WMOD00571

• Title: Static Mode Seeding of Convective Clouds - A Review
• Date: 5/23/1984
• Weather Modification - Doc Type: Report

which report activation of ice crystals following silver iodide seeding for
15 min. and longer.

Seeding agents such as silver iodide-sodium iodide and silver iodide-
potassium iodide which appear to act by the consdensation-freezing mode of
nucleation have somewhat shorter nucleation time constants. Blumenstein
et al. (1983) report that a silver iodide-sodium iodide nucleant produces 90
percent of its ice crystals in about 14 to 20 min. depending on
temperature, in a cloud chamber at water saturation. However, when sub-
jected to transient supersaturations with respect to water, both the
nucleation rate and effectivity of the nucleant increased. In super-
saturated conditions this nucleant produced 90 percent of its ice crystals
in about 4 min. and its effectivity increased by almost one order of magni-
tude. Rilling et al. (1984) found that a silver iodide-potassium iodide
nucleant behaves similarly. Finnegan et al. (1984) showed that the incor-
poration of a hygroscopic salt, .like sodium chloride, into the silver
iodide-silver chloride nucleus composition changed its mode of nucleation
from contact to condensation-freezing, thereby increasing both its effec-
tivity and nucleation rate, especially under transient supersaturation con-
ditions.

The combined effects of a silver iodide seeding agent's nucleation rate and
residence time in a cloud is to make the nucleant's ice crystal yield less
than its effectivity at all temperatures, the amount of reduction depending
on the mode of nucleation and thE~ cloud conditions where and when
nucleation actually occurs. In effect it lowers the temperature threshold
of activity for most silver iodide seeding agents to about _90 to -10 oc.

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