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The 1991 field program measurements have confirmed for the Wasatch Plateau of centra! Utah some of the earlier findings from the Tushar Mountains of southern Utah. Namely, abundant SL W is available during phases of many winter orographic storms which should provide frequent seedable opportunities. Most of the SLW is within 1 km of the mountain crestlines at relatively high temperatures. The c~allenge is to develop the means to routinely target the SL W zone with adequate concentrations of artificially nucleated ice crystals which can start the precipitation formation processes in naturally inefficient clouds. 8.12. Super, A. B., and E. W. Holroyd, 1994: Estimation of effective AgI ice nuclei by two methods compared with measured ice particle concentrations in seeded orographic cloud. J. Weather Modification, 26, 33-40. ABSTRACT The Utah/NOAA Atmospheric Modification Program conducted a field program during early 1991, with additional support from the Bureau of Reclamation. Several aircraft missions were flown over central Utah's Wasatch Plateau to monitor plumes of AgI (silver iodide) and tracer gas, and microphysical changes caused by the AgI seeding. This paper discusses.one mission during which high-altitude, ground- based AgI release resulted in obvious enhancements in ice particle concentration. Fast-response observations of co-released tracer gas, presumably collocated with the AgI plumes, were used to define seeded zones and crosswind control zones. Two methods were used to estimate concentrations of AgI ice nuclei effective at cloud temperatures sampled by the aircraft. One method used tracer gas concentration measurements while the other was based on acoustical ice nucleus counter observations. Both methods were partially based on a cloud simulation laboratory .calibration of the AgI generator done over two decades ago. The methods were compared with the ice particle concentrations apparently caused by the AgI seeding. Both approaches were found to provide a reasonable first approximation for the particular AgI aerosol produced and the sampled cloud conditions. However, caution should be exercised in applying the estimation approaches'to other cloud conditions and AgI aerosols. SUMMARY AND CONCLUSIONS A specially-instrumented aircraft was used to monitor AgI and SF 6 plumes co-released from a high- altitude site on the Wasatch Plate of central Utah during early 1991. An NCAR IN counter measured AgI while a fast-response detector measured the SF 6 tracer gas. A 2D-C particle imaging probe monitored the IPC within and crosswind of the Agl/SF 6 plumes. Other sensors observed cloud temperature, liquid water content, aircraft position and other variables. Obvious increase in lPCwere associated with the SF6 plume on theC} 7 February aircraft mission. Observations from this mission were used to estimate concentrations of AgI IN effective at the sampling temperatures. These estimates were compared with measurements of the IPC enhancement with the SF 6 (and presumably collocated AgI) plume. The IPC enhancement was considered to be the difference between in-plume IPCs and the natural IPCs in crosswind control zones. Two methods were used to estimate effective AgI IN concentrations. One method was based on SF 6 gas concentration measurements and the other on NCAR IN observations. Both methods relied on cloud simulation laboratory calibrations to establish the source strength and temperature dependence of 54