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potential for enhanced precipitation in coastal regions appearing to be greater in <br /> convective cloud regimes. The consistent range of indicated effects in many <br /> regions suggests fairly widespread transferability of the estimated results. <br /> Technological advances have aided winter precipitation augmentation <br /> programs. Fast-acting silver iodide ice nuclei, with higher activity at warmer <br /> temperatures, have increased the capability to augment precipitation in shallow <br /> orographic cloud systems. Numerical modeling has improved the understanding <br /> of atmospheric transport processes and allowed simulation of the meteorological <br /> and microphysical processes involved in cloud seeding. Improvements in <br /> computer and communications systems have resulted in a steady improvement <br /> in remotely controlled cloud (ice) nuclei generators (CNG's), which permit <br /> improved placement of CNG's in remote mountainous locations. <br /> Wintertime snowfall augmentation programs can use a combination of <br /> aircraft and ground-based dispersing systems. Although silver iodide <br /> compounds are still the most commonly used glaciogenic (causing the formation <br /> of ice) seeding agents, dry ice is used in some warmer (but still supercooled) <br /> cloud situations. Liquid propane also shows some promise as a seeding agent <br /> when dispensers can be positioned above the freezing level on the upwind <br /> slopes of mountains at locations adequately far upwind to allow growth and <br /> fallout of precipitation within the intended target areas. Dry ice and liquid <br /> propane expand the window of opportunity for seeding over that of silver iodide, <br /> since they can produce ice particles at temperatures as warm as -0.50 C... For <br /> effective precipitation augmentation, seeding methods and guidelines need to be <br /> adapted to regional meteorological and topographical situations. <br /> Although traditional statistical methods continue to be used to evaluate <br /> both randomized and non-randomized wintertime precipitation augmentation <br /> programs, the results of similar programs are also being pooled objectively in <br /> order to obtain more robust estimates of seeding efficacy. Objective evaluations <br /> of non-randomized operational programs continue to be a difficult challenge. <br /> Some new methods of evaluation using the trace chemical and physical <br /> properties of segmented snow profiles show considerable promise as possible <br /> means of quantifying precipitation augmentation over basin-sized target areas. <br /> Summer Precipitation Augmentation <br /> The capability to augment summer precipitation from convective clouds <br /> has been reasonably well demonstrated. Assessments of some operational and <br /> research programs that have seeded selected individual clouds or clusters of <br /> clouds with either glaciogenic or hygroscopic nuclei have found that seeded <br /> clouds tend to last longer, expand or travel farther to cover larger areas, and are <br /> more likely to merge with nearby clouds and produce more precipitation. Both <br /> dynamic and microphysical changes appear to be involved. <br />