The People's Weather

At this summer’s Beijing Olympics, China puts a 50-year experiment to the test: Officials are betting weather modification can keep the sun shining on the Games. Despite shaky science, the government is confident (not for the first time) that man can best nature. Whatever their chances, there’s plenty at stake—because all that development and urban renewal won’t look so good beneath a curtain of smog.

By Tom Scocca

Beijing under the haze of industry and construction, October 2007. Photo by Frederic J. Brown/AFP/Getty Images

The rain-prevention trial ending that same month was also inconclusive. The technique employed in that effort was a variant on the usual plan to make more rain, which is related to the technique for stopping hail. Both depend on the supply of particles in the air to serve as nuclei for rain formation. In a brewing hailstorm, Zhang says, think of the available droplets of supercooled water as mantou—steamed bread rolls—and the supply of ice-precipitating nuclei as monks. “If you give 1,000 mantou to 100 monks, each of them is going to burst to death,” Zhang said.  (Mantou are notoriously filling.) In hail-formation terms, the overloaded monks would come crashing out of the clouds as dangerously large hailstones. But by firing silver-iodide shells into clouds, you’re adding more monks to the scene. “So in the end,” Zhang said, “each monk gets two or three mantou.” The resulting ice pellets should be small enough to melt on their way down, arriving as raindrops. The metaphor leaves out a few things—hail also requires powerful thermal updrafts to serve as a buffet line that allows for feeding the monks—but it captures the basic strategy. Thus, if you continue to reduce each monk’s portion of mantou, eventually no one gets enough to eat, and the droplets stay in the cloud.

The concentration of nuclei in the air, with and without seeding, is one of the great outstanding questions of weather-modification science. The silver iodide monks are beside the point if the mantou have already been nibbled to bits, and the skies over China are rich with aerosol particles from dust and pollution. In a paper published in Science last year, Yao Zhanyu and a team of researchers concluded that in the mountains near Xi’an, heavy pollution can suppress rainfall by 30 to 50 percent.

In his office at the Academy of Meteorological Sciences, Yao explained the strategy for protecting the National Stadium. China had tried rain-prevention ventures before, Yao said—at the Tiananmen Square celebrations of the 50th anniversary of the People’s Republic in 1999, for instance, and at the 10th National Games in Nanjing in 2005—but the leading practitioner of anti-rain seeding was the former Soviet Union. Yao, a compact and muscular man with thin-rimmed glasses, pointed to a floor tile to represent the Olympic grounds. He traced three semicircles, one inside the next, where the mountains would be. The majority of summer storms, Yao said, come from the northwest, the west, or the southwest. Starting at the outermost line, the modifiers plan to seed approaching storms to encourage rainfall, in the hopes that they rain themselves out. By the nearest line, the goal will be instead to overseed the surviving clouds to suppress rain entirely. So rain seeding and anti-rain seeding “are not two strategies that are contradictory to each other,” Yao said. “We have to use them both."

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Issue 25

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