Q: You sometimes hear about efforts to create rain, and the term "cloudseeding" comes up. What exactly is cloudseeding?

A: Efforts to control the weather – the correct term is "weather modification" – have been around longer than you think, says Brent McRoberts of Texas A&M University. "In the 1700s in England, scientists thought that loud noises, such as the ringing of bells, would divert thunderstorms and hail," McRoberts says. "It didn’t work. In 1892, a Washington, D.C., lawyer got Congress to give him $10,000 to conduct rain making experiments in Texas using balloons rigged with explosives, again thinking that loud noises would produce rain. Again, no luck. Modern rainmaking efforts can be traced to Nov. 13, 1946, in New York. A scientist working for General Electric named Vincent Schaefer tested his theory that dry ice – solid carbon dioxide – dropped into clouds near Albany would produce precipitation. Five minutes later, snowflakes were falling from the clouds. Then in 1947, scientist Bernard Vonnegut found that silver iodide worked well at producing precipitation since its atoms have a similar structure to ice crystals."

Q: So does cloudseeding really work?

A: The results have been somewhat disappointing, says McRoberts. "Silver iodide appears to be the most effective rainmaking substance, but there is considerable debate over the results," he explains. "We’ve learned that storms and clouds are complex, and efforts to produce rain have not been what we’ve hoped for. Only if a portion of a seeded cloud is cold enough will moisture tend to form. Most efforts today are aimed at squeezing more rain out of existing storms, especially in the western U.S. Cloudseeding has also been used to reduce hail in thunderstorms before it becomes too large to cause damage. Some studies have shown that cloudseeding only increases precipitation between 5 and 20 percent, which is not very promising considering the costs involved. So generally, cloudseeding efforts have not been very encouraging."

Weather Whys is a service of the Department of Atmospheric Sciences at Texas A&M University.

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