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---Cloud Seeding Frequently Asked Questions---
Cloud seeding (also known as weather modification)
is the deliberate treatment of certain clouds or cloud systems with the
intent of affecting the precipitation process(es) within those clouds.
Application of this technology is increasing world-wide.
This page addresses some key questions relating to cloud seeding and
its practical uses. References are provided at the end of this document keyed to
numbers at the end of the highlighted topics. A brief summary of the scientific basis for cloud seeding is available here.
application of modern
cloud seeding technology begin?
Attempts to modify the weather have been conducted for centuries.
cloud seeding dates from the late 1940's, springing from a discovery at the
General Electric labs in Schenectady, New York in 1946.
The ability of dry ice shavings to convert supercooled water droplets
(those existing as water at temperatures colder than freezing) to ice crystals
was observed during the conduct of an unrelated experiment.
Later consideration of those observations led to a series of laboratory
trials which demonstrated the nucleating
properties of various materials in certain cold cloud
conditions. Trials in the
atmosphere soon followed, and operational cloud seeding programs began in
Reference: 1, 2.
are the most common applications of cloud seeding technology?
The most common intended effects of cloud seeding include precipitation
increase (rain and/or snow), fog dispersal (visibility improvement) and hail
suppression. Of these, the
majority of operational projects focus on precipitation increase.
Reference: 1, 2.
cloud seeding effective?
Trials conducted by various researchers under laboratory conditions have
documented the effects of cloud seeding materials.
Numerous scientific experiments have been conducted to
investigate/demonstrate the effects produced by cloud seeding in the
atmosphere on various cloud types in a variety of climatic regions.
Evaluations (usually statistical) have been made of many operational
programs. The results of many of
these efforts have been published in the scientific literature and in industry
Summarizing the collective evidence from the various studies and
operational projects, Capability Statements have been published by the Weather
Modification Association, the American Meteorological Society and the World
Meteorological Organization. For
precipitation augmentation, the accepted magnitude of increase to be expected
from well-designed and properly conducted projects ranges from 5% to 20% for
winter precipitation in continental regions and from 5% to 30% for coastal
areas. For warm season
precipitation increase, single-cloud experiments have indicated increases as
large as 100%. Area wide
increases over a project area vary with the frequency of occurrence and
spatial coverage of suitable cloud systems, plus the ability to treat all
favorable clouds. Hail
suppression effectiveness, based upon surface hail data, is estimated to be in
the range of a 20-50% reduction.
conducts cloud seeding activities?
The large majority of cloud seeding projects are conducted by a handful
of highly specialized commercial firms, working under contract to a variety of
sponsors. Some water agencies and
hydroelectric power generation companies conduct their own programs.
Researchers continue to conduct occasional trials within carefully
designed and controlled experimental projects, striving to better understand
the various in-cloud effects of cloud seeding, and to refine
quantitative estimates of cloud seeding effectiveness.
are the most common sponsors of cloud seeding projects?
The most common sponsors of cloud seeding projects include water
agencies, municipalities, operators of hydroelectric power facilities,
agricultural or ranching interests, airports and recreational interests such
as ski areas. An increasing
number of sponsors are incorporating cloud seeding as an integral part of
their ongoing water resource management strategies.
is cloud seeding so attractive to the increasing number of sponsors?
Cloud seeding is a highly portable and flexible technology.
It does not require construction of large, permanent and costly
structures, such as dams or water conveyance systems.
Projects can be mobilized quickly and operations can be regulated as
water needs dictate or suspended very quickly if hazardous weather conditions
develop. Comprehensive laboratory
and field studies have indicated no significant environmental impacts.
Further, the benefit/cost ratios associated with most cloud seeding
projects are typically very favorable, ranging as high as 25-30:1, depending,
in the case of precipitation increase applications, on the value of water.
Reference: 5, 6, 7.
widely used/accepted is cloud seeding technology?
Over its history of 50 years, modern cloud seeding has involved projects
of various types in nearly 50 countries around the world.
Some individual projects have been in operation nearly continuously for
decades, with a few operating for nearly fifty years.
As water needs increase world-wide, the demand for weather modification
services will also increase. Similar
increase in demand will occur in hail-prone regions.
Reference: 5, 8.
is the cloud seeding accomplished?
Cloud seeding materials are released via ground-based and/or airborne
systems (see pictures). Determination of the
best suited method or combination of methods for a given project is based upon
an assessment of a variety of factors. The
seeding materials are applied to the clouds (sometimes targeted very carefully
into very specific portions of clouds) so that the material has adequate time
to affect the precipitation process, so the effect will be focused over the
intended geographic area.
Reference: 1, 4, 5.
are the most commonly used seeding materials?
The materials used in cloud seeding include two primary categories, tied
to the type of precipitation process involved. One category includes those which act as glaciogenic
(ice-forming) agents, such as silver iodide, dry ice and compressed liquid
propane or carbon dioxide, which are appropriate in cloud systems where the
precipitation process is primarily cold
(colder than freezing). Of the
ice-forming materials, the most commonly used is silver iodide.
The second major category is focused on cloud systems where the warm (coalescence) process predominates.
In those environments, hygroscopic (water attracting) materials such as
salt, urea and ammonium nitrate can be utilized. Of the hygroscopic materials, the most commonly used are
cloud seeding work?
In cold cloud seeding. the introduction of an ice-forming nucleating agent,
e.g., silver iodide, into the appropriate cloud regions causes supercooled
liquid water droplets to freeze. Once
these droplets freeze, the initial ice embryos grow at the expense of the
water droplets around them (sublimation) and through contact with these
neighboring water droplets (riming). These
embryos, if they remain in favorable cloud conditions, will grow into
snowflakes, falling to the surface as snow if surface temperatures are below or near freezing, or as
raindrops at warmer surface temperatures.
This process mimics nature where certain airborne substances, e.g.,
soil particles have the ability to act as ice-forming nuclei and initiate the
freezing process. A secondary
effect of this process can occur, wherein the freezing of water droplets
releases latent heat of fusion into the cloud.
This addition of heat, under the right circumstances, can cause the
treated clouds to grow larger and last longer than would have naturally
The first freezing process is often referred to as a static seeding effect, increasing the efficiency of the precipitation process
within the seeded cloud volume. The
second freezing process, resulting from release of additional heat into the
cloud, is often called the dynamic
effect, whereby the treated clouds are invigorated, thus
Although one might initially develop the impression that cold cloud seeding is strictly appropriate to winter clouds, it can work
equally well during the summer when the precipitation forming process is
active in the upper (colder) portions of cumulus clouds and adequate amounts
of supercooled water droplets are available.
Reference: 1, 2, 5.
How does warm
cloud seeding work?
Nature can produce rainfall from clouds that are warmer than freezing.
Tiny water droplets that form during condensation and define the cloud
can grow as they collide with one another within the cloud.
This process is known as collision/coalescence.
Cloud seeding of this type of cloud involves introduction of additional
(e.g., salt particles) which can cause additional water droplets to
condense within the cloud. Various
modeling and research studies have indicated that this type of seeding is
effective in continental clouds, but ineffective in maritime clouds.
Reference: 1, 2, 5.
the commonly used seeding materials pose any direct health or
Many detailed studies have been conducted to address these questions.
These efforts have ranged from chemistry-focused work to broad ranging
environmental investigations. The
bottom line is that no significant environmental effects have been observed.
Seeding materials are applied in very small amounts relative to the
size of the geographic areas being affected, so the concentrations of the
seeding materials in rainwater or snow are very low.
Using silver iodide (the most common seeding material) as an example,
the typical concentration of silver in rainwater or snow from seeded cloud
systems is less than 0.1 micrograms per liter.
This is much below the U.S. Public Health Service=s
stated acceptable concentration of 50 micrograms per liter.
As another example, the concentration of iodine in rainwater from
seeded clouds is far below the concentration found in common iodized table
Reference: 1, 2, 5, 9.
cloud seeding rob
Peter to pay Paul?
The answer to this commonly-posed question is no.
Of the total atmospheric moisture passing over any point, the
proportion falling as natural precipitation is quite small, typically less
than 10-15%. Cloud
seeding-induced increases in precipitation of the order of 5-30% still results
in a small overall proportion (<20%) of the total available moisture
reaching the ground. Further,
especially when cumuliform clouds are present, and over mountainous terrain
where air is forced to rise, the cloud-bearing layer of the atmosphere
undergoes nearly continuous moisture replenishment. Analyses of precipitation data from areas downwind of several
cloud seeding projects have indicated small percentage precipitation increases
extending as far as 100 miles downwind of the intended areas of effect on
projects that had indications of increases in the intended target area.
Reference: 1, 10, 11, 12, 13.
Are cloud seeding activities
subject to regulation or control?
In many jurisdictions, government agencies are responsible for
regulation of cloud seeding in the public interest. These agencies commonly require licenses and/or permits for
cloud seeding, to help assure that the projects are properly designed and that
those conducting such operations are properly qualified. In the U.S., for example, nearly two-thirds of the fifty
states have developed rules and regulations specific to cloud seeding
activities. These regulatory
groups generally also maintain records of cloud seeding activities.
does cloud seeding cost?
The cost of cloud seeding varies greatly, depending on a large number of
factors, such as which seeding methods and materials are appropriate to a
specific application, the frequency of seedable conditions, the size of the
intended area of effect and the duration of the project.
Most cloud seeding projects carry favorable benefit/cost ratios,
ranging over 20:1 in some cases. Cost questions are best addressed via direct discussion with
a well qualified cloud seeding company/consultant.
does the future hold for the cloud seeding field and its sponsors?
As water needs steadily increase worldwide, the demand for weather
modification services will also increase.
Focused research efforts will continue to yield incremental refinements
to the technology. Sponsors will
increasingly enjoy the benefits of cloud seeding at very attractive
benefit/cost ratios, and a growing number of those sponsors will incorporate
cloud seeding as an integral part of their ongoing water resource management
Dennis, A.S. , 1980: Weather Modification by Cloud Seeding.
Academic Press, New York.
Hess, W.N. , 1974: Weather and Climate Modification.
John Wiley & Sons, New York.
Weather Modification Association, 1992: Weather Modification Capability
Statement. Journal of Weather
Modification, Vol. 24, No.
Weather Modification Association, 1997: Some Facts About Seeding
Clouds. Fresno, California.
American Society of Civil Engineers, 1995: Guidelines for Cloud Seeding
to Augment Precipitation. ASCE
Manuals and Reports on Engineering Practice No.
Griffith, D.A. and M.E. Solak, 1999: A Cloud Seeding Program to Enhance
Hydroelectric Power Production from the El Cajon Drainage, Honduras.
on Weather Modification, Chiang Mai, Thailand, February 17-22, 1999.
Stauffer, N.E. and K. Williams,
2000: Utah Cloud Seeding Program, Increased Runoff/Cost
Analysis. Technical Report, Utah
Department of Natural Resources, Division
of Water Resources.
Weather Modification Advisory Board, 1978: The Management of Weather
Resources. Report to the
Secretary of Commerce, 2 volumes.
Klien, D.A. , 1978: Environmental Impacts of Artificial Ice Nucleating
Agents. Dowden, Hutchinson &
Ross, Inc. , Stronsburg, Pennsylvania.
Brown, K.J., R.D. Elliot and M.W. Edelstein, 1978: Transactions of
Total-Area Effects of Weather
Modification. Report to the
National Science Foundation on a
workshop held August 8-12, 1977, Fort Collins, Colorado.
Brown, K. J., R.D. Elliott and J.R. Thompson, 1974: The Seeding of
AMS Fourth Conference on Weather Modification, Ft. Lauderdale, Florida,
November 18-21, 1974.
Long, A. B., 2001: Review of Persistence Effects of Silver Iodide Cloud Seeding.
Journal of Weather Modification, Vol. 33, No.
Solak, M. E., D. P. Yorty, and D. A. Griffith, 2003: Estimations of Downwind Cloud Seeding Effects in Utah.
Journal of Weather Modification, Vol. 35, No.
How can I obtain more specific
The Weather Modification Association (WMA) has produced a basic
informational booklet, Weather Modification: Some Facts About Seeding Clouds.
The WMA can be contacted at 559-434-3486 or via e-mail at email@example.com.
The American Society of Civil Engineers (ASCE) has published a
comprehensive manual, Guidelines for Cloud Seeding to Augment Precipitation,
which contains much useful information.
telephone number is 800-548-2723, and their web site is http://www.asce.org.
For those wishing to investigate the possibility of developing and
implementing a specific project, we
recommend that you contact us, North American Weather Consultants,
an industry pioneer and leader since 1950, to discuss your cloud
seeding needs and interests. As
the longest-standing, continuously-operating cloud seeding company in the U.S.,
and probably the world, we have amassed a track record of many hundreds of
successful research and operational project seasons in a wide variety of
climatic regions world-wide. We
would be happy to discuss your specific needs and interests, and help with all
aspects of cloud seeding issues, from basic questions to feasibility studies to
full service cloud seeding projects. Our
projects are never of the cookie cutter
variety. We will work closely with
you to design and implement a project which is tailored specifically to your
circumstances. Contact information
is provided below.