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or more above the target site. Yet a growing body of evidence shows that most of the SL W is concentrated in the lowest kilometer over the windward slope. Therefore, considerable ice particle growth can occur in this zone. This is also the zone in which most ground-released AgI is transported. So a kilometer-deep vertical "gap" between lowest aircraft observations and surface monitoring will cause considerable uncertainty in the growth, fallout and resulting targeting of ice particles resulting from seeding. For example, the early 1989 physical experiments over the Tushar Mountains (Huggins and Sassen, 1990) suffered from lack of aircraft observations because the rugged terrain made impractical low- level flight over the target site. The ideal mountain barrier for physical cloud seeding experimentation would allow surface sampling on the crestline, would have only marginally higher peaks near the target site, would not have an abrupt crestline which could create serious downwind turbulence, and would have nearby navigational aids such as a VORTAC station. Fortunately, portions of the mountains in the Sevier Basin approximate the ideal except for nearby navigational aids. Any physical experiment requires some targeting scheme to decide when and where to release the seeding material in the case of a fixed target, or when and where to operate the "mobile target" (usually sampling aircraft) in the case of fixed generator locations. The scheme may be no more complicated than using a typical wind velocity for the altitude range in question to estimate transport time, and typical growth rates and fall speeds for the type(s) of ice particles expected. Such approaches are sometimes referred to as "back of the envelope" calculations. On the other extreme, a highly sophisticated three-dimensional time- dependent numerical model may be run on a supercomputer to simulate the entire airflow pattern around the barrier and all important microphysical processes for expected ranges of conditions. Given the uncertainties in certain key processes and impracticality of detailed measurements around mountains, it is probably most reasonable to use a targeting model of modest sophistication that can be run on a computer in the field using real-time input data. The approach used by Rauber, et al. (1988) may be a good compromise. If resources permit, a highly sophisticated model should be run for several combinations of atmospheric conditions believed to cover the range of winter storm conditions in the region of interest. The resulting predictions should be in general agreement with the simpler operationally used scheme or the latter might require some modification. Another requirement for physical experiments is detection of either the seeding material or a simultaneously released tracer to document the seeded zone. Silver iodide can be tracked with an acoustical ice nucleus counter but dry ice is not traceable. Natural variations in IPC (ice particle concentration) can easily mask the ice particles caused by seeding unless the seeding material itself, or a tracer material such as sulfur hexafluoride (SFJ gas, be independently measured to distinguish the seeded volume from natural cloud. In other words, attempting to specify the seeded volume by monitoring ice particles alone may lead to uncertain interpretation of seeding effects; for example, see Deshler and Reynolds (1990). The most detectable characteristic of seeding at ground level is usually the IPC. Ice cryst'll sizes and habits, and silver content in the snow, are also suggestive that seeding affected the precipitation process. However, successful seeding usually substantially increases the concentration of ice crystals to levels well above background. Much of the IPC enhancement will likely be at small crystal sizes, less than a millimeter in diameter. Small seeded crystals are often in the form of hexagonal plates or small columns. An aspirated. particle imaging probe offers a practical means of continuously monitoring ioo crystal 37