http://www.nature.com/news/2010/100502/full/news.2010.213.html
Rain-making lasers could trigger showers on demand
Lasers that stimulate condensation may help to induce rain
artificially.
Zeeya Merali
Laser-assisted condensation / Make rain while the laser shines.J-P.
Wolf / University of Geneva
The rain dance is getting a twenty-first-century revamp using laser
technology. Optical physicists have demonstrated that shooting lasers
into the air can trigger the formation of water droplets, a technique
that could one day help to stimulate rainfall.
For more than 50 years, efforts to try to artificially induce rain
have concentrated 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," says optical physicist
Jérôme Kasparian at the University of Geneva, Switzerland. "There are
also worries about how safe adding silver iodide particles into the
air is for the environment."
Kasparian and his colleagues realized 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, Kasparian explains.
To test whether 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. Immediately after 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. The team's results are
published online in Nature Photonics1.
Rainmaker
The next step for Kasparian and his team was to take the technique
outside. The researchers already have experience using plasma channels
to modify the weather: in 2008, they demonstrated that a beam from
their high-powered portable 'Teramobile laser' could be fired into
thunder clouds, triggering an electric discharge2. The beam was able
to reach its target without being deflected because the generated
plasma channel modifies the speed at which light travels through air —
slowing it down in the centre of the beam and speeding it up at the
sides. This causes the beam to continually self-focus, helping it to
maintain a high intensity across large distances (see 'Bendy laser
beam fired through the air').
This time, Kasparian and his colleagues tested the Teramobile laser
over a number of different nights and in various humidity conditions.
Once again, they detected 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, says Kasparian, suggesting that condensation
droplets were forming.
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Roland Sauerbrey, an expert on laser physics at the FZD Dresden–
Rossendorf Research Centre in Dresden, Germany, says that the team has
the potential to create a "breakthrough technology". "This is the
first time that a laser has been used to cause condensation outdoors,"
he says.
However, the technique is still in its early stages. "We can only
create condensation along the laser channel, so we won't be going out
and making rain tomorrow," Kasparian notes. He and his team are now
investigating whether they can create condensation over a wider area,
by sweeping their laser across the sky.
Thomas Leisner, an atmospheric physicist at the Karlsruhe Institute of
Technology, Germany, remains cautious about the feasibility of scaling
up the technique in this way. "I am sceptical that this could be used
to trigger rain on demand," he says. But he adds that the technology
will have other uses. The researchers should now calibrate the
relationship between the amount of condensation produced by the laser
and the prevailing atmospheric conditions, he says. "They could use
the amount of condensation produced by their laser as a measure of
water saturation to help forecast the chance of rain," he says.
*
References
1. Rohwetter, P. et al. Nature Photonics advance online
publication doi:10.1038/nphoton.2010.115 (2010).
2. Kasparian, J. et al. Opt. Express 16, 5757-5763 (2008).
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