Abstract
The ice-nucleating properties of cloud seeding materials produced by burning acetone solutions of AgI-NaI, AgI-KI and AgI-NH4I have been tested in a wind-tunnel cloud-chamber test facility.
The AgI aerosol produced from 5% AgI-NH4I solutions yields nuclei active at temperatures as warm as −4C and shows an efficiency of 1012 nuclei per gm AgI at −5C. Aerosols from similar solutions of AgI-KI and AgI-NaI produce no ice crystals at temperatures warmer than −6.5 and −8C, respectively. Temperatures of −7.5 and −9C are required to obain efficiencies of 1012 nuclei per gm for these two latter solutions.
The greater ice nucleating ability of the NH4I solution at high temperatures in attributed tentatively to the production of relatively pure AgI crystals rather than to increased particle size but may also result from minor NH4I contamination.
Calculations of the droplet-aerosol particle collision rates show that contact nucleation could explain all ice formation observed at higher chamber temperatures, although other processes may occur simultaneously. On the other hand, the calculations also show that direct ice deposition or condensation-freezing is likely the dominant mechanism below −12C.
The implications of the laboratory results for field applications are discussed briefly. It is concluded that the AgI-NH4I system offers sufficient advantages to justify its adoption in field projects.
