WMOD00298

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

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11

Discussion here is limited to AgI-type ice nucleants to be most

relevant to the present study. For AgI-type nucleants, at least until

recently, there has clearly been a dearth of studies isolating the

action of specific nucleation mechanisms for aerosols as they are

produced in actual cloud seeding. Instead, most earlier studies were

done using thermally produced AgI (for example, heating AgI on metal

wire). Presumably, this was done because studies were motivated by the

desire for a basic understanding of ice nucleation processes, so direct

relevance to cloud seeding was not an important requirement.

Nevertheless, these studies demonstrate some of the characteristics of

the different nucleation modes for AgI aerosols.

Studies of nucleation of ice from the vapor phase onto AgI

aerosols have shown this mechanism to be the least efficient of all,

but it possesses a strong dependence on particle size. Using small

particles (< O.Ol~m radius) produced by reacting iodine vapor. with

silver aerosol deposited onto a gold surface and exposed to relative

humidity near 98%, Edwards and Evans (1960) found less than 0.5% of

these to form ice down to a temperature of -18.50C. For polydisperse

thermally produced AgI in free suspension in ice-thermal diffusion

chambers, both Schaller and Fukuta (1979) and Detwiler and Vonnegut

(1981) inferred nucleation from the vapor as warm as -60C and found

activity to be related at all temperatures to ice supersaturation.

Schaller and Fukuta noted nucleation activities as high as 6.4% of the

particles present (0.3~m median diameter) below -120C, while Detwiler

and Vonnegut (O.l~m median diameter particles) noted 1% activation for

a similar range of ice supersaturation and temperatures.