An estimated 125 Americans are killed by lightning each year, about a dozen of them while out on open water. When a thunderstorm approaches and you're swimming or sailing, the only safe thing to do is to head for land.

Pool managers must clear their pools "immediately at the first sounding of thunder or the first sighting of lightning," says Gerald M. Dworkin, whose Springfield consulting firm, Lifesaving Resources, specializes in aquatics emergency management. And pool patrons, he says, must stay indoors or in their cars -- not under trees or umbrellas, which can attract lightning -- until 15 minutes after the storm has passed.

The same goes for any other water activity, even something as seemingly innocuous as paddle boating. Dworkin remembers the "shock" he felt earlier this month as he passed the Jefferson Memorial one evening on his way home to Virginia. Although a thunderstorm warning had been in effect since 3:30 p.m. and the sky already was dark and threatening, Dworkin noticed several paddle boats in the Tidal Basin.

Just after he crossed the bridge, a true thunderstorm began. "Once the storm did hit," says Dworkin, "it would have taken too long to recall the paddle boats to shore. Every minute the public, as well as those working the concessions, was exposed to the elements, the chances of another lightning injury or death occurring increased."

To protect yourself from a lightning-related accident, follow these safety rules:

Keep a radio on your boat. The marine radio all-weather station can keep you informed of impending storms as soon as they're predicted. Watch the sky. If it becomes dark and threatening, a storm is approaching. Head for shore.

Get out when you hear thunder. Water conducts electricity, and lightning usually heads for good conductors for its strike. If you're in a body of water that gets hit by lightning, you in effect get hit by lightning, too. Don't wait until you actually see the lightning in the sky; give yourself plenty of time to reach cover.

Don't be the tallest object around. Lightning strikes the highest object, which could be you if you're in an open field. Stay away from other tall objects, too, such as isolated trees, umbrellas or the mast of a sailboat. One of the safest places to be is in a car with the windows up; even if lightning strikes the car, the electricity travels directly to the ground and does not touch the passengers. (Contrary to popular belief, rubber tires are not the reason a car is safe, said Jim Belville of the National Weather Service. The lightning is led to the ground by the storm's water running off the car.)

If you're stuck in an open field or on the water, get down. Out on a golf course or a wide body of water, it might be hard to find shelter under a building or a clump of low trees. In a boat, lie down or get below deck, if possible. On a field, get to your knees (don't lie down; you want to minimize direct contact to the ground) and keep your head down until the storm passes.