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... ~ Thus, the optical snow rate sensor appears to provide data at least as con- sistent as other state-of-the-art snow sensors, and it has the advantage of good time resolution. Wind collection and blowing snow factors severely limit the accuracy of snowfall measurements using all conventional tech- niques. Figures 15-17 present typical results of the calibration program. Figure 15 for 5 December 1966 spans a wide temperature ra.nge. The only serious dis- crepancies are near the beginning of the period where there was some melting or probably heavy riming. Figure 16, the case for 25 January 196'7, illus- trates behavior at much colder temperatures. Figure 17 is a plot of the data points of Figure 16. 2. 10 Ice Nucleus Counters Four ice nucleus counters were used for surface measurements and an addi- tional aircraft installation used for plume tracing. This unit has been de- scribed previously (Langer, et. al. 1966) a.nd its performance is fairly well documented. The ground units have been used routinely for storm measure- ments. During the final season of operation the counters were located at Craig (upwind), Steamboat Springs, and Rabbit Ears Pass (target area), and Kremmling (downwind). . 2. 11 Forecast Center Operations .'Ii Routine twice daily forecasts of precipitation, upper wind and vertical profiles of temperature and stability were made (at 0700 and 1900 MST) utilizing the latest available upper air chart plotting information. For the four seasons 1964- 65 through 1967 -68, these forecasts were used as alerts for planning pulsed seeding operations. Operations were actually started when either the ceiling dropped below ridge top level (approximately 10,000 ft. MSL) and/or snow began to fall anywhere over the data network. Once an operation was begun, it continued until snowfall ceased over the area and/ or the winds fell decidedly outside the nominal targeting envelope. . During the 1968-69 operational season the forecasts were used for the actual scheduling of operations, and for determining the mode of seeding (airborne or ground) depending on predicted wind direction and low level stability con- ditions. Operations were scheduled in six hour blocks. Each of these six hour blocks was further broken down into two three-hour blocks designated seeded and unseeded. AgI release either from Mt. Harris or aircraft was made during the first hour of the seeded three hour block in each case. The decision of whether the three hour seeded block was to be the first or last three hours of each six hour block was randomized by random drawing from a reservoir of equally likely choices. Table 5 lists the six hour operational blocks (analysis periods) during which operations were conducted in 1968-69. . .~ 33