Who’ll Stop the Rain?

Close pollution-belching factories around Beijing? Check. Restrict cars and trucks? Oh, yes. Send hookers, beggars, vagrants and street hawkers packing? Done. Wring rain from clouds before they float over Beijing National Stadium (a.k.a. the "bird's nest"), site of the opening and closing ceremonies, as Beijing announced in January it would do? Let's just say that rocket launchers, antiaircraft guns and a veritable armada of small planes stand ready. Too bad that no project in the 60-year history of weather modification has managed to reliably bring about or suppress rain on demand. And that goes for the old Soviet claim that Russian science ensured sunny skies for every May Day parade.

On paper, the recipe for keeping raindrops away from the bird's nest is basically what China has been doing since the late 1950s. With an estimated 30,000 rainmakers, a $100 million budget and more hardware (like those launchers) than it has pointed at Taiwan, China has the largest weather-modification program in the world. Despite China's claims that its cloud-seeding technology can make rain on demand, though, experts are dubious. "They've never really evaluated their results quantitatively," says Roelof Bruintjes, a cloud physicist at the National Center for Atmospheric Research in Boulder, Colo. "It's hard, if you've always told your sponsor that it works, to turn about and say, 'Um, we have to check'."

Beijing has a twofold plan for a blue-sky Olympics. Both rely on the standard technique of seeding rain clouds with either the traditional silver iodide or newer hygroscopic (water-absorbing) particles such as calcium-chloride salts. Particles of silver iodide resemble ice crystals; supercooled water droplets in the clouds—too light to fall as rain—freeze on these "nuclei," causing each ice crystal to grow until it is heavy enough to fall as snow or rain. If rainmakers seed rain clouds approaching Beijing, they hope, the precip will fall well short of the Games. Alternatively, they might use smaller seeds, but more of them. If the water droplets are dispersed among many seeds, no single nucleus will attract enough to form a raindrop.

Contrary to myth, however, there is no scientific basis for either making clouds rain before they reach Beijing or keeping water droplets bottled up inside a cloud. Rainmaking has been marked by "failure to provide scientifically demonstrable successes," Michael Garstang of the University of Virginia, and chair of a 2003 report on weather modification from the National Academy of Sciences, told a Senate panel in 2005. The myth persists because if cloud seeding is followed by an increase in rainfall (or, at the Olympics, no rainouts), the rainmakers claim success. But in fact an increase of even 10 to 40 percent is within the range of natural variability. The myth also persists because experiments starting in the 1990s—in South Africa, Thailand and Mexico—seemed to get statistically significant increases in rainfall using hygroscopic salts. But a new analysis is more downbeat. Although the water-absorbing particles sped up the growth of cloud droplets and therefore the formation of precip-size "hydrometeors" (you have to love the terminology in this field), the rain would have fallen eventually if the clouds had been left alone.

Rainmakers are by no means giving up, though. Beijing's best hope is new research showing that the size and concentration of natural droplet-growing particles in a cloud are keys to success. If there are a lot, they dominate the man-made ones; with so many nuclei to choose from, too few supercooled droplets glom onto any particular one to form a raindrop. Studies also show that larger particles make raindrops form earlier than otherwise, but the rain does not last long. That's no help in combating drought, but may be just what China needs.

And if a tropical storm heads for the Games? At an international symposium in Colorado in April on weather modification, one inventor proposed weakening a hurricane by maneuvering a ship into the eye. Gigantic fans on deck would break up the inner wall, disrupting the storm's structure sufficiently to make it "implode." While Joe Golden of the National Oceanic and Atmospheric Administration calls that "kooky," the basic idea isn't. From 1962 to 1983, Project Stormfury shot silver iodide just outside the eyewalls of four hurricanes, to increase convection and thereby cause the eyewall to re-form at a larger radius. The larger a storm's radius, the weaker its winds. Although at first it seemed to work, scientists now think it was a coincidence that the storms' winds weakened after the seeding. But there are better schemes in the works, says Golden, including shooting heat-absorbing carbon (soot) into the eyewall to change the distribution of heat. The idea won't leave the drawing boards in time to help anyone in the path of Atlantic hurricanes this season, which is already way ahead of schedule in the number of storms. But the Department of Homeland Security, which includes FEMA, was interested enough to cosponsor a workshop this past February on hurricane control. The dream of playing weather god lives, but Olympic fans should still pack an umbrella.

© 2008