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has been established, it has become apparent that problems with seeding plume delivery in many early <br /> experiments may in part account for the failure of such programs to produce significant results. <br /> a. Fog and stratus removal <br /> Operations employing glaciogenic seeding to dissipate supercooled fog and low stratus have become routine at <br /> some airports. The ability to admit more solar radiation to reduce heating requirements through the dissipation <br /> of such clouds and fogs appears promising. <br /> The dissipation of warm(nonsupercooled) fogs can often be accomplished by more expensive thermal <br /> techniques,but this has proven cost effective at only a few major airports. More reliable and economical warm- <br /> fog dissipation techniques have not yet been established. <br /> b. Precipitation increase <br /> There is considerable evidence that,under certain conditions,precipitation from supercooled orographic clouds <br /> can be increased with existing techniques. Statistical analyses of precipitation records from some long-term <br /> projects indicate that seasonal increases on the order of 10%have been realized. The cause and effect <br /> relationships have not been fully documented; however,the potential for increases of this magnitude is <br /> supported by field measurements and numerical model simulations. Both show that SLW exists in amounts <br /> sufficient to produce the observed precipitation increases and could be tapped if proper seeding technologies <br /> were applied. The processes culminating in increased precipitation have recently been directly observed during <br /> seeding experiments conducted over limited spatial and temporal domains. While such observations further <br /> support statistical analyses,they have to date been of limited scope, and thus the economic impact of the <br /> increases cannot be assessed. <br /> Recent experiments continue to suggest that precipitation from single-cell and multicell convective clouds may <br /> be increased, decreased, and/or redistributed. The response variability is not fully understood,but appears to be <br /> linked to variations in targeting,cloud selection criteria, and assessment methods. <br /> Heavy glaciogenic seeding of some warm-based convective clouds (bases about+10°C or warmer)can <br /> stimulate updrafts through added latent heat release (a dynamic effect), and consequently increase precipitation. <br /> However,convincing evidence that such seeding can increase rainfall over economically significant areas is not <br /> yet available. <br /> Seeding to enhance coalescence or affect other warm-rain processes within clouds having summit temperatures <br /> warmer than about 0°C has produced statistically acceptable evidence of accelerated precipitation formation <br /> within clouds,but evidence of rainfall change at the ground has not been attained. <br /> Although some present precipitation augmentation efforts are reportedly successful, more consistent results <br /> would probably be obtained if some basic improvements in seeding methodology were made. Transport of <br /> seeding materials continues to be uncertain,both spatially and temporally. Improved delivery techniques and <br /> better understanding of the subsequent transport and dispersion of the seeding materials are needed. Current <br /> research using gaseous tracers such as sulfur hexafluoride is addressing these problems. <br /> There are indications that precipitation changes, either increases or decreases, can also occur at some distance <br /> beyond intended target areas. Improved quantification of these extended(extra-area) effects is needed to satisfy <br /> public concerns and assess hydrologic impacts. <br /> 3 <br />