Swiss scientists use a laser to create clouds over Germany

Geneva, Switzerland--Swiss scientists from the University of Geneva have used a laser both in the lab and in the skies over Berlin, Germany to create clouds, according to a May 2 story in New Scientist from Colin Barras at http://www.newscientist.com/article/dn18848-laser-creates-clouds-over-germany.html?DCMP=OTC-rss&nsref=online-news.

Barras explains that people have experimented with cloud seeding for decades in the hope of boosting rainfall, usually by sprinkling silver iodide crystals into clouds high in the atmosphere. These crystals encourage large water droplets to form around them, and the droplets then fall as rain--in theory, at least. "The efficiency of this technique is controversial," says Jerome Kasparian at the University of Geneva, one member of a research team that think lasers may be a better way to trigger rain on demand.

In the lab the team fired extremely short pulses of infrared laser light into a chamber of water-saturated air at -24°C. Linear clouds could be seen to form in the laser's wake, like a miniature airplane contrail. Kasparian says that the laser pulses generate clouds by stripping electrons from atoms in air, which encourage the formation of hydroxyl radicals. Those convert sulphur and nitrogen dioxides in air into particles that act as seeds to grow water droplets.

Each laser pulse packs a 220-millijoule punch into just 60 femtoseconds--an intensity "equivalent to the power of 1000 power plants," says Kasparian. Analysis of the air in the chamber after the laser was switched off showed that the total volume of condensed water droplets inside had increased by half, and that within the clouds the volume of condensed water had risen 100-fold.

Kasparian’s team also sent pulses into the autumn sky, focused 60 meters up. Nothing could be seen with the naked eye, but weather light detection and ranging (LIDAR), which uses lasers to measuring light scattering in the atmosphere, confirmed that the density and size of water droplets spiked when the laser was fired. "As in the lab, the effect is clearly detected," Kasparian says. "It does not require saturation of the atmosphere." The article goes on to say that Kasparian’s team will now try to boost the effect by optimizing the laser wavelength, its focus and the pulse duration, which could produce droplets large enough to fall as rain.

--Posted by Gail Overton; [email protected]; www.laserfocusworld.com.