WMOD00319

• Title: North American Interstate Weather Modification Council - Fiscal Year 2009 Request
• Prepared By: McClintock Associates, Inc.
• Date: 1/1/2007
• Weather Modification - Doc Type: General Information

sponsoring companies monitor streamflow and reservoir storage. Although weather modification increases are
small compared to natural precipitation variability, there might be some concern about snow removal from roads
and snow loading on roofs.

Q. Does cloud seeding decrease precipitation downwind?
A. No. The idea that precipitation increases in one area cause decreases elsewhere is a misconception. The
amount of atmospheric moisture passing over a mountain barrier that is converted to precipitation is usually 10%
or less. If this natural precipitation is increased by 10% by cloud seeding, only 1 % of the original atmospheric
moisture supply is depleted1 . Moreover, winter cloud seeding is done on clouds on the upwind side of mountain
ranges. These clouds usually dissipate on the downwind or lee side of the range, because of a natural effect called
the "rain shadow." This is the reason that lee side areas like the Colorado Front Range and Nevada are much drier
than on the upwind side of the mountains. So the atmospheric moisture supply on the lee side of the mountain
range will not likely precipitate anyway. Finally, precipitation data from a number of long-term cloud seeding
projects have been examined in detail for evidence of "extra-area" effects. These examinations do not show that
seeding clouds with silver iodide causes a decrease in do~nw~n~ fr~ipitation; in fact, sometimes there may be an
increase as far as 100 miles downwind of the target area13.14.15.16.1 .18.19.

Q. What about interference with nature?
A. These questions often ignore the fact that human activities have caused inadvertent weather modification for
many centuries. A recent National Research Council report20 states that "there is ample evidence that inadvertent
weather and global climate modification (e.g., greenhouse gases affecting global temperatures and anthropogenic
aerosols affecting cloud properties) is a reality." Even the simple act of cultivating a farm field alters local
climate. Intentional weather modification, particularly of the form practiced in winter seeding, alters the
environment far less than the accumulated effects of inadvertent weather modification. Indeed, cloud seeding in
California may have been partially compensating for precipitation losses from the inadvertent weather
modification brought on by air pollution21.

REFERENCES

1 Super, Arlin B. and James A. Heimbach, 1983: Evaluation of the Bridger Range winter cloud seeding
experiment using control gages. J. Applied Meteorology, 22, 1989-2011.

2 Ryan, B. F., and W. D. King, 1997: A critical review of the Australian experience in cloud seeding. Bull. Amer.
Meteor. Soc., 78, 239-354.

3 Mielke, P. W., G. W. Brier, L. O. Grant, G. J. Mulvey and P. N. Rosenzweig, 1981: A statistical reanalysis of
the replicated Climax I & II wintertime orographic cloud seeding experiments. J. Applied. Meteorology, 20, 643-
659.

4 Bureau of Reclamation, 1977: Project Skywater, A program of Research in Precipitation Management. Final
Environmental Statement (INT PES 77-39).

5 Harris, Edward R., 1981: Sierra Cooperative Pilot Project - Environmental Assessment and Finding of No
Significant Impact. U.S. Department of the Interior, Bureau of Reclamation, Denver, CO, 208 pp.

6 Howell, Wallace E., 1977: Environmental Impacts of Precipitation Management: Results and Inferences from
Project Skywater. Bull. American Meteorological Society, 58, 488-501.

7 Donald A. Klein, 1978: Environmental Impacts of Artificial Ice Nucleating Agents, Dowden, Hutchinson &
Ross, Inc., Stroudsburg, 256 pp.

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