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

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North American Interstate Weather Modification Council
www.nalwmc.org

"Facilitating the exchange of interstate/interprovincial atmospheric
resource management information. "

Answers to Frequently Asked Questions about
Cloud Seeding to Augment Mountain Snowpacks*

Q. Does cloud seeding for snowpack augmentation really work?
A. Yes! 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. 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. Experiments in Colorado, Montana and Australia showed statistically significant
1-2-3
increases of 10% or greater ' , .

Q. What are the environmental and health effects of seeding material and its effect on clouds and
precipitation?
A. 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 Bureau of Reclamation
(Reclamation) extensively studied environmental and health impacts4;s:6. The toxicity of silver and silver
compounds (from silver iodide) was shown to be of low order. According to Reclamation, 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 enou~h to measure above natural background7. A 1995 environmental assessment in the Sierra
Nevada of California and a 2004 study in Australia confirmed these earlier findings.

Q. What are the impacts of additional snow?
A. An environmental assessment report in Californias investigated the impacts of an assumed weather
modification-induced precipitation increase 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 weather modification 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"lO. An exhaustive five-
year ecological studyll 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