MIAMI — In 1995, a bolt of lightning from a sunny sky hit 7-year-old Jenna Bernardo during softball practice at a Lighthouse Point park. The strike left her in a coma. Three years later, she died.

In Florida, where lightning kills about 10 people a year and injures dozens more, governments are increasingly investing in an electronic system that claims to be able to warn when lightning is imminent.

Broward County has spent about $136,000 to install the devices in six county parks and will spend more to add the equipment at seven other parks by March. The Thor Guard equipment is operating near swimming pools and athletic fields in a handful of Broward cities and at Fort Lauderdale-Hollywood International Airport and the University of Miami.

The Sunrise-based company claims that since it went into business in 1974, no one has been struck by lightning in an area covered by Thor Guard's equipment.

"It provides a safety margin for people using the parks," Thor Guard President Bob Dugan said. "Every single park should have it. Every single school should have it."

Lightning occurs when liquid and ice particles collide and create an electrical charge in the atmosphere. Positive charges rise to the top of a cloud while the negative charges fall to the bottom of the cloud. Typically, lightning occurs when negative charges from the bottom of the cloud interact with positive charges on the ground.

Thor Guard — named for the Norse god of thunder — says its equipment senses the energy changes that occur before lightning strikes. A computer database determines when the energy level needed for lightning to occur is within two miles. The system triggers a loud horn and emits a light, warning park visitors to seek shelter at least eight minutes before a strike.

Despite their growing popularity, not everyone is convinced that such gadgets are necessary.

"There is a lot of garbage in the lightning detection and protection field," said Richard Kithil Jr., founder of the nonprofit National Lightning Safety Institute. "There is a lot of hysteria and advertising which tends to overshadow science."

For recreation supervisors, Thor Guard has a peripheral advantage, said Tara Dean, who works at the Davie Pine Island Park Multipurpose Center. Closing the pool in poor conditions has become much easier, she said, because the equipment — not an employee who could be a teenager — makes the call.

Youngsters on the swim team "get a little annoyed because it looks like there is nothing there, but parents are happy because it's obviously detecting something the parents can't see," Dean said.

Thor Guard initially was sold to the military and airlines, then began marketing to the golf industry in 1993. Thor Guard was used at the Atlanta Olympics in 1996.

Now about two-thirds of its 4,500 customers are local governments and schools. Several companies offer lightning warning systems, but Thor Guard claims it has the only equipment that can predict lightning, rather than simply detect it. There are no published scientific studies about Thor Guard, said Dugan, whose privately held company has no interest in sharing details of its system with competitors.

Strike Guard, manufactured by Tucson-based Wxline, sounds when lightning is a few miles away. Company President Christoph Zimmermann says technology can measure conditions conducive to lightning but "predicting when and where lightning strikes, I don't think the technology exists to give you that information," he said.

Martin Uman, professor of electrical and computer engineering at the University of Florida, said a network of devices such as the one set up the Kennedy Space Center are a much better indicator of lightning than a single device.

"If you have a network and look at what each instrument says, you can get a reasonable idea of where thunderstorms are," said Uman, an internationally recognized lightning expert. "If you have one instrument, it's very easy to make mistakes."

It isn't possible to predict lightning — only that a thunderstorm is charging.

"You don't know if it's going to keep charging, you don't know if it's charging and is not going to produce lightning," Uman said. "There are too many unknowns in the whole business. A device like this on average can give you some information. How accurate it is, I don't know."

UF athletic officials use a lightning detection system operated by Tucson-based Vaisala that involves sensors throughout North America mapping lightning locations. Game officials track lightning bolts on a computer so they know if a storm is nearby. Uman developed an early version of the system and now consults for the company.

Vaisala's system is used by the National Weather Service.

Broward County had used a hand-held detection system for a few years.

"It was becoming outdated," said Toni Peyton, safety manager for Broward parks and recreation. "The detector tells you you are right now, this very moment being struck by lightning. It takes 15 to 20 minutes to evacuate a water theme park. We needed to be proactive."

Critics say Thor Guard goes off so often it can shut down parks for an unreasonable amount of time. In South Florida, a Thor Guard device could go off 200 to 300 times a year, Dugan said.

"People will eventually start ignoring the warnings and won't comply with the system," Zimmerman said.

At the Davie pool, Dean said the alarm does go off frequently during rainy weeks, but before they had the equipment, they closed the pool in such weather anyway.

Lightning deaths have been declining in recent years, said Jim Lushine, meteorologist and severe weather expert with the National Weather Service in South Florida.

Last year, 43 people were killed by lightning nationwide, while the average was 73 between 1984 and 2003. In Florida last year, eight people died and 32 were injured.

Lightning detection equipment could be a factor in the decline, Lushine said.

Lushine said he can't endorse a product, but there is evidence that a buildup in charge can be observed.

"You can know something is going to happen before it's going to happen," he said. "I am an advocate of having some kind of warning."