Laser creates clouds over Germany

Colin Barras in New Scientist: A laser has been used to generate small clouds on demand in lab, and real-world experiments suggest this could be a way to call down rain when it's needed. 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 Jérôme Kasparian at the University of Geneva, Switzerland, one member of a research team that think lasers may be a better way to trigger rain on demand.

Kasparian and colleagues have just reported the first successful use of this technique to summon clouds from air both in the lab and in the skies over Berlin, Germany. 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….

Cirrocumulus stratiformis and blue blue sky, shot by Simon Eugster, Wikimedia Commons, under the Creative Commons Attribution ShareAlike 3.0 License