WMOD00413

• Title: Validation of Precipitation Management by Seeding Winter Orographic Clouds in the Colorado River Basin
• Date: 9/1/1993
• Weather Modification - Doc Type: Report

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2. BACKGROUND

This section discusses recent weather modification policy statements and presents a historical
overview of winter orographic cloud seeding experimentation. Several factors are explored
which now make it possible to scientifically demonstrate that cloud seeding can enhance
mountain snowfall and subsequent runoff in the Colorado River. I

2.1 Weather Modification Policy Statements

Since 1957, several weather modification panel reports have stated that some winter
orographic cloud seeding projects appeared to augment precipitation (Advisory Committee on
Weather Control, 1957; National Academy of Sciences-National Research Council, 1966;
National Academy of Sciences, 1973; Weather Modification Advisory Board, 1978). 'lWo recent
policy statements provide current scientific views on the status of cloud seeding technology.

The most recent weather modification policy statement of the 10,000 member AMS (American
Meteorological Society), published in the March 1992 issue of the Bulletin oftheAMS, indicates
that scientific proof remains elusive. The policy statement notes that, "There is considerable
evidence that, under certain conditions, precipitation from supercooled orographic clouds can be
increased with existing techniques. Statistical analyses of precipitation records from some
long-term projects indicate that seasonal increases on the order of 10 pct have been realized.
The cause and effect relationships have not been fully documented; however, the potential for
increases of this magnitude is supported by field measurements and numerical model
simulations." The policy statement further notes that, "The processes culminating in increased
precipitation have recently been directly observed during seeding experiments conducted over
limited spatial and temporal domains." These brief seeding experiments which affeicted small
areas were similar to, but less comprehensive than, the direct detection experiments discussed
in section 4.

The newest policy statement of the WMO (World Meteorological Organization), affiliated with
the United Nations, was approved in July 1992. Both the AMS and WMO policy statements
were prepared by panels of experts, the latter from several countries worldwide. 'The WMO
statement notes that, "In our present state of knowledge, it is considered that the glaciogenic
seeding of clouds or cloud systems either formed, or stimulated in development, by air flowing
over mountains offers the best prospects for increasing precipitation in an economically viable
manner." The policy statement notes that statistical analyses suggest 10 to 15 pct seasonal
precipitation increases in certain project areas. The statement continues, "Physical studies
using the new technology highlighted above, give convincing evidence of the production of an
effective seeding agent, the tracing of the agent to SLW portions of the cloud, the initiation and
development of ice crystals to precipitation size particles, and the fallout of additional
precipitation on mountain slopes in favorable situations over limited areas. Numerical
simulations of the process corroborate the physical studies. This does not imply the problem of
precipitation enhancement in such situations is solved Much work remains to be done in
pursuit of the goals of strengthening the results and producing incontrovertible statistical and
physical evidence that the increases occurred over a wide area, over a prolonged period of time
and with minimum, or positive, extra area effects." The goal of the CREST is to provide such
statistical and physical evidence in the Basin.

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