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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advection southward along the western U.S., not evolving into a closed cir-
culation but moving eastward at moderate speed. The third storm type of
importance, which produces light to moderate precipitation, is caused by a
mild disturbance in the circulation around a large closed low pressure
system located over cent ral Canada. The fourt h storm type characteri zed,
which generally yields light precipitation, originates from a propagating
mild disturbance embedded in westerly or northwesterly flow occurring over
the eastern Pacific Ocean and the western U.S.

The suggestion in some prior studies that precipitation percentage increase
from cloud treatment may depend on event amount, suggested data analyses
which included stratification of historical events by precipitation amount.
Furthermore, since small-amount events generally occur with higher fre-
quency, their study was vital. Comparisons were made among selected wet,
normal and dry winters.

Two primary parameters, the temperature at the 500 millibar pressure level
(approximately 5.6 kilometers or 18,000 feet) and the wind speed at the 700
millibar pressure level (about 3 kilometers or 10,000 feet) were employed
in analyses with precipitation measurements. These parameters have been
found to be a useful index of seedability in other projects, though the
physical basis has not yet been determined.

Results of analyses indicated that, for mountain stations studied, about 60
percent as many precipitation days (24-hour period with more than a trace
of precipitation) occurred during the dry winter as compared to the wet
one. Also, the wet winter experienced more large events. Of precipitation
cases with 500 millibar temperatures and 700 millibar wind speeds within a
selected seedability window (categories suggested in previous studies as
favorable for cloud treatment), about 20-25 events occurred during a dry
winte~ and 35-40 events in a wet winter on the Mogollon Rim that may have
had treatment potential during some phasE~ of their lifetime. In com-
parison, the number of opportunities dropped by about 30 percent in the
Chiricahua Mountains and 50 percent in the Mule Mountains to the south.
Additionally, more large events occurred in the southern mountains. These
results suggest, assuming that the indication of seedable clouds by this

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