WMOD00298

• Title: Quantifying Ice Nucleation by Silver Iodide Aerosols
• Date: 5/1/1990
• State: CO
• Weather Modification - Doc Type: Report

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Various studies have differed on the degree of water

supersaturation required to trigger condensation-freezing nucleation by
AgI aerosols, or to cause the immersion of a particle in a cloud

droplet that might freeze at some colder temperature. Edwards and Evans

(1960) showed that supersaturation had to exceed 10% for condensation

alone on their small particles. Langer et a1. (1978) claimed similar

results for nearly monodisperse "pure" AgI particles with diameters

between 0.02 and 0 .l2~m. When condensation was forced on these AgI

aerosols, nucleation activity was as much as 100 times the activity by

deposition. In contrast, Schaller and Fukuta (1979) found condensation-

freezing to readily operate at small finite water supersaturations,

increasing sharply with higher supersaturations.
Contact-freezing nucleation appears to be a very efficient

nucleation mode for AgI aerosols, rate limited by the collision process

with cloud droplets. This mechanism has been examined by allowing

droplets to settle through an aerosol cloud or by using very high cloud

droplet concentrations in mixing chambers. Typically, it has been found
that the activity by contact-freezing exceeds the activity by
immersion-freezing at similar temperatures (Ghokale and Goold , 1968;
Langer et al., 1978). In the study by Ghoka1e and Goold, the surface

nucleation of millimeter-sized droplets was found to occur even at

-50C, while the purposeful submersion of the particles in drops did

not lead to efficient nucleation at this warm temperature. Contact-

freezing activity has been found to increase with particle size and

decreasing temperature. Sax and Goldsmith (1972) showed fractional

activation to approach 1 at -180C for polydisperse (0.02 to 0.03}.'m

diameter) thermally generated AgI. They also demonstrated that the