“Although 40 years have passed since the first NAS report on weather modification, this Committee finds itself very much in concurrence with the findings of that assessment we conclude that the initiation of large-scale operational weather modification programs would be premature. Many fundamental problems must be answered first. It is unlikely that these problems will be solved by the expansion of present efforts, which emphasize the a posteriori evaluation of largely uncontrolled experiments. We believe the patient investigation of the atmospheric processes coupled with an exploration of the technological applications may eventually lead to useful weather modification, but we emphasize that the time-scale required for success may be measured in decades.” [1]
Committee on the Status and Future Directions in U.S. Weather Modification Research and Operations. National Research Council, Division on Earth and Life Studies, Board on Atmospheric Sciences and Climate. Michael Garstang (chair), Roscoe R. Braham Jr., Roelof T. Bruintjes, Steven F. Clifford, Ross N. Hoffman, Douglas K. Lilly, Robert J. Serafin, Paul Try, Johannes Verlinde.
The report is the latest in a series of assessments of weather modification carried out by the National Academies; all of these reports have concluded that the science underlying attempts to modify the weather is too weak to prove that such attempts actually produce results. Since the last Academy report on the subject in 1973, however, there have been significant advances in observational, computational, and statistical technologies, allowing a more thorough investigation of the complex processes in the atmosphere, the new report says. Still, little of this collective power has been applied in any coherent way to weather modification.
The national program recommended in the report would conduct research on cloud and precipitation microphysics, cloud dynamics, cloud modeling, and cloud seeding. The program would focus on using new tools to carry out experiments, working to improve models of clouds and precipitation, and developing partnerships among research groups and select operational programs. The new technologies include millimeter-wave cloud radar, which has high sensitivity and resolution, enabling researchers to observe the fine-scale structure of clouds, snowstorms, and rainfall. This radar also holds great promise for revealing the physical transformations in the seeded regions of clouds, the report says. [2]
Despite the lack of hard, consistent and repeatable evidence to support it, many government agencies and private firms continue to take part in cloud seeding. In its 2003 report, "Critical Issues in Weather Modification Research," a panel of the National Academies found that 10 U.S. states were conducting at least 66 cloud-seeding programs. "We know that human activities can affect the weather, and we know that seeding will cause some changes to a cloud," noted the report. "However, we are still unable to translate these induced changes into verifiable changes in rainfall and hail fall, and snowfall on the ground, or to employ methods that produce credible, repeatable changes in precipitation."
The report did note progress in some areas. Research suggests that cloud seeding can be effective in those mountainous regions where upslope winds often induce clouds that are inefficient in producing snow on their own.
In addition, the hygroscopic seeding of summertime convective clouds shows some promise to enhance rainfall. However, in times of serious drought, weather modification may be of little use. When the skies are clear, there are no clouds to seed. [3]
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