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riming (freezing of SL W droplets) on snowflakes. It was certainly known from such measurements that SLW existed some of the time in winter orographic clouds. But the frequency, durations, amounts and horizontal and vertical positions of SL W were not well documented. As a consequence, seeding projects implicitly assumed frequent SL W presence during presumably "seedable conditions." Such conditions were often assumed to exist based on some indication or forecast of cloud top temperature. Cloud tops colder than about -20 to -250C were believed to indicate naturally efficient stonns while stonn periods with wanner cloud tops were thought to be seedable. This opinion was based in part on the statistical suggestions of the Climax Experiments, conducted near Leadville, Colorado, during the 1960s. These results were widely accepted for several years but more recently have been seriously challenged in the scientific literature. Most commercial seeding operators still attempt to forecast seedable conditions, some still using an estimate of cloud top temperature, without documenting that SL W actually exists. This may be an adequate operational approach if SL W cloud is often present, and often targeted by sufficiently high concentrations of seeded crystals. But it is unlikely to be an optimum seeding approach. Even large earlier experimental programs like the Colorado River Basin Pilot Project (CRBPP), conducted in the San Juan Mountains of southwest Colorado during the five winters 1970/71 through 1974/75, did not routinely monitor SL W cloud presence. Given the fact that SL W is necessary for seeding to be effective, failure to monitor it now seems shortsighted, but suitable instrumentation was not generally available until about 1980. The then limited knowledge concerning winter orographic SL W distributions and variability can be evidenced by the book on weather modification by Dennis (1980) which provides little in the way of observational results. Nor does a major planning document for the CRBPP contain much insight into SLW availability (Grant et at. 1969). The Bridger Range Experiment (Super and Heimbach 1983), conducted in the early 1970s, did not monitor SL W, nor did a number of concurrent experiments. These comments are not meant as criticism but simply illustrate previous limitations in knowledge and instrumentation. An important exception which monitored SL W cloud was the then cutting edge experimental work conducted over the Cascade Mountains of Washington from 1969 to 1974 (Hobbs 1975a, Hobbs and Radke 1975, Hobbs 1975b). Attempts were made to heavily seed the orographic clouds with Agl and dry ice in order to both increase and redistribute snowfall, producing more on the drier downwind mountain slopes. The average distribution of SL W cloud, based on 22 aircraft flights, was given as Fig. 3 of Hobbs (1975a). The figure shows that, . the maximum SL W was at lowest sampling altitudes over the windward slopes, . SL W was found at highest altitudes over the crestline, and . SL W decreased rapidly with increasing altitude. This 1975 general portrayal of the SL W distribution over mountains has since been observed over several other mountain ranges, and extended to mountain surface altitudes by improved instrumentation. Much of the earlier knowledge ofSLW was based on interpretation of raw ins on de relative humidity and temperature data, shown to have serious instrument-caused flaws by Hill (1978). Rawinsondes have the additional shortcoming of usually ascending upwind of the primary SLW zone now known to exist near the windward slopes and crests of mountains. Aircraft observations of icing were also commonly used (e.g., Hill 1982a), but such measurements were typically made well above the barrier-induced SL W zone because of Federal Aviation Agency (FAA) flight restrictions. 13