National Academy of Sciences urges further research


A recent proposal by PG&E to begin a five year cloud seeding project east of Mount Shasta to increase rainfall is on hold pending a special use permit from Siskiyou County. The proposal has raised concerns regarding whether the procedure is environmentally safe with citizens proposing that the Mount Shasta city council pass a controversial, and perhaps unenforceable, ordinance that is aimed at preventing the project.
Cloud seeding is controversial, with concerns as to silver iodide’s safety and the long term effects of weather manipulation. A recent report by the National Academy of Sciences, however, raises an issue that could trump safety concerns as the study concludes there is no credible scientific evidence that cloud seeding even works.
The NAS study concluded that “Every assessment of weather modification dating from the first National Academies’ report in 1964 has found that scientific proof of the effectiveness of cloud seeding was lacking.”
“The Committee concludes that there still is no convincing scientific proof of the efficacy of intentional weather modification efforts,” the study says. “In some instances there are strong indications of induced changes, but this evidence has not been subjected to tests of significance and reproducibility.”
The NAS report also recommends that cloud seeding research should include social and environmental concerns.
Cloud seeding is a procedure by by which silver iodide vapor is released into the air, typically by plane or ground based generators, causing water droplets in clouds to freeze and clump. The added weight of the larger water droplets would then cause the droplets to fall. In effect, cloud seeding theoretically forces clouds that would normally stay overhead to fall as rain.
The purpose of cloud seeding is, of course, to produce rain and the need for more water in the world has never been greater. According to Oxfam, more than 4 billion people will face water shortages in the near future. The US Commerce Department's National Oceanic and Atmospheric Administration satellite program reports that 20 percent of the earth’s surface is in a state of severe drought.
Several studies, including the report from the National Academy of Sciences, conclude that there is not enough evidence to support cloud seeding as an effective rain making tool. Other studies claim cloud seeding actually suppresses rainfall or simply redistributes it.
Cloud seeding supporters say there is no question the procedure causes increased precipitation.
Hundreds of millions of dollars are spent worldwide on the procedure, with China having spent over $300 million on cloud seeding since 1993 with current annual expenditures at $60 to $90 million a year.
The State of Utah, among many states that collectively spend tens of millions of dollars annually on cloud seeding, claims the procedure increases precipitation. The Utah Division of Water Resources claims that cloud seeding has increased precipitation 14 to 20 percent. The departments website at contains studies that appear to support this contention.
The NAS, however, says that “numerous methodological advances thus have not resulted in greater scientific understanding of the principles underlying weather modification.”
“Despite this lack of scientific proof, operational weather modification programs to increase rain and snowfall and to suppress hail formation continue worldwide based on cost versus probabilistic benefit analysis,” the NAS?study states. “Among the factors that have contributed to an almost uniform failure to verify seeding effects are such uncertainties as the natural variability of precipitation, the inability to measure these variables with the required accuracy or resolution, the detection of a small induced effect under these conditions, and the need to randomize and replicate experiments.”
The NAS concedes that cloud seeding may increase precipitation, but states that intensive, carefully controlled studies are needed to support whether the procedure works.
“The Committee recommends that a coordinated national program be developed to conduct a sustained research effort in the areas of cloud and precipitation microphysics, cloud dynamics, cloud modeling, and cloud seeding; it should be implemented using a balanced approach of modeling, laboratory studies, and field measurements designed to reduce the key uncertainties,” the NAS states.
In addition, the NAS?suggests that cloud seeding studies should include impacts beyond precipitation.
“It must be acknowledged that issues related to weather modification go well beyond the limits of physical science,” the study says. “Such issues involve society as a whole, and scientific weather modification research should be accompanied by parallel social, political, economic, environmental, and legal studies.”