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immediately to the lee of barrier crests, and descent may even begin just upwind of the crest, with the result that the air mass warms, causing the tiny SL W cloud droplets to rapidly evaporate downwind of crestlines. The downwind disappearance of orographically-produced SL W is often accelerated by natural conversion to snowfall. Accordingly, the best observing locations for both surface icing sensors and microwave radiometers are on top of mountain crestlines rather than some distance upwind. That is because SL W measured upwind of barriers may be partially or totally converted to natural snowfall before the moist ascending airflow passes over crestlines. Therefore, observations over upwind slopes may result in overestimates of cloud seeding potential. The same is true of crestline observations but to a lesser extent. The situation over generally flat-topped barriers is more complex. For example, mobile microwave radiometer observations ofSLW were made over the Wasatch Plateau (Huggins 1995). Subsiding air motions were implied immediately downwind of the Plateau top's upwind edge. But downward motions over such terrain may not be as abrupt as over more typical narrow crestlines, which could provide more time for natural conversion of SL W to snowfall. Huggins (ibid.) showed a decrease of SL W across the Wasatch Plateau top, thought to be due to a combination of conversion to snowfall and descending airflow. But the number of cross-barrier observations was limited, and additional locally-produced SL W was apparent over a minor ridgeline midway across the Plateau top. Weak embedded convection may also enhance SLW. These remarks are made to caution the reader that estimating seeding potential from SL W observations, wherever made, is a complex and imperfect undertaking. Yet since SL W is the necessary "raw material" for cloud seeding to work, improved knowledge of its spatial and temporal variations is important for improving cloud seeding practices. Several published articles and project reports have been reviewed and briefly summarized in Appendix A to present current understanding of SL W availability over Colorado. Observations have been used from Colorado where available, and also from Utah, to expand the knowledge base. As previously noted, Utah measurements are believed to be reasonably representative of Colorado mountains. 6. Transport and Dispersion of Ground Seeding with Emphasis on Valley Releases As discussed in Sec. 5, wintertime SL W cloud is primarily found in the lowest few thousand feet above the windward slopes and crests of mountain barriers. It was also noted that a thin layer of SL W frequently exists at cloud top and sometimes near cloud base as well. Evidence for the primary SL W zone was provided from a number of sources. The cloud top and cloud base zones are based primarily on the work of Rauber and Grant (1986), briefly reviewed in Appendix A. A conceptual portrayal of the three SLW zones is given in Fig. 13 of their paper. More recent work has shown that gravity waves sometimes produce abundant SL W zones over or downwind of mountain barriers. These transient zones may contain more SL W than found near windward mountain slopes, but often can only be seeded by aircraft, as also true for the thin layers found at cloud tops. Targeting of seeded snowfall from gravity wave-produced SL W would be difficult unless a mountain barrier existed an appropriate downwind distance from the wave-produced SL W zone. The facts that the presence of such SL W zones are transient, and that they shift their positions with time, further complicates any attempt to seed them in a manner that produces additional snowfall on a desired mountain target area. Perhaps future research will show such SL W zones to be seedable under some circumstances, but such evidence does not currently exist. Consequently, seeding of gravity wave SL W zones will not be considered further. 15