The Rain Maker: Laser-induced Water Condensation

No, I am not talking about John Grisham novel, rather I am talking about a new study published today in Nature Photonics which uses laser energy to induce rain! Researchers at Freie Universität Berlin, Université de Genève and CNRS France have developed a technique called Laser-induced Water Condensation where they use a high energy pulsed femtosecond laser to form a guided filament which can be used to induce water-cloud condensation in free sub-saturated atmosphere . They conducted laboratory as well as field experiments to test the technique and the researchers are very optimistic that with further optimization of various parameters, it can be used to seed the clouds and form rain in parched dryer parts of the world without any danger of any side effects to the environment. Usually, conventional techniques for seeding the cloud include adding silver iodide molecules to upper atmosphere or sodium, potassium, lithium based salts in lower atmosphere to induce cloud condensation. But theses techniques have issues due to side-effects it can cause to the environment. This new technique described in the current study has no such side effects. The pulsed femtosecond laser can form a long filament shaped plasma by altering the refractive index of the air and self focusing nature of such laser beams. These plasma filaments when interact with air molecules ionize the Nitrogen and Oxygen molecule and provide the seeding material for condensation process. In their lab experiments , the researchers were able to increase the volume of condensed water droplets by 50% by laser-induced process. You can see in the video here showing the effect of plasma filament on the number and size of condensed particles in the saturated chamber (230% saturation). The flashes you see are the result of increase in the scattering of probe laser due to increased size and number concentration of water droplets in the chamber due to laser-induced water condensation process.

Even though the researchers feel that with proper optimization and further study, this method can be used in real world, but  some researchers are not too optimistic about it due to certain issues. The major issue being the relative humidity condition, the lab experiments were performed at relative humidity of about 230% which is never the case in atmosphere ( maximum ~ 101 %). So, the lab results can not be expected to be achieved in the atmospheric conditions. To counter that, the researchers in current study also performed experiments in open atmosphere in Berlin and LIDAR data showed the enhancement in condensation process using laser. Another issue can be the laser span, in order to increase the laser-induced effect to larger volumes of the cloud, an effective laser spanning technique will be needed. Nonetheless, it’s an interesting study, atleast from the proof of concept point of view.
Source: Nature Photonics : 2 May 2010 | doi:10.1038/nphoton.2010.115

Picture Credit: Jean-Pierre Wolf/University of Geneva  and New Scientist

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