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|Seeding Clouds to Increase Rainfall||Back|
Since World War II, many programs in various parts of the United States have attempted
to increase local precipitation by seeding clouds with chemicals. Although most of these projects
have been in areas of drought, prolonged dry periods, or in the more arid West, Illinois State
Water Survey (ISWS) scientists also conducted a major field experiment to assess seeding
potential from 1984 to 1995. Today, cloud-seeding projects are routinely conducted in North
Dakota, Colorado, and California.
|A seeded cloud|
How Is Seeding Done?
Quite simply, successful seeding increases the number of raindrops that form inside
certain clouds, including cumulus and stratus. Silver iodide (AgI), a common chemical, has been
used for this purpose. When burned, AgI produces tiny particles, called ice nuclei, that capture
water vapor in a cloud and form droplets.
Three techniques have been used to insert AgI particles into clouds:
• Burning AgI in ground-based generators in hopes that AgI particles will rise and enter passing clouds.
• Using an airplane that carries burners for generating AgI aloft and at the cloud base where air flows into the cloud (the updraft).
• Flying an airplane through a cloud and burning AgI where raindrops already are forming.
Seeding projects conducted in Illinois have used all three techniques. Use of ground-based generators is the least expensive and the least effective method. Seeding by an airplane
flying inside a growing cumulus cloud is considered the best approach, but it is also the most
|A plane seeding a cloud|
Projects in Illinois
Farmers in the Vandalia area raised funds to hire a cloud-seeding firm for a project in
summer 1963. Ground-based generators were used. At the end of the project, the summer rainfall
total at the Weather Service raingage in Vandalia was slightly higher than in raingages in
surrounding counties. The difference could have been due to natural conditions, however. Survey
scientists were asked to monitor the project.
|Average yearly yield increase from 25% summer rain enhancement|
During 1969–1980, farmers in four Illinois areas raised funds for cloud-seeding projects
over two summers (Vermilion County) and three summers (McLean
County, Mattoon area, and
Harrisburg area). Reputable cloud-seeding firms were hired to deliver AgI at cloud bases. Each
project involved establishing a seeding headquarters at a local airport in the project area, staff to
forecast rain conditions and direct operations, two piloted airplanes equipped with AgI burners,
and a radar to guide the planes to approaching showers. Annual costs for each project ranged
from $85,000 to $100,000 (1980 dollars), or over $1 million to support the 11 project years of
operations in Illinois. Using raingage data and radar data to evaluate project results, ISWS
scientists calculated rainfall increases of 10 percent in the seeded areas in most years but no
increase in a few years.
|Study area for the 1989 PACE project|
Interest in using cloud seeding to increase Illinois’ water resources and provide additional
moisture for crops during dry periods led ISWS scientists to design a multi-year field experiment
to test seeding under controlled scientific conditions. Federal funding was used to start the costly
project in 1971, but agency budget reductions ended the support before field tests got underway.
However, studies of the possible impacts of seeding showed crop yields and surface water
supplies would benefit greatly from potential increases in summer rains without environmental
effects from release of minuscule amounts of AgI.
|Growth of ice crystals from drop evaporation|
Adequate funding became available in 1984, and field tests were conducted in the
summers during 1986–1990. An airplane randomly seeded the AgI into growing cumulus clouds
so that both seeded and unseeded clouds were available for comparison. A sophisticated Doppler
weather radar measured precipitation growth inside the cloud before and after seeding, and also
the amount of rain deposited on the ground by the individual clouds. Certain types of cumulus
congestus clouds produced 5 to 15 percent more rain when seeded than unseeded clouds of the
|Silver iodide flares on seeding aircraft|
Current interest in cloud seeding in Illinois is low, even though modest increases in
summer rain are possible. Many potential sponsors think that the substantial project costs do not
outweigh the benefits of receiving slightly more rain. This situation could change if the economy
improved and value of the added rainfall was perceived to exceed the project costs.
|Survey Doppler RADAR covered by radome|
In concert with the Illinois Farm Bureau, ISWS leaders worked with the Illinois General
Assembly, and legislation was enacted in 1972 that controlled the use of weather modification and
established a board that included scientific expertise to issue project permits for quality projects
based on adequate plans, staff, and equipment. Permits were issued for the Illinois projects
conducted during 1973–1980. Other states that frequently employ cloud seeding have similar
regulations regarding weather modification efforts. Although this Illinois regulation was
terminated in the 1980s because of a lack of use of cloud seeding, it is an important issue to
consider if interest in cloud-seeding projects is revived.
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Last Modified: May 8, 2013
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