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have markedly greater IPC than would natural cloud. Much of the IPC increase can be expected to consist of relatively small newly formed crystals, with habits appropriate for the sampling-level temperature. An acoustical ice nucleus counter (Langer, 1973) will be used to measure AgI concentrations and detect the approximate boundaries of the AgI plume or seedline by the method of Super et al. (1988). While not essential, a PMS 2D-P probe would be valuable for monitoring snowflakes larger than about a millimeter in diameter. The 2D-C probe has limitations in observing these larger particles which exist in much lower concentrations than do the small ice crystals. Yet most of the precipitation mass is contained in the larger particles. I i j 1 Other cloud physics aircraft instrumentation will include sensors for monitoring pressure altitude, horizontal position, temperature, dew-point temperature, indicated airspeed, and heading. Horizontal wind velocity will be calculated from heading, true airspeed, and position. A radar altimeter will indicate terrain clearance. A forward-looking video camera will document cloud structure. Both graphic and text displays of current and recent observations will be available to the aircraft scientist to aid in conduct of the mission (experiment). The scientist and other crew members (pilot and data system operator) will have intercom communication plus radio communication with project personnel on the ground. 1.5.4 Seeding Aircraft. - The seeding aircraft will carry an Aero Systems Model E-1 G generator, which uses compressed gas to pressurize a solution of AgI-in-acetone. The solution flows through a no:zzle, where it is atomized, into the burning chamber, where the acetone is burned and the AgI vaporized. In the exhaust region, the AgI crystallizes, producing high concentrations of tiny AgI particles. Since no other fuel is used, monitoring the temperature in the burning chamber provides sure evidence of whether the generator is on or off. The seeding aircraft also will have limited instrumentation consisting of time-referenced recordings of J-W liquid water content, air temperature, dew-point temperature, pressure altitude, horizontal position, heading, indicated airspeed and AgI generator temperature. Approximate horizontal wind velocity will be calculated from the heading, airspeed, and position information. A forward-looking video camera will document general cloud conditions over the experimental area. The seeding aircraft will be equipped to release radar chaff several times during each mission in order to mark the locations of seeding agent release. Only the pilot will fly on this aircraft which, like the cloud physics plane, must be equipped for flight into known icing conditions. Both aircraft will use state-of-the-art Loran-C receivers for primary in-cloud navigation, A flight test in September 1989 indicated that conventional VOR-DME equipment would not receive signals from the Flagstaff and Winslow VORTAC stations within 300 m elevation of the highest terrain in the experimental area. Other navigational alternatives are much more expensive. The Mogollon Rim is presently in the "mid-continent gap" for Loran-C, but new stations scheduled for installation during 1991 should eliminate this problem. Moreover, recently marketed receivers can triangulate from more than one Loran chain. 1 1.5.5 Project Operations Center. - A scanning Doppler radar, sensitive enough to observe very light snowfall (-10 dBz) at 5-km range, will be located at the operations center above the northeast shore of Mormon Lake in the largest clearing on the heavily forested crestline area. The site is the only clearing large enough to allow the radar antenna to be tilted to low elevation angles without blockage by terrain or trees. This will permit near-surface scanning through seeded volumes 14