May/June 2010

Laser makes rain on demand

Ultra-fast pulses from a powerful laser can create droplets of water out of thin air, according to a study published in Nature Photonics. With the right conditions and large enough droplets, the researchers said, the technique could be used to make rain on demand.

Rain forms when water condenses around tiny particles in the atmosphere. Most of the time, dust or pollen do the job, but humans have long attempted to speed the process by seeding clouds with chemicals like silver iodide. Those chemicals provide the so-called "condensation nuclei" that trigger the consolidation of water into raindrops.

Unfortunately, such methods are difficult and could have environmental side effects, said Jérôme Kasparian, an optical physicist at the University of Geneva, Switzerland, who was on the team that demonstrated the laser-triggered condensation.

"The potential advantage of laser is that it can work continuously," Kasparian said. If lasers can trigger rain on a large scale, he said, it would also be more efficient and cheaper than spraying silver iodide out of airplanes or shooting it into the sky from rockets.

Researchers have long known short, strong laser pulses can ionize air molecules, creating pathways of ionized gas called plasma channels. Kasparian and his team wanted to find out whether those plasma channels could be of use to wannabe rainmakers.

"Our idea is to use the laser to ionize the air, and the ions that are produced can then serve as the condensation nuclei," Kasparian said.

To test the idea, the researchers first used an atmospheric cloud chamber, a box that enabled them to vary temperature and humidity. After saturating the air in the chamber, the team flipped on a several-terawatt
laser (one terawatt is a trillion watts) and watched as visible water droplets formed. Three seconds after the laser pulsed, the droplets swelled to diameters of 80 micrometers, smaller than a raindrop but larger than expected.

The next step was to take the laser outside. Using a weaker laser to monitor the formation of foggy air, the team blasted their multiple-terawatt laser into the sky of Berlin. They saw particles coalescing in the atmosphere.

"This means that the laser can trigger the formation of droplets inside a cloud chamber, [but also] in the real atmosphere," Kasparian said. "Now the challenge is to find conditions that will allow the droplets to grow further into the size where they will fall and get turned into rain."