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made above the plateau (Doppler acoustic sounder or profiler), but some estimation errors will still exist. Silver-in-snow concentrations sometimes will be measured at each gauge site to test the estimation method for deciding whether particular gauges were seeded or not. Relatively high relationships can be expected during nonseeded periods between target gauges along the center (target) portion of each line, and control gauges near the north and south ends of each line. The terrain is reasonably unifonn and the distance along each gauge line will be limited (about 6 miles along the windward edge and about 15 miles along the lee edge line). Relationships established during nonseeded periods with similar meteorological conditions will be used to estimate precipitation at target gauges during physical seeding experiments. Any significant differences from that predicted will be considered due to the seeding. Many nonseeded periods used to establish target-control gauge relationships will be from nighttime when safely considerations preclude low-level aircraft flight. While daytime in-cloud flight also relies on aircraft instruments, emergency landing is much safer during daylight Conventional weighing precipitation gauges, modified with orifices providing 5 times the area of 8-in diameter gauges, have been successfully used on the Grand Mesa of Colorado and the Mogollon Rim of Arizona. They provided a resolution of about 0.002 in (0.05 mm), and, with daily rotation gears, a time resolution of about 5 min. Both should be adequate when seeding produces significant snowfall so similar gauges will be used on the Wasatch Plateau to monitor physical seeding experiments. The duration of a physical seeding experiment is partially detennined by the temporal stability of the atmosphere and partially by the practical flight duration of an instrumented aircraft. The latter is about 3 hours on-station time. Thus, a typical seeding experiment will release AgI for about 2 hours, commencing shortly before arrival of the sampling aircraft. The aircraft will continue to sample after generator turnoff to document changes in cloud microphysics as the concentration of seeding material decreases with time.. This approach, used by Super and Heimbach (1988), can provide convincing evidence of changes due to seeding in both space and time. It is important that measurements be made to monitor natural temporal changes in cloud conditions that are unaffected by seeding. These observations help correctly identify both natural and seeding-caused changes in IPC, precipitation rate, etc. Of course, the purpose of the control gauges is to monitor natural changes in snowfall with time. However, radar coverage of the experimental area also can be helpful by documenting the passage of any mesoscale features (e.g., "precipitation bands") during the course of an experiment Monitoring available SL W is very important because its presence is necessary for cloud seeding to be effective. A physical seeding experiment would not commence unless SL W were present. Moreover, any natural changes in SL W during the course of an experiment should be known for proper interpretation of results. A zenith-pointing microwave radiometer should be operated on top the plateau, crosswind of the seeded zone, to provide a time history of SL W amount throughout each experiment If a second radiometer were available, it should be located in the seeded zone as often as practical to search for reductions in SL W caused by seeding. A truck-mounted radiometer, such as used in the 1991 field effort, could be driven back and forth along the upwind highway to sample both natural and seeded cloud. Effective seeding should reduce the SL W in the seeded zone. It is estimated that about three winter field programs, each 4 to 5 months long, would be required for the physical experiment phase. Analyses of the many experiments conducted during that time should provide 51