Firing extremely powerful laser pulses through humid air can stimulate the formation of clouds, according a team of European scientists. They say that the effectiveness of this method is much easier to gauge than traditional cloud-seeding techniques and that it could provide a practical means of triggering rainfall.

Cloud seeding is practised in many countries around the world and usually involves adding small particles to the atmosphere from ground stations, aircraft or rockets, in order to increase rainfall or reduce hail. This can be done by using molecules of silver iodide as nuclei around which supercooled water in higher-altitude clouds freezes, forming ice crystals that fall from the sky when heavy enough. Alternatively, compounds such as sodium, lithium and potassium salts can be released into lower altitude clouds in order to encourage the aggregation of small water droplets.

Although cloud seeding could have major practical benefits, it remains controversial because scientists have not been able to establish whether it really does change rainfall significantly. Among the many uncertainties are limitations in both our understanding of natural rainfall fluctuations and our knowledge of the extent to which pollutant aerosols stimulate precipitation.

Filaments of light

Philipp Rohwetter of the Free University of Berlin and colleagues in Germany, Switzerland and France believe that they can overcome these problems to some extent by seeding clouds using laser beams. To demonstrate their idea they used the portable Teramobile infrared laser with beam pulses lasting just 10–13 s and a power of 5 × 1012 W. Such pulses are intense enough to modify the refractive index of air, which causes the beam to focus itself. This further increases the intensity, producing filaments of light that are intense enough to ionize the air and initiate condensation.

The researchers fired the laser into both the atmosphere and into a controlled environment – a cloud chamber filled with ambient air. In both cases they illuminated the trajectory of the beam with a second, lower-powered laser, which would experience greater scattering if more droplets were present.

That is indeed what they found – the scattering of the second laser increased each time a pulse from the first laser was fired. They observed this pattern in over 900 laser flashes, providing, they say, a clear proof of the pulsed laser's cloud seeding capability that cannot be established for traditional seeding techniques.

Sweeping the beam

According to team member Jerôme Kasparian of the University of Geneva, several years will be needed to turn this physical demonstration into a practical technique. In particular, he says, a more powerful laser will probably need to be developed to take advantage of a sweeping effect that they have noted – that ionization continues for a few seconds after the laser has stopped flashing and so by sweeping the beam it should be possible to seed a larger volume of air.

More importantly, the researchers need to establish the physics behind the effect in order to know how to optimize the laser's wavelength, pulse duration and other parameters. They are certain that the ions in the laser-induced plasma contribute to condensation but they also believe that condensation might occur on molecules of sulphuric acid and nitric acid, which are formed when electrons from the plasma generate the OH radical that then oxidizes sulphur dioxide and nitrogen, respectively.

The experiments, however, do not convince other researchers. Bill Cotton of Colorado State University in the US describes the results as "intriguing" but maintains that Rohwetter and colleagues have "grossly overstated their case for impacts on real cloud formation and, especially, on precipitation". In particular, he points out that the air in the cloud chamber had a relative humidity of 230% whereas that in the atmosphere rarely exceeds 101%, meaning that droplet formation in the chamber would not necessarily imply droplet formation in the atmosphere.

This view is supported by Dan Breed of the National Center for Atmospheric Research in Colorado, who says that, on the other hand, laser-enhanced condensation in air with a relative humidity of less than 100% would be very transient and therefore unlikely to generate significant amounts of new cloud droplets, let alone precipitation. "The leap to modifying clouds and even larger jump to influencing precipitation is very speculative and I believe fairly unrealistic," he claims.

The research is described in Nature Photonics doi:10.1038/nphoton.2010.115.