Could Laser Beams Induce Rain?

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Charles Dudley Warner, a friend of Mark Twain, wrote, "Everybody talks about the weather, but nobody does anything about it." He'd have thought differently if he had laser beams.

A review in the Institute of Physics' Journal of Physics D: Applied Physics, says that it might be possible to use high-powered laser beams to make it rain. This might be an alternative to seeding clouds, either with dry ice or silver iodide particles. While such seeding is common, it isn't always clear how effective it is and the results can vary a lot. It's also unclear what the consequences are to the atmosphere in those areas where seeding is done.

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The paper, by Jérôme Kasparian, Philipp Rohwetter, Ludger Wost and Jean-Pierre Wolf, outlines methods of controlling how water condenses in the atmosphere and how it might be controlled with high-powered laser beams.

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They write that a powerful enough laser beam fired into the atmosphere would break up the atomic ozone and nitrous oxide, which ultimately form nitric acid. Particles of nitric acid are big enough to support droplets and that could lead to rain.

The beam itself doesn't last long, as the flash is measured in femtoseconds, or millionths of a billionth of a second. That's about how long it takes a beam of light to move 300 nanometers, or the length of a very fine dust particle.

Kasparian, of the University of Geneva, led a team that demonstrated using lasers to induce condensation in 2010, using femtosecond laser pulses at powers measured in terawatts. The laser caused water droplets to form in humid air. The droplets were too small to fall as rain but the experiment showed that the idea is feasible.

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There are still questions, though. Even though powerful lasers are a lot cheaper than they were even a decade ago it isn't clear this method would be cost-effective. The laser also has to be swept over a large volume of air very quickly. And it is still not known what power levels are needed to get a given volume of rainwater.

Photo: True-color image of laser-induced condensation in a cloud chamber, illuminated by a green auxiliary laser. Credit: Institute of Physics

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