A group of engineers plan to fire supersonic jets of salty water towards storm clouds in order 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 U.S., killing 100. The majority of victims are in sports grounds or playgrounds when struck. The strikes lead to $5 million in insurance claims every year, so predicting when and where lightning will strike is important.

In the 1960s, scientists working for NASA used large rockets to trail copper wire into storm clouds near the Kennedy Space Center, and succeeded in attracting lightning bolts. But the idea was ruled out as too dangerous to try near spacecraft.

Now Doug Palmer, of BoltBlocker in San Diego, believes 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 water jet from breaking apart into a stream of droplets. Any lightning that is about to form will be attracted instead towards the water jet. When the lightning strike is triggered, the 10,000-amp current will pass down the jet, grounding itself on the copper cone surrounding the water nozzle.

The Navy accidentally tested a similar effect in the 1960s by detonating an experimental depth charge, which sent an enormous plume of salty seawater into the sky, inducing a lightning strike from nearby storm clouds.

Palmer says you could set up mobile water cannons around the perimeter of sports arenas to protect them from lightning strikes. Detectors rigged to the cannons would sense when the electric fields were high enough to make lightning strikes probable. The water jets would then be fired as temporary lightning conductors.

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NASA scientists have learned a lot about lightning from using satellites to monitor it worldwide. Lightning avoids the ocean, but often hits Florida. It?s attracted to the Himalayas and to central Africa, and almost never strikes the north or south poles.

?For the first time, we?ve been able to map the global distribution of lightning, noting its variation as a function of latitude, longitude and time of year,? says Hugh Christian, project leader of the lightning team at NASA?s Marshall Space Flight Center.

The information on lightning comes from two satellite-based detectors: the Optical Transient Detector (OTD) and the Lightning Imaging Sensor (LIS). ?The OTD and the LIS are two optical sensors that we?ve flown in lower Earth orbit,? says Christian. ?The OTD was launched in 1995 and we got five good years out of it. The LIS was launched on the Tropical Rainfall Measuring Mission satellite in 1997 and it?s still going strong.Basically, these optical sensors use high-speed cameras to look for changes in the tops of clouds, changes your eyes can?t see.? The LIS satellite can spot brief lightning flashes even under daytime conditions.

Before OTD and LIS, global lightning patterns were known only approximately. Ground-based lightning detectors used radio-frequency sensors to provide local measurements. But because such sensors have a limited range, oceans and low-populated areas had been poorly sampled.

The new maps show that Florida has an unusually high rate of strikes. Dennis Boccippio, an atmospheric scientist with the lightning team, says it?s because ?Florida experiences two sea breezes: one from the east coast and one from the west coast.? The ?push? between these two breezes forces ground air upward and triggers thunderstorms.Within thunderclouds, turbulence caused by updrafts causes tiny ice crystals and water droplets to bump around and collide. For reasons not fully understood, a positive electric charge accumulates on smaller particle, while negative charges grow on the larger ones, leading to lightning.

Another area with lots of lightning is in the Himalayas where the extreme local topography forces the convergence of air masses from the Indian Ocean. But lightning strike most frequently in central Africa. ?There you get thunderstorms all year ?round,? Christian says. ?[It?s a result of] weather patterns, air flow from the Atlantic Ocean, and enhancement by mountainous areas.?

Areas such as the Arctic and Antarctic have very few thunderstorms and, therefore, almost no lightning at all. ?Oceanic areas also experience [a dearth of lightning],? Christian says. ?People living on some of the islands in the Pacific don?t describe much lightning in their language.? The ocean surface doesn?t warm up as much as land does during the day because of water?s higher capacity for absorbing heat. Heating of low-lying air is necessary for storm formation, so the oceans don?t experience as many thunderstorms.

The satellite data also track patterns of lightning intensity over time. In the northern hemisphere, most lightning happens during the summer months. But in equatorial regions, lightning appears more often during the fall and spring.

This seasonal variation means that lightning ignites many of North America?s late-summer wildfires, while wildfires in South America are most often started by humans. This is because lightning in South America happens during a season when the ground is damp. Summertime lightning bolts in North America strike when the ground is dry and littered with fuel for fires.

Most global patterns aren?t influenced by human activity. Some people believe that tall buildings and metal towers increase the overall frequency of lightning strikes. But ?lightning that does make it to the ground is pretty much creating its own channels,? Boccippio says. ?The likelihood that we are changing the amount of cloud-to-ground strikes with construction of towers is very slim.?

A new lightning detector, called the Lightning Mapper Sensor or LMS, is being planned by NASA. It would circle our planet in a orbit over the United States, detecting all forms of lightning. It could provide valuable data to weather forecasters. ?The same updrafts that drive severe weather often cause a spike in the lightning rate [at the onset of] a storm,? says Boccippio. Measuring the rate of lightning flashes is one way to identify dangerous storms before they become deadly.

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