WMOD00299 ‎(2)‎

• Title: Scientific Weather Modification Experimentation in the United States
• Date: 8/25/1984
• Weather Modification - Doc Type: Report

constants of nucleation. Their nucleation rate is dependent on Brmmian coagu-
lation which is a function of temperature and the size and concentration of both
the silver iodide aerosols and cloud droplets. They showed that at temperatures
warmer than -16 oC it takes from about 15 min to 1 h for 90 percent of the
silver iodide-silver chloride aerosols to nucleate ice crystals by the contact-
freezing mode, the exact times being dependent on the specific chemical com-
positon and cloud chamber conditions used. At temperatures colder than -16 oC,
DeMott et al. concluded that the deposition nucleation mode was dominant with
even longer nucleation time constants. These findings are generally consistent
with field observations (Dye et al., 1976; Strapp et al., 1979; English and
Marwitz, 1981) 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 condensation-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 subjected to tran-
sient supersaturations with respect to water, both the nucleation rate and
effectivity of the nucleant increased. In supersaturated conditions this
nucleant produced 90 percent of its ice crystals in about 4 min and its
effectivity increased by almost one order of magnitude. Rilling et al.,
(1984) found that a silver iOdide-potassium iodide nucleant behaves simi-
larly. Finnegan et ale (1984) showed that the incorporation 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 effectivity and
nucleation rate, especially under transient supersaturation conditions.

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.
These factors may help to explain the diverse results of some of the past
cloud seeding experiments.

c. Physical evaluation of experiments

HIPLEX-1 represents a significant advance in our ability to design, carry
out and evaluate a randomized seeding experiment to improve our
understanding of precipitation development in natural and seeded clouds.
It specified in advance and attempted to verify by observations during the
course of the experiment each step leading to additional precipitation from
seedi ng. It was simil ar in approach to experiments conducted by Braham et
al., (1957) and Bethwaite et al., (1966) but more of the steps in the chain
of physical events could be included because of recent developments 'in
observing, measuring and real-time data processing capabilities.

Among the important concepts that were employed, some for the first
time in weather modification, in designing, executing and evaluating
HIPLEX-1 are the following (Smith et al., 1984):

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