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Edmond W. Holroyd III, Jack T. McPartland, and Arlin B. Super


A series of wintertime airborne tracing experiments was examined to determine some characteristics of the plumes of silver iodide smoke released either from the ground or from an aircraft over the Grand Mesa of Colorado. The plumes were identified in nearly every experiment by detecting the airborne AgI particles and often also by observing resulting ice particle plumes in essentially the same airspace. The lateral and vertical plume positions of Wound-released AgI from eight sites were determined for several wind, cloudiness and stability conditions. The instantaneous ground-released plume had a median spreading angle of 15° and meandered within a median angle of 38°. The median plume height above the crest exceeded 500 m. The lateral spreading rates of aircraft-released AgI were estimated at over 2 m s−1 for cloudy conditions and less in clear conditions. The implications for future cloud seeding strategies are discussed.

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Arlin B. Super, Jack T. McPartland, and James A. Heimbach Jr.


A field method of estimating the persistence of a commonly used silver iodide seeding agent is described. The method involved measurement of the AgI plume structure at two downwind distances from the ground generator(s). Distances between the nine available pairs of downwind measurement planes ranged from approximately 10 to 100 km. An NCAR acoustical ice nucleus counter in a light twin aircraft was used to sample the AgI plumes. A series of passes was made through the entire vertical and horizontal extent of the plume at each downwind distance. These measurements, together with pilot balloon observations, permitted calculation of the flux of AgI through each vertical cross-sectional plane. The difference in flux measurements yielded an estimate of the persistence of the seeding agent over the period of transport between the two vertical planes.

This method was applied at three separate locations, during different seasons, and with various degrees of cloudiness. Resulting estimates of deactivation rates of the ice nucleating ability of AgI ranged from no loss to 70% loss per hour. The implications for possible cloud seeding effects beyond the intended target area are discussed.

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