently cold SLW clouds. Some of these ice 5