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overhead passage of three of the seedlines. Two ground-based seeding experiments were reported that also resulted in substantial microphysical effects at aircraft altitudes, Light snowfall was observed to reach the mesa top from one of these experiments and surface snowfall was probably caused in the other. These experiments offer some of the most convincing physical evidence available that seeding winter orographic clouds can, under some conditions, result in precipitation increases at the surface. 1.2.7 Sierra Nevada Experiments. - Many attempts were made to conduct physical experiments during the course of the SCPP (Sierra Cooperative Pilot Project) in the Sierra Nevada of California (Reynolds and Dennis, 1986). A survey of 36 SCPP experiments is given by Deshler et al. (1990). A trajectory model was used to position the seeding aircraft so that resulting ice particles were calculated to settle on and around a well-instrumented target site well up the windward slope of the barrier. A cloud physics aircraft, weather radar, and aspirated ice particle imaging probe in the target were the primary means of documenting key links in the chain of physical events expected to follow seeding of the postfrontal stratus and stratocumulus clouds, The success rate for detecting seeding effects in the 36 experiments is stated as 35 percent for the aircraft, 4 percent for the radar, and 17 percent for the imaging probe on the surface. Only two cases were claimed to document all links in the chain of events resulting from seeding. The authors noted that, "A variety of reasons contributed to the poor success at measuring seeding effects." They pointed to factors such as the limited and variable SL W content of the clouds, the high na tural IPC (due to ice multiplication) masking increased concentrations of ice particles due to seeding, and the logistics of coordinating the various measurement platforms which sometimes led to long delays in initiating seeding. A more promising case study was recorded during the final SCPP field season (Deshler and Reynolds, 1990). A seedline of AgI was released by flying across the wind in a stratiform cloud with an acetone-fueled generator in operation. The research aircraft followed the seedline for over 90 minutes (100 km) with an acoustical ice nucleus counter, although passes between 19 and 61 minutes after seeding apparently were below the seedline. Enhanced IPC was associated with the seedline position from 64 to 90 minutes after seeding. Unfortunately, it was impractical to target the instrumented ground site and no indication of seeding effects was found in the precipitation gauge network, ~ f 1.2.8 Tushar Range Experiments. - Physical experiments were attempted on the Tushar Mountains of Utah during early 1989. Preliminary analysis of these experiments is discussed by Huggins and Sassen (1990). Silver-in-snow measurements suggest that the seeding material rarely reached the target. Examination of aspirated ice particle imaging probe data showed little success in demonstrating seeding effects at the intended target. The basic approach in the Tushar experiments was to operate one to three high-altitude AgI generators, basing targeting on upwind rawinsonde winds, in attempts to affect snowfall and cloud structure (revealed by radar, lidar and microwave radiometer) at and above the target. Generators were pulsed on and off at 1- to 2-hour intervals, and time changes in snowflake characteristics, precipitation rate, and snow silver content were expected to vary accordingly. In the absence of wind observations near the mountainous terrain, or aircraft tracking of the seeded volume, it seems unlikely that it will ever be determined where the AgI was transported. 1.2.9 HIPLEX Summer Experiment. - HIPLEX-1, while dealing with summer convective rather than winter orographic clouds, is worth noting because of the well-thought-out and detailed seeding hypothesis and well-designed field program, which monitored each key step in the physical 5