We hate to give comfort to all those conspiracy theorists who believe the US government is controlling the world's weather, but progress is finally being made on the old dream of finding a way to make it rain.

 

Most efforts towards artificial precipitation have relied on chemical means, using dry ice or silver iodide to provide the particles on which moisture in the air can condense to become large enough to start to fall. 

 

However, most efforts have had a mediocre payoff at best. Lasers offer another path. Charged particles attract water to become raindrops, which is why the charged stream left behind by the first lightning bolt of a storm can bring on rain, and one paper suggests a single lightning stroke can be responsible for around 7000 tonnes of rain. Electrical discharges in the laboratory have been found to cause water droplets to form on the walls of glass chambers.

 

A recent study concluded that laser pulses could break N₂and O₂ particles into excited atoms that would then react to form NO and O₃. As endothermic reactions these draw heat out of the atmosphere, triggering condensation while ionized particles formed in the process become seeds for rain. Unfortunately, this paper assumes a peak intensity over 500m. Instead the beam's energy dissipates with distance and past efforts have not stretched 500mm, let alone meters. 

 

However, a paper in Nature Photonics demonstrates that it is possible to refuel the laser beam through a second laser wrapped around the central beam, acting as what the researchers call an “energy reservoir”. Using this method the length of the plasma column can be extended by a factor of ten or more. “Our approach offers an efficient and viable route towards the generation of extended light strings in air without inducing premature wave collapse or an undesirable beam break-up into multiple filaments,” the paper says.

 

"Because a filament creates excited electrons in its wake as it moves, it artificially seeds the conditions necessary for rain and lightning to occur," says co-author Matthew Mills, a graduate student at the University of Central Florida. Single beams have been enough to create “electrical events” within clouds, but not managed to make lightning or rain.

 

"What would be nice is to have a sneaky way which allows us to produce an arbitrary long 'filament extension cable.' It turns out that if you wrap a large, low intensity, doughnut-like 'dress' beam around the filament and slowly move it inward, you can provide this arbitrary extension," Mills says. "Since we have control over the length of a filament with our method, one could seed the conditions needed for a rainstorm from afar. Ultimately, you could artificially control the rain and lightning over a large expanse with such ideas." 

 

Even with the greatly extended length Mills and his colleagues have achieved the pulses still reach little more than 2m, but they are working on extending this.

 

"This work could ultimately lead to ultra-long optically induced filaments or plasma channels that are otherwise impossible to establish under normal conditions," says co-author Professor Demetrios Christodoulides. “"In principle such dressed filaments could propagate for more than 50 meters or so.”

 

Something that can bring the rain draws attention, and will really matter if it can be done on a scale that brings relief to drought plagued lands, but other applications are being considered as well. The authors list remote sensing, spectroscopy, channeling microwaves and lightening protection as possibilities. They also see potential in the combined laser technique for attophysics, the process of using pulses lasting 10-18s to probe the dynamics of electron behavior around atoms.

Like all weatherwork however, rain-bringing lasers carry the risk of unintentional effects elsewhere. Some experiments in seeding clouds found that they did often trigger increased rainfall, but at the price of reduced rain nearby, which tended to make the farmers who didn't benefit rather annoyed, although more recent trials appear to have avoided  this problem.