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DECEMBER 1983 ARLIN B. SUPER AND JAMES A.HEIMBACH, J,R. 1995 hours even in bright sunlight, as discussed by Super et al. (1975). b. Pibal investigations Further evidence of proper targeting of the AgI was gained by dual-theodolite tracking of pibals released from the seeding sites. As discussed more fully in Part , I, pibals of known ascent rate were simultaneously tracked from the two seeding sites on many occasions. These data were used to estimate average vertical and horizontal wind speeds between the launch sites and Main Ridge. This was done for each pibal for which data existed within the zone from the crestline to 0.9 km west of it and withn 450 m above it; Correlation coefficients and linear regression equations were cal- culated between all pairs of vertical wind speed (VWS) and the component of the horizontal wind speed nor- mal to the Main Ridge (HWS).f These are given in Table 1. A correspondence between vertical motion and the horizontal cross-barrier flow is evident. A sim- ilar correspondence has been shown by Hill (1980a) for a north-south oriented barrier in Utah. The ratio of change in ground elevation with hor- izontal distance between the seeding sites and Main Ridge is 0.09 and 0.28 for the southern and northern sites respectively. These are reasonably close to the slopes of their regression lines suggesting that, to a first approximation, the terrain slope determined the ver- tical air motion. The average VWS for the data of Table 1 was 0.6 m S-I for the southern seeding site, located 4.7 km west of the Main Ridge and 1.6 m S-I for the northern site, only 1.5 km west of it. Updrafts of this magnitude can be expected to produce abundant condensate from air saturated with respect to water. . The positions of the pibals were noted at 1 min intervals. Eighty and 90% of the resulting 1 min changes in balloon position indicated an updraft component between the southern and northern sites, respectively and the Main Ridge. The pibals were also examined to determine how well the 10 000 ft wind represented the mean wind in the layer from the seeding sites to 10 000 ft, i.e., the layer in which the AgI was generally transported until beyond the Main Ridge. As discussed in Part II, it was found that the 3050 m (10000 ft) wind direction was TABLE 1. Correlation coefficients and regression equations for vertical wind sPeed (VWS) versus horizontal wind speed (HWS). Number Correlation Regression equation Launch site of pairs coefficient (m 5-1) Southern AgI generator 55 0.64 VWS = -0.25 + 0.10 HWS Northern AgI generator 134 0.70 VWS= -0.47 + 0.39 HWS a very good predictor of the mean wind direction for this entire layer, with mean differences ofless than 50. With this in mind, it is instructive to consider the distribution of 700 mb (approximately 3050 m) wind directions. All 359 available rawins were considered and it was found that 80% of the cases were between 240 and 3200. Winds in this range should transport AgI toward the intended target area (see Fig. 1). Only 6% of the 700 mb wind observations had an easterly component. In summary, the pibal data are quite consistent with the concept that the AgI was transported rapidly up the west slope of the Bridger Range, crossed the Main Ridge and moved toward the intended Bangtail Ridge target area. Further, they suggest that substantial liquid water should be produced by saturated air flowing up the west side of the Main Ridge in the probable region of the AgI plumes. c. Silver concentrations in snow Warburton and Young (1968) discussed a neutron activation procedure for analysis of the silver in pre- cipitation. Snow samples from the Bridger Range were sent to Warburton who supervised their silver analysis. Samples collected prior to any known seeding on the Bridgers and near the Gallatin Range control gage dur- ing the BRE, revealed background silver concentrations that were approximately 1 X 10-11 g mrl. Seasonal samples were obtained . from snowpits dug during March 1971 and 1972 at seven sites on the Bangtail Ridge as described in Part I. Values for these ranged from 3 to 10 times the average background value. These data .and snowpack water equivalents were used to estimate the total amount of silver in the Bangtail Ridge snowpack above the 1800 m (6000 ft) elevation contour, an area of 218 km2. The estimates suggest that, ofthe total silver emitted by both generators only during periods with snowfall occurring near the center of the Bangtail Ridge, about 70% and 40% of the silver was in the snowpack during March 1971 and 1972 respectively. Comparable figures for all hours of gen- erator operations are 18 and 14%. Increased silveri.n the snow is not proof that seeding altered the snowfall amount. Scavenging by natural snowfall or direct deposition could also explain in- creased silver concentration. However, failure to find increased silver would indicate that proper targeting was not being achieved. The increased silver concen- trations found on the Bangtail Ridge lend further sup- port to the eviden~e of proper targeting previously discussed. 5. Experimental design and operations The BRE had been planned to commence on 1 November 1969, using radio-controlled generators at two remote sites on the west slope of the Bridger Range.