View inside the cloud chamber before (a) and after (b) firing a set of three laser shots at 100-ms intervals. Image credit: Nature Photonics, doi:10.1038/nphoton.2010.115
(PhysOrg.com) -- Optical physicists in Europe have shown that lasers can be used to create tiny water droplets when they are fired into the air. The idea could eventually develop into an alternative to cloud seeding as a way of stimulating rainfall.
Physicist Jérôme Kasparian from the University of Geneva in Switzerland, and Philipp Rohwetter of the Free University of Berlin, realized it is not really known how effective is the technique of cloud seeding using silver iodide particles as condensation nuclei, and there are also concerns about the environmental effects of injecting silver iodide into high clouds, so they thought lasers might offer a more “environmentally friendly” solution.
Kasparian, Rohwetter and their colleagues reasoned that shooting a laser into the air would ionize oxygen and nitrogen molecules around the laser beam forming a “plasma channel” of ionized molecules, which could then act as condensation nuclei. They had already used plasma channels to modify weather in 2008, when the team fired a high-powered “Teramobile” laser into thunder clouds, which triggered an electric discharge.
To test the rainmaking idea, the researchers used a laboratory cloud chamber and fired short pulses of infrared laser light into the chamber in conditions of low temperature and very high (230%) humidity. A second low-powered laser was used to illuminate the chamber and allow them to observe what was happening, and measure any water droplets produced.
They discovered that after the laser was fired 50 μm droplets formed immediately along the plasma channel, forming a miniature linear cloud, and the drops then coalesced to form larger droplets around 80 μm in diameter during the three seconds after firing. The total volume of condensed water in the chamber was increased by 50% and the cloud could be seen with the naked eye.
The team then tested the laser technique outside, firing the laser into the sky over Berlin on different nights and in conditions of varying humidity. They again used a low-powered laser to measure the condensation produced. The results showed that no droplets formed when the humidity was low, but when it was high the low-power laser measured almost 20 times more back-scattering when the laser was fired than when it was not, which suggested droplets were being formed, although no cloud was visible to the naked eye.
a, Experimental set-up (schematic). b, Measured vertical temperature (blue, right scale) and derived supersaturation profile (red, left scale) in the chamber. Image credit: Nature Photonics, doi:10.1038/nphoton.2010.115
Kasparian said the research still has a long way to go because condensation is only formed on the plasma channel, but their next step is to use the laser in a sweeping motion across the sky to try to induce condensation in a wider area. They will also try to boost the effectiveness by adjusting the laser’s focus, wavelength and pulse duration.
The paper is published online in Nature Photonics.
More information: Laser-induced water condensation in air, Nature Photonics, Published online: 2 May 2010. doi:10.1038/nphoton.2010.115
作者: 乏味 时间: 2010-7-6 10:49
目前的人工降雨技术一般是在空中播撒碘化银颗粒作为凝结核，促使水蒸气凝结，而一项最新研究显示，可以使用激光将空气分子离子化，使之成为天然的凝结核，从而达到人工降雨的目的。
