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003192 winter orographic cloud temperatures as warm as 0 oC appears feasible (Reynolds, 1989; Reynolds, 1991), so many winter stonns too warm to treat with ground-released AgI may be seedable with propane. Greater appreciation now exists of the logistics involved in conducting seeding experiments over mountainous terrain, especially in regard to the need for low level measurements by aircraft and remote sensing systems. Experimental areas must be chosen that pennit aircraft observations into the lowest kilometer above the terrain where most SLW is concentrated and where ground-released seeding plumes are found. Evidence that most SLW is located near mountain swfaces includes observations (Hobbs, 1975a; Holroyd and Super, 1984; Hill, 1986; Thompson and Super, 1987; Heggli and Rauber, 1988) and results of numerical model runs (Young, 1974; Bli.unenstein et aI., 1987). Experimental areas should also pennit reasonable swface access to the barrier top, where a variety of sensing systems must be operated. Recent improvements in instrumentation have made it practical to monitor the key physical processes involved in cloud seeding. For example, microwave radiometers can remotely monitor SLW above mountain barriers. Two-dimensional laser imaging probes can be used on aircraft and on the ground to observe and analyze vast numbers of ice particles. Tracer gas and radar chaff can be released and tracked to monitor positions of seeding plumes and resulting ice crystals. Doppler radars can provide wind fields over mountain barriers. Wind-profiling radars using the RASS (radio acoustic sounding system) technique can continuously monitor vertical profiles of wind and virtual temperature. Automatic weather stations can provide swface measurements of wind, temperature, and moisture over mountain barriers. Chemical analysis methods are sensitive enough to detect silver in snow from AgI seeding. , ~ l Reynolds (1988), in a review of winter snowpack augmentation, showed that a consistent relationship is emerging between physical studies and statistical results. For example, Reclamation and other scientists have provided convincing evidence that the physical seeding hypothesis was correct in a limited number of experiments in recent years. Super and Heimbach (1988) confirmed microphysical changes in seeded clouds over the Bridger Range that presumably increased snowfall (no swface observations were made in the limited 4-week study). Super and Boe (1988b) showed evidence of precipitation changes at aircraft sampling levels and on the swface during a 2-mo study period over the Grand Mesa, Colorado. Deshler et aI. (1990) demonstrated seeding-induced microphysical changes at aircraft levels in about 35 pct of their experiments, but, as previously noted, following seeding effects to the ground proved difficult in the Sierra Nevada Deshler and Reynolds (1990) presented a case study in which the effects of aerial seeding were followed for over 90 min and 100 km. These direct detection physical experiments have been encouraging. However, too few have been conducted to demonstrate how often stonn conditions permit the seeding hypothesis to operate, or how much additional snowfall might result from routine seeding. Moreover, the technology exists to design improved direct detection experiments to better determine appropriate seeding rates and approaches for given cloud conditions. Improvements in numerical modeling of winter orographic clouds have significantly aided the understanding of airflow and microphysical processes (Young, 1974; Cotton et aI., 1986; Bruintjes et aI., 1992). Incorporating observations from a particular mountain region into a numerical model adapted for that region can markedly increase understanding of the key processes involved and how seeding influences them. Numerical model simulations can be run with each set of intensive observations to continually improve experimental design, and to test ~ 9