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<br />expected for cloud seeding operational days. One of the flights <br />was made as stable atmospheric conditions prevailed in the valley. <br />The particulate materials used for tracing was silver- <br />iodide, fluorescent zinc-sulfide particles and sulfur hexa- <br />fluoride gas. All three of the tracers were released for sampling <br />during the flight of December 15, 1970. <br />The tracer materials were released from two sites, Minturn <br />and Redcliff during four of the flights, and from Minturn during <br />the last three flights. <br />The aircraft used for the sampling flights was the Aero- <br />commander 500-B. The instrumentation consisted of the NCAR <br />ice-nucleus counter, a Mee Industries model 110 automatic fluore- <br />scent particle counter, millipore filter sampling system for <br />ice nuclei and a simple sampling tube and bottle system for <br />collecting sulfur hexafluoride. <br />4. Field Results <br />On the basis of these limited field data some tentative <br />generalizations can be made regarding the atmospheric conditions <br />which prevail during storm periods selected for cloud seeding and <br />how these atmospheric conditions may effect the dispersal of the <br />seeding material. <br />Storm events associated with the northwest winds exhibited <br />a strong orographic effect in the Eagle River Valley area. Visual <br />observations of the local weather conditions supported these <br />conclusions. <br />A diurnal variation in static stability was observed during <br />the storm events. Neutral or unstable stability conditions existed <br />in the lower 1000 meters during the late morning and afternoon <br />hours. More stable conditions existed during the night-time hours. <br />However, there may be storm situations that have quasi-steady state <br />stability conditions for 12 hours or more. The field sampling was <br />not frequent enough to make any definite conclusions concerning <br />the duration of specific stability regimes. <br />The wind direction and speed was typically variable. The <br />ridge-top wind speeds (-13000 ft/msl) varied between 1 to 16 m/s <br />but the most frequent wind speed was between 8 to 12 m/s. <br />Significant cross-valley flow was observed several times <br />and may occur quite frequently during the winter. Yet, events with <br />little (<300) directional shear were also observed a number of times. <br />The dual constant-volume balloon flights3showed that the <br />total dispersion rate approached and exceeded t and t4 for <br />short periods during the flights (Fig. 3-2). <br />Calculations of the eddy diffusivities from the super- <br />pressure balloon data showed that on this particular day (December <br />12, 1969) the vertical eddy diffusivity changed with height, from <br />104 cm2 sec-l within the valley to 106 cm2 sec-l near the surrounding <br />mountain summits. This indicated a strong vertical eddy flux out <br />of the valley due to a strong shear flow at ridge level. Zonal and <br />meridional eddy diffusivities were on the order of 105-106 cm2 sec-l <br />(Ref. 30). <br />Aircraft sampling flights of the silver-iodide seeding <br />material showed that the range of vertical transport of seeding <br />material was of the order <br /> <br />~ <br /> <br />~ <br /> <br />~ <br /> <br />28 <br />