WMOD00284

• Applicant: Thompson, J.R., G.W. Wilderson, and D.A. Griffith, North American Weather Consultants
• Sponsor Name: USBR
• Title: Cloud Seeding Data Collection, and Analysis Assoc. with the Colo River Augmentation Demonstration Program
• Prepared For: USBR, Divison of Atmospheric Research
• Prepared By: Thompson, Wilderson, Griffith
• Date: 12/1/1987
• State: AZ
• Weather Modification - Doc Type: Scientific Study

These findings, that episode durations tended to be longer
in Colorado, but precipitation totals were greater within
shorter time periods in Arizona, may be due to the fact that,
in Colorado, several episodes consisted of Mesoscale Stratiform
(MS) cloudiness. This condition, which has been observed
over the Grand Mesa on many occasions, can produce long periods
of very light precipitation. None of these cloud conditions
were observed in Arizona during the field season. In addition,
convection, which can produce significant precipitation in
a relatively short time, was observed to occur in several
of the Arizona episodes, at least for short intervals.

Of the 217 episodes identified in Arizona, 152 (70 percent)
of them had durations of 6 hours or less with these 152 episodes
accounting for 395 precipitation hours. However, they only
accounted for 11 percent of the total precipitation. Only
7 percent of the 152 episodes had precipitation totals of
0.10 inch or more with the greatest being 0.375 inches during
a 5 hour period. Over a third of these episodes (34 percent)
had durations of only one hour, while well over half of the
episodes (56 percent) lasted two hours or less. Average precipi-
tation per gauge per episode for these short term periods
was only 0.04 inches while the average duration for all episodes
6 hours or less was 2.5 hours (median 2 hours).

In light of the above statistics, it may be that enhancement
of precipitation during these short term durations will not
produce significant increases in precipitation over a watershed
because of the small amounts of precipitation that are likely
to be encountered. However, as noted earlier, episodes that
last up to 12 hours do contribute as much as 33 percent of

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