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Could China combat smog with artificial rain?

China plans to open the heavens to bring back its blue skies.

According to local news reports, a document released by the China Meteorological Administration says that from 2015, local weather authorities will be allowed to use cloud seeding to create rain and clear the country's notorious smog. It's part of the government's plan to invest 1.7 trillion yuan ($277 billion) in tackling air pollution. But will it work?

Programmes to generate artificial rain have been running since the 1950s. In recent years, the controversial technique has gained some credibility. "There is indeed something to be said in favour of cloud seeding, provided it is very targeted," says Roger Stone, a climate scientist from the University of Southern Queensland in Toowoomba, Australia.

China has used cloud seeding before on a sporadic basis, for example, to ensure clear skies for the opening ceremony of the Beijing Olympics. The technique the Chinese use involves firing rockets carrying a payload of silver iodide particles into the clouds. Ice crystals then form from super-cooled liquid water in the clouds, using the particles as nucleation points. The crystals fall as rain or snow depending on the temperature below. The idea is that the artificial precipitation should clear the smog below.

There are other methods that deploy different chemicals designed to work in warmer clouds, but the "glaciogenic" method the Chinese use is considered the most effective, says Steven Siems from Monash University in Melbourne, Australia.

However, he doubts cloud seeding will be effective in smoggy conditions. This is because smog is made of particulates of soot and dust, which should already have acted as ice nuclei. If it hasn't rained already, Siems reasons, then adding more particulates in the shape of silver iodide won't help.

Satellite data suggest China doesn't have the right atmospheric conditions for cloud seeding, Siems says. "I can pretty much confidently say they don't." And he adds that the skies over China already contain a lot of ice nuclei from factory pollution and dust from the Gobi desert.


Smog rockets: make it rain, make it rain, make it rain <i>(Image: Wang Shuangzheng/Xinhua Press/Corbis)</i>

Smog rockets: make it rain, make it rain, make it rain (Image: Wang Shuangzheng/Xinhua Press/Corbis)


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