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Developments have expanded our abilities to understand and document precipitation processes. For example, <br /> the development of microwave radiometers and their application to the measurement of supercooled liquid <br /> water(SLW)has revealed the existence of low altitude SLW within winter orographic clouds above the <br /> crestlines of mountain barriers. Aircraft-based sensing platforms often fail to detect low-level SLW because <br /> aircraft cannot safely fly sufficiently close to the mountain barriers. Vertically pointing radiometers sited at or <br /> near barrier crestlines have often observed quantities of SLW sufficient to give useful additional snowfall if <br /> successfully precipitated by cloud seeding. <br /> Multiparameter radars,which can distinguish between liquid and ice-phase hydrometeors in clouds, offer new <br /> prospects for remote sensing of seeding signatures. Such radars have been successful in differentiating regions <br /> of graupel and hail from rainfall. <br /> Complementing such physical measurements, demonstration seeding trials have confirmed seeding effects from <br /> both aerial and ground-based releases. Observations of increased ice-particle concentrations in seeded air <br /> volumes also tagged with tracer gases such as sulfur hexafluoride have increased confidence that observed <br /> effects follow from efforts to stimulate precipitation. <br /> Furthermore,recent advances in computer hardware, software, and physical understanding have allowed <br /> improvements in two- and three- dimensional numerical models which simulate cloud processes. These models <br /> continue to provide an ever-improving understanding of the complicated interactions within both natural and <br /> seeded clouds.Numerical simulations are now able to replicate many of the details observed in actual clouds. <br /> The impacts on cloud and precipitation development resulting from slight variations in cloud microphysical or <br /> dynamic characteristics can be produced for the same cloud, for both natural and seeded circumstances. This is <br /> the only manner in which identical clouds can be studied in both treated and untreated versions, which may lead <br /> to more confident prediction of seeding effects and thus improved selection criteria for candidate clouds. <br /> Similarly, our ability to assess inadvertent modifications of our atmospheric environment has also improved. <br /> Atmospheric changes that might have passed unnoticed, or have been dismissed as inconsequential just a few <br /> years ago, are now often found to have broader ramifications. Some of these changes are quite subtle; for <br /> example, increased cloudiness associated with condensation trails from jet aircraft may modify the radiation <br /> budget at the ground. Others, such as acid rain are more obvious; structures, vegetation, and lake water quality <br /> have all been adversely affected. Air quality and visibility are often locally degraded by increased <br /> anthropogenic pollutants, and urban effects on temperature, humidity,wind, and precipitation have been well <br /> documented. <br /> Top of Document <br /> 2. Status of planned weather modification <br /> There is growing evidence that glaciogenic seeding(the use of ice-forming materials) can, under certain <br /> weather conditions, successfully modify supercooled fog, some orographic stratus clouds,and some convective <br /> clouds. Recent research results, utilizing both in situ and remote measurements in summer and winter field <br /> programs provide dramatic though limited evidence of success in modifying shallow cold orographic clouds and <br /> single-cell convective clouds. Field studies are beginning to define the frequencies with which responsive <br /> clouds occur within specific meteorological regimes. <br /> Successful treatment of any suitable cloud requires that sufficient quantities of appropriate seeding materials <br /> must enter the cloud in a timely,well-targeted fashion. As the need for stringent spatial and temporal targeting <br /> 2 <br />