Does cloud seeding for snowpack augmentation really work?
Yes, if the seeding is
done in a scientifically sound manner. Not all seeding approaches are
The Weather Modification Association, World Meteorological
Organization (WMO), American Meteorological Society (AMS) and the
National Academy of Sciences (NAS) all state that there is strong
evidence for seasonal precipitation increases over natural
precipitation. The AMS further states that increases of about 10% are
feasible. Experiments in Colorado,
Montana and Australia showed statistically significant increases of 10%
or greater1;2;3. To achieve such increases, well-designed and conducted
projects must be operated on a long-term and continuing basis, not just
during droughts. Such projects will help fill reservoirs for use during
times of greater need, such as droughts.
What are the environmental and health effects of seeding material?
Minimal. The most common seeding material,
silver iodide, is used in very minute amounts. The typical
concentration of silver in rainwater or snow from a seeded cloud is
less than 0.1 microgram per liter (one part in 10,000,000,000). This is
well below the acceptable concentration of 50 micrograms per liter, set
by the U. S. Public Health Service. Many regions have much higher
concentrations of silver in the soil than are found in precipitation
from seeded clouds. The concentration of iodine in iodized salt used on
food is far above the concentration found in precipitation from a
seeded storm. National Environmental Policy Act compliance for all
cloud seeding environmental impacts has been demonstrated by past
studies. The U.S. Bureau of Reclamation (USBR) extensively studied
environmental and health impacts4;5;6. The toxicity of silver and silver
compounds (from silver iodide) was shown to be of low order. According
to the USBR, the tiny amounts of silver used in cloud seeding are
100 times less than industry emissions into the atmosphere in many
parts of the country, or individual exposure from tooth fillings.
Accumulations in the soil, vegetation, and surface runoff have not been
large enough to measure above natural background7. A 1995 environmental
assessment in the Sierra Nevada of California8 and a 2004 study in
Australia confirmed these earlier findings.
What are the impacts of additional snow?
An environmental assessment report in California5
investigated the impacts of an assumed precipitation enhancement of 5 - 7.5% on weather elements, hydrologic and
physiographic phenomena, plant and animal communities, the human
environment, and land and water resource use. The report concluded that
there would be no significant impact on these environmental sectors.
The percentage increases from precipitation enhancement are much smaller
than inter-annual variability of natural precipitation, which can be
several hundred percent9. Research conducted in the Uinta Mountains of
Utah indicated that “An increase of 10% in the average snowpack is
estimated to prolong the 75% snow-free date by 0.7-1.5 days”10. An
exhaustive five-year ecological study11
in the San Juan Mountains of
Colorado concluded that “…there should be no immediate, large-scale
impacts on the terrestrial ecosystems of these mountains following an
addition of up to 30 percent of the normal snowpack…” Furthermore, all
operating projects in the Western U.S. have suspension criteria
designed to stop cloud seeding anytime there is a flood threat.
Additionally, water management personnel from sponsoring companies
monitor streamflow and reservoir storage. Although precipitation
enhancements are small compared to natural precipitation variability,
there might be some concern about snow removal from roads and snow
loading on roofs.
Does cloud seeding decrease precipitation downwind?
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 10% by cloud seeding, the seeding depletes only 1% of the original
atmospheric moisture supply, the remainder of which (99%) is available for precipitation downwind12. 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 downwind precipitation;
in fact, sometimes there may be an increase as far as 100 miles
downwind of the target area13;14:15;16;17;18;19.
What about interference with nature?
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
What are the scientific sources for this FAQ?
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.
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.
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 FES 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.
8 Parsons Engineering Science,
Inc., 1995: Environmental Assessment for the Pacific Gas and Electric
Company and the U.S. Forest Service, Stanislaus National Forest.
Snowy Hydro Limited, 2004: Snowy Precipitation Enhancement Research
Project Questions and Answers. Online at
Harper, K. T., 1981: Potential ecological impacts of Snowpack
augmentation in the Uinta Mountains, Utah. Brigham Young University
Report to Utah Division of Water Resources, 291 pp.
Steinhoff, Harold W., and Jack D. Ives, Eds., 1976: Ecological impacts
of snowpack augmentation in the San Juan Mountains, Colorado. Final
report of the San Juan Ecology Project to the Bureau of Reclamation,
12 Hindman, E. W., 1986: An atmospheric water balance over a mountain barrier. J. Climate and Applied Meteorology, 25, 180-183.
Grant, L. O. and P. W. Mielke, 1990: An assessment of extra-area cloud
seeding effects on the Uinta Mountains and Basin of Utah. Final Report
to Utah Department of Natural Resources from Colorado State University,
November 30, 36 pp.
14 Griffith, Don. A., John R. Thompson, and
Dan A. Risch, 1991: A winter cloud seeding program in Utah. J. Weather
Modification, 23, 27-34.
15 Long, Alexis B., 2001: Review of
downwind extra-area effects of precipitation enhancement. J. Weather
Modification, 33, 24-45.
16 MacCracken, J.G., and J. O’Laughlin,
1996: California cloud seeding and Idaho precipitation. J. Weather
Modification, 28, 39-49.
17 Orville, H.D., and James R. Miller,
1992: On the cloud seeding potential of the Black Hills. J. Weather
Modification, 24, 66-79.
Solak, M. E., D. P. Yorty and D. A. Griffith, 2003: Estimation of
downwind cloud seeding effects in Utah. J. Weather Modification, 35,
Brown, K.L., et al., 1975: Large scale effects of cloud seeding. Final
report, 1970-1974 seasons. Reclamation contract number 14-06-D-6841,
Aerometric Reseearch Inc., Goleta, CA.
20 National Research
Council, 2003: Critical Issues in Weather Modification Research.
National Academy Press, Washington DC, 123 pp. Online at
A. and D. Rosenfeld, 2005: Separation between cloud-seeding and
air-pollution effects. J. Applied Meteorology, 44, 1298–1314.
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