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Seeding Clouds to Increase RainfallBack
 
A cloud
A seeded cloud
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.

 

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.

 

A cloud seeding plane
A plane seeding a cloud
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 difficult approach.

 

 

Projects in Illinois

    Average yearly yield increase from 25% summer rain enhancement
Average yearly yield increase from 25% summer rain enhancement
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.

 

A map showing the study area in Illinois  
Study area for the 1989 PACE project
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.

 

Seeding Experiments

    A radome used in the detection of weather and rainfall
Growth of ice crystals from drop evaporation
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.

 

Equipment mounted on an airplane used to seed clouds  
Silver iodide flares on seeding aircraft
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 same type.

 

Future Issues

    A radome used in the detection of weather and rainfall
Survey Doppler RADAR covered by radome
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.

 

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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