Page last updated at 14:59 GMT, Monday, 2 November 2009

Scientists 'cause' Beijing snow


Quentin Sommerville, who is in Beijing, says winter arrived early this year

Chinese meteorologists say they brought about Beijing's earliest snowfall in a decade, after seeding rain clouds with silver iodide to ease a drought.

The Weather Modification Office sprayed clouds with 186 doses of the chemical to bring rain for the wheat crop, the Beijing Evening News said.

But the arrival of a cold front caused heavy snow to fall, disrupting road, rail and air travel.

Cloud seeding is often used in China in an attempt to bring on rain.

The country's north is prone to droughts, while the south is often flooded.

In Anhui province, which has been having a drought since September, there was 4cm (1.5in) of rainfall at the weekend.

Much of the country's farming still relies on rainfall as many of its communities have a poor irrigation system.

Cloud seeding graphic
1. Silver iodide is fired into cloud using flares on planes or from the ground
2. Water droplets then attach to these particles
3. They fall as snow if surface temperatures are below or near freezing, or as raindrops at warmer temperatures
4. Heat released as the droplets freeze boosts updrafts, which pull more moist air into the cloud
Despite the use of the cloud seeding technique, many scientists remain sceptical of its effectiveness

In February, snow fell after the authorities seeded clouds over Beijing in an attempt to alleviate the dry conditions.

However, many scientists - particularly in the UK - remain highly sceptical of the effectiveness of cloud seeding.

Even if it is theoretically possible, one of the problems for proponents has been to demonstrate that a rainfall or snowfall was caused by the seeding or simply occurred spontaneously.

In addition to cloud seeding, the government is building a huge network of tunnels and waterways that will funnel water from the south to the north, but the project is still five years from completion.

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