The unseasonal snow that fell on Beijing for 11 hours on Sunday was the earliest and heaviest there has been for years. It was also, China claims, man-made. By the end of last month, farmland in the already dry north of China was suffering badly due to drought. So on Saturday night China's meteorologists fired 186 explosive rockets loaded with chemicals to "seed" clouds and encourage snow to fall. "We won't miss any opportunity of artificial precipitation since Beijing is suffering from a lingering drought," Zhang Qiang, head of the Beijing Weather Modification Office, told state media.
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http://www.guardian.co.uk/environment/2009/nov/04/controlling-the-weather-china