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 Winter Cloud Seeding FAQ
Does cloud seeding for snowpack augmentation really work? Yes, if the seeding is done in a scientifically sound manner.  Not all seeding approaches are equally effective.   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 pollution21.

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.

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 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.

9 Snowy Hydro Limited, 2004: Snowy Precipitation Enhancement Research Project Questions and Answers. Online at http://svc097.wic010v.server-web.com/files/SPET_QA.pdf

10 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.

11 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, 489 pp.

12 Hindman, E. W., 1986: An atmospheric water balance over a mountain barrier. J. Climate and Applied Meteorology, 25, 180-183.

13 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.

18 Solak, M. E., D. P. Yorty and D. A. Griffith, 2003: Estimation of downwind cloud seeding effects in Utah. J. Weather Modification, 35, 52-58.

19 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 http://books.nap.edu/books/0309090539/html/index.html

21 Givati, A. and D. Rosenfeld, 2005: Separation between cloud-seeding and air-pollution effects. J. Applied Meteorology, 44, 1298–1314.

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