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1174 JOURNAL OF APPLIED METEOROLOGY VOLUME 27 experiment a single airborne line of AgI was released approximately perpendicular to the wind at a constant distance upwind of the Snow Lab at 3.8 km altitude. After each seeding arc was completed, a series of passes was flown over the Snow Lab and parallel to the wind at the 3.8 km level. These passes were intended to monitor the evolution of the AgI plume and the resulting cloud microphysical changes during transport over the mesa. Surface observations of snowfall char- acteristics were made at the Snow Lab during each experiment, primarily with the ice crystal photography system (see Part I). In addition, high resolution weigh- ing precipitation gages with 45.4 cm orifices and Alter- n shields were operated in protected clearings at seven sites (Fig. 1). The gage charts were read each 15 minutes to the nearest 0.025 mm with the aid of magnification. The seeded zones in these airborne experiments were defined for each pass parallel to the wind according to marked increases in IPC above the general background and/ or the presence of AgI as detected by the acoustical 1300 1250 860318 Experiment 1 Airborne Seeding 37 km Upwind counter. The presence of AgI was at times observed exclusive ofIPC in regions devoid ofSLW, while IPC was occasionally observed without AgI, probably due to scavenging and/ or transport of the AgI plume to levels above or below that of the aircraft. The following criteria were used in determining seeded and nonseeded zones: 1) regions containing the AgI plume were considered seeded; 2) regions having an IPC significantly above the preseeding natural con- centrations observed over the mesa were considered seeded if associated with an AgI plume; and 3) regions having an IPC significantly above natural concentra- tions were also considered seeded in the absence of an AgI plume if consistent in position with the advection of a volume previously defined as seeded. The non- seeded or natural ( control) cloud was considered to be cloud volume free from any effects of AgI, including the possibility of contamination from aloft resulting from speed shear of the horizontal wind. Figure 8 shows the relative ease with which approximate plume I 60 per liter II 2D-C Ice Cone. 0 .10 3 .02 Liquid Water Content Ig m- I 1~9 Ice Nuclei IA91I, raw counts FIRST DETECTION OF PLUME North 40 20 DISTANCE FROM SNOW LAB lkml FIG. 8. Ice particle concentrations and SLW content plotted as functions of time and distance from the Snow Lab target for the first airborne seeding experiment. Also shown are the pass-total ice nucleus counts and the estimated locations of the AgI plume edges.