Silver Iodide Plume Characteristics Over the Bridger Mountain Range, Montana

Abstract

A modified NCAR acoustical ice nucleus counter was used in an airborne mode to measure the characteristics of silver iodide plumes released in mountainous terrain. These measurements were a supplement to a winter randomized cloud seeding experiment conducted in the Bridger Range, Montana. It was found that the silver iodide was generally transported upslope from the seeding sites, over the Main Ridge of the Bridger Range some 1400 ft higher, and toward the intended target area. Plume widths were found to average 28° above the Main Ridge, while most of the seeding agent was confined to the lowest 1500 ft above the ridgeline. Ice nuclei concentrations were typically in the range of 100?1000 liter?1, effective at ?20C. This is estimated to correspond to about 10?100 nuclei liter?1 at the warmer temperatures prevalent in the lowest 1500 ft above the Main Ridge during winter storms. The flux of ice nuclei was estimated on three occasions. Agreement was good, both from day to day and with the generator output as calibrated in a large isothermal cloud chamber.