In their own studies, Chinese scientists have concluded that their cloud seeding increases rainfall by 10 to 25 percent. They have seeded clouds not only to offset drought and fill reservoirs but even to fight forest fires. Talks have been underway with officials in Spain and Egypt, who are said to be interested in the purchase of modification instruments, and in 2005 China signed a bilateral agreement with Cuba to begin operations there. “We’re not that far ahead of other countries,” the WMO’s Zhang explains. “It’s just because we’re still working at it continuously, trying to tackle these problems, that we have results."

The greatest recent triumph of weather modification in Beijing wasn’t planned as a weather-control operation at all. In fall 2006, Beijing hosted a pan-African summit. It was preceded by a rushed beautification job in which workers hung floating red lanterns and photomural billboards along major roadways and filled in medians with new sod and saplings. To prevent congestion, the city’s traffic authorities banned most government vehicles from the roads, cutting traffic by a quarter. An obliging west wind swept away traces of the old gridlock just before the summit. The sky turned a gorgeous autumnal blue—a Hudson Valley sky, not a Huabei Plain one. The azure stayed all week. It was beyond anything the Meteorological Bureau had ever accomplished.

In August 2007, the city tried a repeat performance. While the Meteorological Services Center utilized its rain-fighting artillery, Beijing tried an even more drastic traffic cutback—alternately allowing only odd- or even-numbered license plates on the road. But what was announced as a two-week trial only ran for four days because of a bureaucratic miscommunication. The haze remained.

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.

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Posted on Apr 17, 2008 at 7:57 AM
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