Deposition Ice Nucleation on Cloud Seeding Agents

Seeding cold clouds with the intent of artificially enhancing rain, producing snow or cloud dispersion, is worldwide practice. Since the start of cloud seeding operations, several seeding agents have been experimented on to achieve optimal results. In common to all seeding agents is their high ice nucleation onset temperature, but little information on their humidity dependent activity is available.
We report ice nucleation experiments, carried out with size selected particles of several cloud seeding agents including AgI, PbI₂ and Phloroglucinol. The temperature and humidity conditions at which the different materials are active ice nuclei are explored between 213 and 268K and from ice to water saturation, covering the whole range of potential deposition ice nucleation conditions in the troposphere.
The experiments show that high onset temperature do not coincide with high activity at low humidity in all cases. The substance and particle size dependent isolines of activity in the temperature-humidity space, can be idealized into three regimes. With decreasing temperature, the dependence on humidity changes from a positive slope, to independent, to negative. The extent of the regimes depends on the substance.
To investigate the mechanisms how deposition ice nucleation in the three regimes works, the experimental data is compared to classical nucleation theory and a newly developed adsorption ice nucleation theory. The input parameter for both theories, the contact angle and the affinity of the ice nucleating materials to adsorb water are determined experimentally. Finally, the substance specific, latent heat of water adsorption in dependence of the number of adsorbed layers of water on the cloud seeding agents is determined. Based on these measurements, we suggest that the state of adsorbed water determines the substance specific onset of ice nucleation in the low temperature regime.