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<br />165 <br /> <br />In 1977, the opportunity arose to make use of an inex- <br /> <br /> <br />pensive small propeller driven research aircraft, .,ith the <br /> <br /> <br />capability of reaching altitudes of 9 km or more, for a <br /> <br /> <br />trial clear-air seeding program of our own. We decided to <br /> <br />try and use silver iodide. It was hoped that greater <br /> <br />flexibility would be pOSSible with silver iodide than with <br /> <br />dry ice. Conceivably, the silver iodide could be released <br /> <br /> <br />in unsaturated air and then could nucleate at some later <br /> <br />time as the air cooled and saturated. <br /> <br />Through the courtesy of the Naval Weapons Center at <br />China Lake, California, we were able to obtain a supply <br />of LW-83 silver iodide pyrotechnic flares for the trials. <br /> <br />These flares produce a nearly pure hydrophobic silver iodide <br /> <br /> <br />aerosol and had previously been noted to nucleate ice clouds <br /> <br /> <br />in clear air when dropped from aircraft (St.-Amand, personal <br /> <br /> <br />communication). When ignited in still air they produce 109 <br /> <br /> <br />to 1010 ice nuclei per gram active in a _lSoC supercooled <br /> <br /> <br />cloud. When burned in an airstream moving at roughly SO m/sec <br />the ice nuclei production is two or three orders of magni- <br /> <br />tude higher (Garvey, personal communication) . <br /> <br />6.1 PROGRAM AIMS <br /> <br />The aims of the trial programs were to (1) establish a <br /> <br /> <br />seeding technology by which silver iodide could be used in <br /> <br /> <br />clear air supersaturated with respect to ice but not <br /> <br /> <br />saturated with respect to the liquid water phase to nucleate <br />