WMOD00308 ‎(2)‎

• Title: Operation and Evaluation Report of the Santa Barbara County Cloud Seeding Program, 1977-78 Winter Season
• Date: 8/1/1978
• State: CA
• Weather Modification - Doc Type: Report

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4. PROGRAM EVALUATION

Evaluations of the results of a cloud seeding program are unfor-
ttmately rather difficult. The seemingly simple problem of detennining
the effects of cloud seeding has received considerable attention over
the years. The primary reason for the difficulty stems from the large
natural variability in the amounts of precipitation that occur in a
given area. Since cloud seeding is only feasible when there are clouds
and usually only when there are clouds that are near to or are already
producing precipitation naturally, the question then becomes "did the
seeding increase (or d.ecrease) the precipitation that ,vas observed and
if so by how much"? The ability to detect a seeding effect then becomes
a function of the size of the seeding increase compared to the natural
variability in the precipitation pattern~ Larger seeding effects can be
dectected easier and with a smaller number of se:eded cases than are
required to detect small increases.

The question has been addressed in research seeding programs in the
Uni ted States and elsewhere by employing a technique knOlm as randomiza-
tion. With this tedmi.que, an objective method is developed to identify
seeclable situations. The decision whether to seed or not seed each
situation is made randomly. Usually, half the situations over a period
of time are seeded and the remaining half left unseeded. Once an adequate
number of cases have been acquired, comparisons can be made between
observations of such parameters as precipitation for the seeded and
lUlseeded cases. Any apparent differences are tested for significance
using various statistical tests to determine whether there is a high
probability.that the differences are important. Results from such research
programs are available for a variety of programs.

The approach that has been adopted in this evaluation is to utilize
the data from the Santa Barbara research project as a base for the develop-
ment of a "target" and "control" comparison since the seeding teclmique
used in the operational program was essentially the same as the one
developed for the research project.

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