The ability to trigger precipitation on demand would present huge socio-economical benefits for many arid countries around the world. Ju et al. at the Shanghai Institute of Optics and Fine Mechanics in China and Laval University in Canada have now reported an optical approach that can be used to induce the formation of rain and snow (Opt. Lett. Doc. ID 157795; 2012).

'Cloud-seeding' using silver salt particles as condensation nuclei is the most common way of artificially inducing rain, although its effectiveness is questionable. In 2010, Jérôme Kasparian and co-workers demonstrated a more environmentally friendly approach that uses self-guided ionized filaments generated from ultrashort 220 mJ pulses at a repetition rate of 10 Hz (Nature Photon. 4, 451–456; 2010).

Credit: © 2012 OSA

In contrast, Ju et al. used a relatively low-energy femtosecond Ti:sapphire laser to deliver 9 mJ pulses at a repetition rate of 1 kHz. They say that such high-repetition laser pulses could provide a more efficient way of inducing macroscopic water condensation and snow formation.

The laser pulses were focused by an f/70 concave mirror into a 50 cm × 50 cm × 20 cm diffusion cloud chamber filled with ambient air, where they generated filaments of around 10 cm in length. A 532 nm probe beam from a semiconductor laser was co-propagated with the femtosecond laser beam to allow observation of the filamentation-induced event via Mie scattering. A vertical temperature gradient was maintained in the chamber; the bottom base plate was held at −46 °C while the top of the chamber was kept at room temperature.

Continuous heating of the filaments in the chamber generated an intense updraft of warm, moist air. This air cooled as it travelled upwards, resulting in further water condensation via convectio n and cyclone-like action to form particles with diameters of 40–300 μm. The researchers say that this process can be seen with the naked eye.

After 30 minutes of irradiation, approximately 13 mg of snow was scattered below the laser filament centre across an area measuring 2.0 cm × 1.5 cm. This snow had an HNO3 concentration of 0.032 mol L−1, which confirms efficient H2O–HNO3 ice nucleation due to the photo-oxidative chemistry of nitrogen triggered by filamentation.