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.. i some in the descending airflow on the lee side of the barrier. Ice particle growth time becomes a key con~ern to the design phase of the cloud seeding project. For seeding to be effective in orographic clouds, there must be excess SL W that nature is not converting to snowfall, or is converting to snowfall too slowly to grow and reach the ground. The excess SL W is one "raw material" needed for seeding to enhance snowfall. The cloud schematic shows silver iodide (Agl) release near the bottom of the SL W on the windward side of the barrier. The released Agl creates ice particles in the SL W that will grow to sizes large enough to survive descent to the surface. An alternative mode of seeding the SLW zone is the release of propane in the bottom portion of the SLW to create ice particles by a process known as homogeneous nucleation (discussed in appendix A, chapter 3). Laboratory and field measurements have shown that large numbers of ice particles (10ll to 1012 ice particles per gram of released propane) can be produced with propane release. An important advantage of propane use over Agl is that large numbers of ice particles can be created at temperatures as warm as -0.5 oC, a feature of importance in the Headwaters Region where cloud SL W is sometimes present at between -0.5 and.,6 oC, temperatures too warm for Agl seeding. The project design phase will develop procedures and equipment for the automatic determining of cloud seedability, and the relay of commands to seeding equipment to initiate cloud seeding when appropriate. Reynolds (1989, 1991, 1992, 1996), a former Reclamation scientist, first used automated, radio- controlled, liquid propane dispensers for seeding winter orographic clouds in California. Super et al. (1995) used dispensers in a totally automated system that released liquid propane only when SL W was detected. A data logger opened and closed a value controlling propane flow according to whether SL W cloud was detected by a commercial icing detector. Propane flow rate and temperature within the expanding propane plume were monitored for system status that was routinely relayed via radio to a central location. The cloud seeding process described above is known as the "static mode" of seeding (Dennis, 1980). Observations of Park Range winter clouds point to the "static mode" as appropriate most often for that area. The "static mode" of cloud seeding (as opposed to the "dynamic mode" where the objective is to invigorate clouds to process more water through latent heat release from converting water droplets to ice particles by seeding in large amounts), seeks to increase the cloud precipitation efficiency. Cotton and Pielke (1995) states that physical and cloud seeding studies suggest, "there exists a much more limited window of opportunity for precipitation enhancement by the static-mode of cloud seeding than was originally thought," and that the "window of opportunity for cloud seeding appears to be limited to: 1) clouds which are relatively cold-based and continental; 2) clouds having top temperatures in the range - I 0 oC to -25 oC; 3) a time scale limited by the availability of significant supercooled water before depletion by entrainment and natural precipitation processes." Park Range observations (Rauber et aI., 1986; Rauber and Grant, 1986; Rauber, 1987) indicate the first two conditions are frequently met by Park Range winter clouds. The time scale limitation presents a challenge to the program design phase. Time limitations will vary according to storm phase and character, location along the barrier, barrier characteristics, and moisture variation within cloud and the approaching air mass. Cloud conditions will vary substantially in time and space. Cotton and Pielke (1995) state that field personnel must be skillful in selecting suitable clouds for treatment. Use of a properly scaled automatic cloud seeding process that uses current local measurements to assess seedability is expected to produce proper treatment timing. 7