Sunday, 15 July 2012

Chinese scientists create snowfall in a lab

Team in Shanghai were able to manipulate air using lasers; technique could be used to fight climate change

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Guanyu 道 said...

Chinese scientists create snowfall in a lab

Team in Shanghai were able to manipulate air using lasers; technique could be used to fight climate change

Stephen Chen
15 July 2012

On the sidelines of "serious" military research, optics Professor Liu Jiansheng accidentally created something he never expected to see in his laboratory: a snowfall.

Playing around with a laser beam at the Shanghai Institute of Optics and Fine Mechanics at the Chinese Academy of Sciences, Liu and his team triggered a tiny flurry in their lab's cloud chamber.

"We had not anticipated the event at all. It was beautiful," Liu said.

For more than a decade, scientists have known a powerful laser beam can manipulate the nature of air, but all they could achieve was condensation barely visible to the naked eye.

As other researchers elsewhere have done, Liu's team fired a laser into the cloud chamber, a sealed environment containing vapour, in bursts one quadrillionth of a second apart.

When energy is released in such short bursts, it can tear apart molecules, charge particles and create plasma. Experiments conducted by teams overseas have focused on low-frequency blasts, while Liu fired his pulse at a higher one.

The higher frequency proved key - as the laser beam pierced the cloud chamber, it severed electrically charged nitrogen, oxygen and hydrogen atoms from air component molecules, such as carbon dioxide. These atoms provided the essential "seeds" for condensation and then snow.

Liu and his team published their findings in the April edition of Optics Letters, an international journal run by the Optical Society of America.

"An intense updraft of warm moist air is generated owing to the continuous heating" by the laser, they wrote. "As it encounters the cold air above, water condensation and large-sized particles spread unevenly across the whole cloud chamber via convection and a cyclone-like action on a macroscopic scale."

The turbulence blew snow all over the chamber, making the sight "spectacular", Liu said.

"It was difficult to believe that we could create such extreme atmospheric phenomenon using a laser, but we did it," Liu said. "The technology may have some military applications, but I think its biggest use will be in the battle with climate change."

Liu, who has twice received the nation's top prize for advances in military science and technology, hopes to get extra funding to further his research. It could lead to the development of large laser cannons set up in drought-hit areas, replacing the current expensive, polluting practice of using silver iodide to seed clouds and trigger rain.

The central government has long sought to control the weather, with the most publicised incident occurring during the Beijing Olympic Games in 2008, when rain was delayed for several hours until after the opening ceremony. But the use of chemicals has prompted concern from other nations. Japan has pointed to pollutants drifting into its territory, and suggested manipulating weather over an area as large as the mainland may lead to atmospheric imbalances - lower rainfall for instance - elsewhere.

So far the technology used by the government has only affected weather within a small area and for a short time.

After firing the beam for about two hours, Liu's team had created 13 micrograms of snow. While not much, they were excited that the snowfall had not only occurred in areas immediately close to the laser beam, but extended to a relatively large area around it.

"It means we can use very fine laser beams to create a thick belt of snow. If we shoot many beams into the air, it could create a snowfall zone of considerable scale and scope," Liu said.