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Lightning: The formation and risk to swimmers


Lightning bolts represent a tremendous discharge of electricity: 100-300 million volts and 30,000 amps. Most often, they occur between clouds, but 25 percent occur from cloud to ground.

Lightning begins within clouds as updrafts carry positively charged water particles while hail and ice particles descend in downdrafts that are negatively charged, resulting in a vertical separation of electric charge within the cloud: positive near the top and negative at the base.

Beneath the cloud, a positive charge develops at the Earth’s surface and follows the cloud horizontally. As vertical charge differences between cloud and ground increase, positively charged particles rise up from tall objects and lightning is formed when the cloud’s negative charge meets the positive charge from below.

It is sometimes possible to witness the positive charge “leaping up” to connect with the negative charge in an overhead storm cloud. This can be alarming for an observer as sparks begin to fly upward from an antenna or when a person’s hair stands on end. I’ve experienced this phenomenon at sea during an intense lightning storm. If this occurs, take cover immediately as a strike is imminent and you may become the lightning rod.

Not all lightning forms at the cloud base. Some originates at the top of a thunderstorm, which carries a large positive charge. This “positive lightning” is particularly dangerous because it can strike the ground far from the storm, either ahead or behind the storm’s active center, where people do not expect lightning risk.

Positive lightning can strike ground up to 10 miles from a storm, even with blue sky overhead. Strikes from “negative lightning” are the more common, typically occurring within 5 miles of a storm. Learn to use thunder as a safety indicator.

Thunder is an acoustic shock wave caused from rapid heating of air (to 54,000 degrees F) by lightning. Sound travels through air at the rate of 1 mile in 5 seconds, equivalent to 6 miles in 30 seconds. If there is a lightning flash and then thunder less than 30 seconds later, take cover.

When lightning hits the sea, most of the electrical current spreads radially outward on the surface. Because seawater is a good conductor, the remaining current penetrates hemispherically downward and fully dissipates less than 10 feet below the surface. It is believed that lethal current spreads horizontally only 20 feet from the position of strike impact.

Someone swimming in fresh water is at greater risk of death from lightning than someone in seawater. Because fresh water is a poor conductor of electricity, a human body becomes the sponge for electrical current from a nearby lightning strike.

In contrast, saltwater disperses electrical current rapidly in all directions and a body would receive less current from a nearby strike. Although a lightning strike carries a huge current, it is brief so the damage to an immersed body can be less than one may imagine.

Some wonder if they should jump overboard if they expect lightning will strike their vessel. This is the proverbial “Yes, but…” scenario. If it’s highly probable that a vessel will be struck by lightning, someone onboard could swim at least 50 feet from the vessel and dive 10 feet underwater until the risk passes.

This is, of course, inconvenient and challenging unless the person is a scuba diver already geared up with a full air tank. (Even a diver is at risk if the strike occurs close by as the metal air tank will attract the electrical current.)

The best way to weather a lightning strike onboard a vessel is to go below and stay away from anything metal, including through-hulls, engines, tanks, wires and electronics.

All swimmers and divers who have been killed by lightning were close to the surface and near the impact location. Fish also would have been killed in this proximity to the strike. The fact that large numbers of fish are not killed at sea is because the electrical charge is lethal only near the surface and close to the impact. It’s strictly a probability matter as most fish and mammals remain well below the surface most of the time.

If someone has been struck by lightning, it is safe to immediately help them with CPR because they do not retain an electrical charge. With immediate attention, most victims can survive a lightning strike, although cardiac arrest, burns and nerve damage are common and psychological effects can be devastating.

Scott E. McDowell has a doctorate degree in ocean physics, is a licensed captain and author of Marinas: a Complete Guide, available at Comments are welcome at

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