London: Shooting lasers into the air can elicit the formation of water droplets, and scientists claim that the technique could one day help to induce rain artificially.
For a long time, efforts to artificially trigger rain have focussed on ``cloud seeding`` — scattering small particles of silver iodide into the air to act as ``condensation nuclei``, or centres around which rain droplets can grow.
"The problem is, it`s still not clear that cloud seeding works efficiently. There are also worries about how safe adding silver iodide particles into the air is for the environment," Nature quoted optical physicist Jerome Kasparian at the University of Geneva, Switzerland, as saying.
The researchers thought that there might be a more environmentally friendly alternative.
Firing a laser beam made up of short pulses into the air ionizes nitrogen and oxygen molecules around the beam to create a plasma, resulting in a ``plasma channel`` of ionized molecules.
These ionized molecules could act as natural condensation nuclei, explained Kasparian.
To see if this technique could induce droplets, the researchers fired a high-powered laser through an atmospheric cloud chamber in the lab containing saturated air (see video).
They illuminated the chamber using a second, standard low-power laser, enabling them to see and measure any droplets produced.
As soon as the laser was fired, drops measuring about 50 micrometres wide formed along the plasma channel.
Over the next three seconds, the droplets grew in size to 80 micrometres as the smaller droplets coalesced.
Next, the researchers took the technique outside.
They tested their high-powered portable ``Teramobile laser`` over a number of different nights and in various humidity conditions.
And they detected that the amount of condensation induced by monitoring how much the light from a second laser was back scattered by any droplets.
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, said Kasparian, suggesting that condensation droplets were forming.
"This is the first time that a laser has been used to cause condensation outdoors," said Roland Sauerbrey, an expert on laser physics.
The results are published online in Nature Photonics1.