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graupel). Aggregation is yet another growth process for ice crystals in which many small crystals chain together into large snowflakes. The concentration of ice nuclei required to optimize seeding is a complex function of SLW availability, , temperature and time dependence of crystal nucleation by the ice nuclei, ice crystal growth and fallout rates, distribution of SL W relative to the topography, and other factors. These complexities are best dealt with in a numerical model. However, seeding usually must be able to create at least several ice crystals per liter at prevailing temperature and moisture conditions in order to have any significant impact on snowfall rates. For example, the data of Super and Heimbach (1988), from January 15, 1985, indicated that seeding created about 10 to 20 ice crystals per liter over the target area with typical liquid water contents of about 0.1 g m-3 and cloud temperatures near -10oC, Yet calculated precipitation rates at the lowest aircraft sampling level were only about 0.06 to 0,09 mm h-1 and there was no clear evidence that the seeding decreased the cloud SLW. Model calculations by Young (1974) and others also suggest that it is unlikely that significant snowfall rates can be achieved under most cloud conditions with less than about 10 effective ice nuclei per liter, Significantly higher concentrations often may be required to optimize conversion of cloud water to precipitation. Snowflakes can settle to the ground if they grow large enough and fall far enough before being carried downwind of the mountain barrier. Typically downward motion of the airstream to the lee of the barrier will evaporate the cloud droplets and sublimate the ice crystals so cloud seeding involves a "race" to get snowflakes to the ground before the zone of downward motion is reached. Dennis (1980) presents a more detailed discussion of the physical processes involved in the fonnation of snowfall in winter orographic clouds. The conceptual model of what is expected to happen following seeding of a winter orographic cloud has not changed markedly since Ludlam (1955) wrote his classic' article on the subject. Similar ideas were restated in different fonn by Super and Heimbach (1983) in discussing what general statements could be made about artificial seeding. They noted that, "In order for cloud seeding to increase snowfall f~m winter clouds over mountainous terrain, several links in a chain of physical events must exist. First, seeding material must be successfully and reliably produced. Second, this material must be transported into a region of cloud that has supercooled water or ice supersaturation in excess of that which can be converted to ice by naturally produced ice crystals. Third, the seeding material must have dispersed sufficiently upon reaching this region so that a significant volume is affected by the desired concentration range of ice nuclei or the resulting ice crystals. In the case of AgI seeding, this requires, fourth, that the temperature be low enough for substantial nucleation to occur. Once ice crystals fonn, they must remain in an environment suitable for growth long enough to enable fallout to occur, generally prior to their being carried beyond the mountain barrier where downslope motion, cloud evaporation and ice crystal sublimation typically exist." J 1 A 2.3 Feasibility Issues of Winter Orographic Cloud Seeding In the light of current understanding of the physical processe~ in winter orographic clouds, a cloud seeding feasibility study should address a few basic questions. First and foremost, does excess SL W exist during significant portions of at least some winter stonns? More specifically, is there a significant flux of SL W over the mountain barriers under consideration during the course of a typical winter? The flux or flow of SLW is approximately the vertically integrated amount of SLW multiplied by the mean windspeed in the layer containing the SLW. A properly conducted cloud seeding project might convert a significant 4