New technique beams lasers into clouds to make it rain

Researchers have developed a new technique that may one day aim a high-energy laser beam into clouds to make it rain or trigger lightning.
Researchers at the University of Central Florida's College of Optics & Photonics and the University of Arizona found that surrounding the beam with a second beam to act as an energy reservoir can sustain the central beam to greater distances than previously possible.
The secondary "dress" beam refuels and helps prevent the dissipation of the high-intensity primary beam, which on its own would break down quickly.
Water condensation and lightning activity in clouds are linked to large amounts of static charged particles. Stimulating those particles with the right kind of laser holds the key to possibly one day summoning a shower when and where it is needed, researchers said.

Lasers can already travel great distances but "when a laser beam becomes intense enough, it behaves differently than usual - it collapses inward on itself," said Matthew Mills, a graduate student in the Center for Research and Education in Optics and Lasers (CREOL).
"The collapse becomes so intense that electrons in the air's oxygen and nitrogen are ripped off creating plasma - basically a soup of electrons," Mills said.

At that point, the plasma immediately tries to spread the beam back out, causing a struggle between the spreading and collapsing of an ultra-short laser pulse.
This struggle is called filamentation, and creates a filament or "light string" that only propagates for a while until the properties of air make the beam disperse.

"Because a filament creates excited electrons in its wake as it moves, it artificially seeds the conditions necessary for rain and lightning to occur," Mills said.
Other researchers have caused "electrical events" in clouds, but not lightning strikes.
The question researchers had to address was how to get close enough to direct the beam into the cloud without being blasted to smithereens by lightning.
"What would be nice is to have a sneaky way which allows us to produce an arbitrary long 'filament extension cable'. It turns out that if you wrap a large, low intensity, doughnut-like 'dress' beam around the filament and slowly move it inward, you can provide this arbitrary extension," Mills said.

"Since we have control over the length of a filament with our method, one could seed the conditions needed for a rainstorm from afar. Ultimately, you could artificially control the rain and lightning over a large expanse with such ideas," Mills said.
So far, Mills and fellow graduate student Ali Miri have been able to extend the pulse from 10 inches to about 7 feet. They are working to extend the filament even farther.
The study was published in the journal Nature Photonics.