Sports arenas of the future could be protected by water cannons - not to control rampaging fans, but to deflect lightning strikes. In June, a group of American engineers plan to fire supersonic jets of salty water towards storm clouds in a bid to trigger lightning. If it works, they say their system could ultimately be used to protect people and property from lightning strikes.

The National Severe Storms Laboratory in Norman, Oklahoma, says lightning hits around 600 people each year in the US, killing 100. The majority of victims are in sports grounds or playgrounds when struck. The strikes lead to insurance claims of an astonishing $5 billion per annum. So predicting when and where lightning is going to strike is crucial--especially if you're in the business of launching fuel-laden spacecraft.

In the 1960s, scientists working for NASA used large firework-like rockets to trail earthed copper wire into storm clouds near Florida's Kennedy Space Center, and succeeded in attracting lightning bolts. But the idea was ruled out as too dangerous to try near spacecraft. And you certainly couldn't have spent rockets landing on crowds at open-air concerts or sports fields, either.

Ultra-thin jet

Now Doug Palmer, founder of a company called BoltBlocker in San Diego, California, reckons lightning could be drawn to a safe spot by squirting an ultra-thin jet of water--mixed with salt and soluble polymers--towards the storm cloud. The salt boosts the water's conductivity while the long-chain polymers help prevent the jet breaking apart into a stream of droplets.

Palmer's idea is to propel a jet with a diameter of just 1 centimetre around 300 metres into the air. His idea is that any lightning about to form will be attracted towards the conducting water jet. When the strike is triggered, the 10,000-amp current will pass down the jet, hopefully safely earthing itself on a hefty copper cone surrounding the water nozzle.

While it sounds on the frontiers of feasibility, Palmer's idea might just work, says Charles Moore, an atmospheric physicist at the New Mexico Institute of Mining and Technology in Socorro. The US Navy accidentally demonstrated a similar effect in the 1960s, he says. Detonating an experimental depth charge sent an enormous plume of salty seawater into the sky--inducing a lightning strike from nearby storm clouds.

To protect places like sports stadiums from strikes, Palmer says you could set up mobile water cannons around the perimeter. Detectors rigged to the cannons would sense when the electric fields were high enough to make lightning strikes probable. The water jet would then be fired to act as a temporary lightning conductor.