WMOD00285 ‎(2)‎

• Title: Feasibility Study of Wintertime Cloud Seeding to Augment Arizona Water Supplies
• Date: 1/1/1987
• State: AZ
• Weather Modification - Doc Type: Report

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3. Weather Modification Techniques

3.1 Ba ck g rou nd

Research in cloud physics and precipitation formation processes for
nearly a half-century eventually 1 ed to pralcti cal methods of cloud modi-
fication (e.g. Schaefer, 1951; Mason, 1975) in the 1940's. Research by
Bernard Vonnegut (1947) for substances whose crystalline structure
resembles ice led to the discovery of AgI (silver iodide) as an artifi-
cial ice nucleant. His work yielded procedures that enabled the produc-
tion of up to 1016 particles per gram of AgI. Some of the nuclei led to
ice crystal formation at temperatures as warm as -5 oC. Si nce the cost
of seeding was so inexpensive with operation of ground-based nuclei
generating equipment only $2 to $3 per hou'r and aircraft operation only
about $25 per hour, by 1950 nearly 10 percent of the United States was
to receive cloud seeding (Dennis, 1980). Due to beneficiary dissatis-
faction with operational project results and few experiments suggesting
a positive treatment effect (Farhar, 1978), controversy developed
regarding the efficacy of cloud seeding.

Scientists soon recognized that a better understanding of cloud preci-
pitation processes was necessary to obtain predictable results.
Consequently, there have been a number of efforts aimed at increasing
such knowledge (e.g., Mielke et al., 1971; Grant and Elliott, 1974) but
only in the past 10 years has the instrumentation emerged and the
knowledge of scientists grown adequate, to develop more sophisticated
and detailed conceptual models of orographic clouds likely to have
potential for augmentation, and to develop more detailed cloud treatment
and seeding agent delivery models (Grant clnd Rauber, 1985).

3.2 Winter cloud types with modification potential

Currently, the cloud types considered to possess potential for preclpl-
tation augmentation are the shallow orographic cloud (figure 1), the
embedded convective cloud and the isolated convective cloud. The
o rographi c cloud, best known of the three types, is formed by the

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