Atmosphere And Weather

How Lasers can Create Rain



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In the 1956 movie "The Rainmaker," Burt Lancaster convinces a drought-stricken farm town to pay him $100 to make it rain. Lancaster is a con man, yet after a series of dramatic twists and turns, rain does fall by the end of the movie—although it's a serendipitous event.

The 21st Century version of the rainmaker is the laser.

Recently, physicists discovered that the pulse from an infrared laser light created water droplets when aimed at air. The way it works is really like a basic physics experiment that could have appeared on TV's "Ask Mr. Wizard."

The heat of the infrared laser's coherent light ionizes the air. This ionization process causes water droplets to condense creating a cloud that's illuminated by a low-power green laser to measure the amount of droplets created. During the experiment clouds developed in the chamber that resembled the contrails made by high altitude jet aircraft.

For more than half a century the only method used to "create rain" was cloud seeding—a method of creating tiny focal points gathering moisture that becomes raindrops. The most common way to achieve that is scattering silver iodide into the atmosphere.

Silver iodide cloud seeding has never been proven to work; the results are unpredictable. In recent years worries about the environmental impact have also arisen.

Dr. Jèrôme Kasparian who heads the team from the University of Geneva explained the laser method addresses the shortfalls of cloud seeding: it does not harm the environment and it's much more predictable.

When the team initially tested their laser triggering hypothesis they fired a beam through an atmospheric cloud chamber set up in a lab. They found the laser ionized the molecules in the same fashion that silver iodide does. Yet it exceeded silver iodides effects. Water droplets that formed along the humid plasma channel of the chamber grew by two-thirds in volume.

After the lab experiment, under controlled conditions, the Kasparian's team decided to test their theory in the real world. Using a high-powered laser called a "Teramobile," they shot energy bursts into the sky above Berlin on several autumn nights with variable humidity.

During the nights of higher humidity droplets formed  following the path of the laser beam within the plasma channel.

"In low humidity conditions, the Teramobile laser did not induce droplets. But when the humidity was high, the team measured up to 20 times more back-scattering after the Teramobile laser was fired than before," notes Kasparian, "suggesting that condensation droplets were forming."

Roland Sauerbrey, a physics expert who specialized in high-energy lasers, declared himself impressed. Interviewed by Nature.com at his office at the FZD Dresden-Rossendorf Research Center in Germany, he said, "This is the first time that a laser has been used to cause condensation outdoors."

The next experiment to be carried out will determine if condensation can be created across a broader area by sweeping the laser beam across a large swath of the sky. Kasparian also plans to  boost the laser's effect by optimizing its wavelength. Theoretically, that might produce rain.

"As in the lab, the effect is clearly detected," Kasparian said. "It does not require saturation of the atmosphere."

If rainmaker Burt Lancaster had this technology back in 1956 he might have charged more than $100 for his services.

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