WMOD00011

• Applicant: Don Griffith North American Weather Consultants, Inc.
• Sponsor Name: City and County of Gunnison
• Project Name: Cloud Seeding Evaluation
• Title: The Development, Conduct and Evaluation of a Cloud Seeding Program for the Gunnison River Basin, Colorado During the 2003-2004 Season
• Prepared For: City and County of Gunnison
• Prepared By: North American Weather Consultants
• Date: 8/1/2004
• County: Gunnison
• Weather Modification - Doc Type: Report

I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I

4.0 EVALVA TIO:'iS or SEEDI:'iG EFrECTlVE:'iESS

The task of determining the effects of cloud seeding has received considerable attention
over the years. Evaluating the results of a cloud seeding program for a single season is rather
difficult. The primary reason for the difficulty stems from the large natural variability in the
amounts of precipitation that occur in a given area, and between one area and another during a
given season. Since cloud seeding is normally feasible only when existing clouds are near to (or
already are) producing precipitation. it is not usually obvious if. and how much. the precipitation
""'as actually increased by seeding due to this large natural variability. The ability to detect a
seeding effect becomes a function of the magnitude of the seeding increase and the number of
seeded events. compared with the natural variability in the precipitation pattern. Larger seeding
effects can be detected more easily, and ,",'ith a smaller number of seeded cases. than are required
to detect small increases_

Historically. the most significant seeding results ha\.e been observed in wintertime
seeding programs in mountainous areas. However. the apparent differences due to seeding are
relati\-ely small. being of the order of a 5.20 percent seasonal increase_ In pan. this relatively
small percentage increase accounts for the significant number of cases required to establish these
results (often live years or more).

Despite the difficulties invoh.ed. some techniques arc available for evaluation of the
effects of operational seeding programs. These techniques are not as rigorous or scientifically
desirable as is the randomization technique used in research. where roughly half the sample of
stornl events is randomly left unseeded. ~10st of!\A WC's clients do not wish to reduce the
potential benefits ofa cloud seeding project by half in order to better document the effects of the
cloud seeding project. The less rigorous techniques do. however. offer helpful indications of the
long-ternl effects of seeding on operational programs.

A commonly employed technique. and the one utilized by !'\A we in this assessment, is
the "target" and "control" comparison. This technique is one described by Dr. Arnett Dennis in
his book entitled "Weather 7\lodification by Cloud Seeding"(l980). This technique is based on
the selection of a variable that would be affected by seeding (such as liquid precipitation or

28