Advances of Silver Iodide Seeding Agents for Weather Modification

Silver iodide (AgI) is the most widely-used seeding agent in field experiment and operations of weather modification. There are several nucleation modes for AgI seeding agents, and the nucleation process is affected by many factors including atmospheric temperature, humidity, particle size, composition of seeding agent and its particle generation method. The nucleation efficiency, nucleation modes affect the number of nucleated crystals, and thus affect the seeding effect.Through laboratory research of cloud chamber and theoretical calculation, the critical embryo sizes required for different nucleation mechanisms, nucleation threshold temperature, nucleation mechanisms, factors effecting nucleation for AgI and with some other added materials are analyzed. It is currently accepted that there are four nucleation mechanisms, including deposition, condensation freezing, contact freezing and immersion freezing nucleation. And it is recognized that nucleation processes depend on the varied temperature, humidity, and cloud conditions that can be encountered in the atmosphere. Immersion nucleation refers to nucleation of freezing by a particle immersed in water, and deposition nucleation refers to nucleation of the ice phase from the vapor, contact nucleation refers to the nucleation of the freezing induced by a particle during first contact with supercooled water, and condensation freezing is the nucleation of the freezing from the condensation of vapor to liquid droplet.The ice-nucleating properties and nucleation effectiveness of cloud seeding materials produced by burning acetone solutions or pyrotechnics of AgI and other materials are tested in a wind-tunnel cloud chamber test facility along with isothermal cloud chamber and dynamic cloud chamber. Through laboratory test, different formulations are compared, and high nucleation efficiency of seeding materials are selected, and nucleation features for four nucleation modes separately are obtained.Silver iodide seeding cloud model are based on a combination of theory and laboratory results. The ice nucleation schemes employed in cloud models vary widely. Hsie's seeding scheme simulated both contact-freezing and deposition nucleation on the laboratory measured effectiveness spectra of nucleus. Meyers seeding scheme considers all the four processes on cloud chamber results of ice-forming processes by AgI. In China, AgI seeding models are developed since the 1990s, either similar with Hsie's seeding scheme on three nucleation mechanisms, or similar with Meyers seeding scheme on four nucleation mechanisms.