Controlling the weather is a long-standing ambition of mankind. Throughout human history, tremendous efforts have been made to find ways of making rain. More recently, efforts to encourage rainfall (or suppress fog) have focused on using aircraft or rockets to seed clouds with silver salt particles or dry ice. The particles act as ‘ice nuclei’ around which raindrops can form.
In this latest research, led by Dr Jérôme Kasparian of the University of Geneva in Switzerland, a unique instrument called the Téramobile femtosecond-terawatt laser used extremely powerful and ultra-short laser pulses to generate ‘self-guided ionised filaments’. These ions induced condensation first in a cloud chamber and then, one cold, autumn evening, in the skies above the German capital city of Berlin. In follow-up laboratory experiments, the team explored the mechanisms behind the process, including photo-oxidative chemistry and electrostatic effects.
‘This is the first experiment demonstrating that a laser is capable of generating condensation,’ explained Dr Kasparian. His team believes that the laser technique can be used to modify local weather conditions. ‘The reaction that we have obtained constitutes the first step in the formation of rain, and we foresee the possibility of replacing current techniques,’ he added.
The researchers hoped to imitate the role of dust and pollen as ‘droplet seeds’ in the condensation process using laser-generated ions. They first directed the laser pulses at clouds generated in an experimental chamber. In that experiment, the ions served as ‘hooks’ for the moisture, and a condensation trail could be seen along the laser’s path. But the droplets were too small to constitute a real downpour; it was more like a fine mist.
Next, the team launched Téramobile laser pulses straight up into the air on a chilly evening when the humidity was registered at around 90%. They took LIDAR (light detection and ranging) measurements to monitor the aerosol content of the atmosphere, and watched as meteorological conditions changed: visibility decreased, humidity climbed to 93% and the concentration of water aerosols increased.
‘We have experimentally demonstrated that self-guided filaments generated by ultra-short laser pulses can assist water condensation, even in an under-saturated free atmosphere,’ the study concludes. ‘The phenomenon provides a new and attractive tool for remote characterisation of the humid atmosphere and cloud formation. In addition, it may even provide the potential to influence or trigger water precipitation using continuously operating lasers rather than rockets.’
The next step for the team is to refine their technique and make more rain.