WMOD00572

• Title: Status of Precipitation Augmentation and Hail Suppression Experiments
• Date: 2/20/1990
• Weather Modification - Doc Type: Report

Printed January 30, 1990

Cloud seeders have often used an ice crystal concentration of 1 per liter as a desirable objective in
their attempts to optimize precipitation efficiency in orographic clouds. However, as crystal
concentration usually increases markedly with height, it is not possible to achieve any specified
concentration over any appreciable cloud depth. In any case, Jiusto and Weickmann (1973) have
found that the optimum concentration is a function of conditions near cloud base, and may be much
higher than 1 per liter if cloud base is warmer than OOC.

It has been hypothesized that application of excessive amounts of glaciogenic seeding agents could
decrease precipitation by creating numerous fine ice crystals with such low fall speeds that none of
them could reach the ground. Seeding in this manner is called overseeding. Some orographic
clouds contain enough natural ice crystals to optimize precipitation efficiency so, in theory, any
seeding at all would constitute overseeding. In practice, overseeding is very difficult to achieve,
because ice crystals tend to form aggregates of as many as 50 to 100 crystals, which have
appreciable fall speeds of as much as 1 m.s.1 (Jiusto and Weickmann, 1973).

Although overseeding does not appear to be a serious problem in orographic clouds, the question
of seedability deserves close attention. It was suggested over 20 years ago that orographic clouds
with cold tops would generate enough natural ice crystals to support an efficient precipitation
process, and therefore would not be seedable. Grant and Elliott (1974) reported some
observational data to support this view, and spoke of a "cloud seeding temperature window"
extending from about -10 to about -250C. The upper bound of _lOoC was set because most silver
iodide generator products are relatively inactive above that temperature. The lower bound of -
250C seemed reasonable, as some observations indicate that the concentration of active ice nuclei '1'f~
reaches 1 per liter at a temperature near -20oC. However, because of the phenome~n of ice
multiplication, some clouds with top temperatures as high as -10 or -150C may contain enough ice
particles, say 10 to 100 per liter, to utilize effectively the supply of condensate. Therefore Grant
and Elliott's (1974) window is only a rough guide to seedability.

Engineering studies of glaciogenic seeding of winter orographic clouds have included investigations
of the transport and diffusion of seeding agents, the nucleation of additional ice crystals, the
availability of SL W to support crystal growth, and the trajectories of both artificial and natural
snowflakes as they grow and fall to the ground. Two examples will now be presented.

Grand Mesa. Super and Boe (1988) have described the results of experiments using both aerial and
ground-based silver iodide generators to affect clouds over the Grand Mesa in western Colorado.
An instrumented aircraft flying 500 m above the top of the mesa measured the concentrations of
small snowflakes produced by the silver iodide crystals and thereby provided for tracking of the
generator plumes. Super and Boe's paper includes figures showing the distributions of ice crystals
in terms of both size and crystal habit (shape) at various times after seeding (slides). The data
were used to estimate the rate at which the artificial snow accumulated on the ground. The rate
remained low, typically the equivalent of 0.5 mm. h.1 of water, but significant when one considers
that non-precipitating, supercooled, orographic clouds persist over Grand Mesa for over 100 hours
each winter.

The Sierra Cooperative Pilot Proiect. The objectives of the Sierra Cooperative Pilot Project
(SCPP) included the testing and refinement of techniques for increasing precipitation in the Sierra

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