A Staff Report from the Straight Dope Science Advisory Board

Is lightning really that dangerous to swimmers? Will my choice of bicycle get me struck by lightning?

September 11, 2007

I like to swim in a lake. Whenever there's lightning anywhere nearby, everyone warns me of my imminent electrocution. It seems logical that if lightning struck very near to me, there'd be trouble. On the other hand, if lightning struck the lake a mile away while I was swimming, I'm not sure it would even be noticeable. How near does a lightning strike have to be to harm me? And if it can harm me from a distance, how about all the fish? Does it have to do with depth below the surface? Otherwise, why aren't there dead fish everywhere after a strike? Is the real danger that if I were in the middle of a large body of water, I'd be the tallest thing around and act as a lighting rod? I need the straight dope, please.

Living in New Mexico and being a mountain biker I find myself discussing which frame material would be most likely to be struck by lightning: steel, aluminum, carbon fiber, or titanium. I believe the Ti bike would be likely to get zapped first since titanium is an excellent conductor. Am I being selfless by riding a Ti frame so that I can save my buddies on their aluminium rigs from being shocked, or should I buy a carbon fiber frame to save my own butt?

You've actually figured out the answers to most of your own questions, Tom, but with a potentially high-stakes topic like this we might as well go through it carefully.

The chances you'll actually be struck by lightning while swimming are in fact fairly trivial compared to your chances of meeting dozens of other fates. Over the course of a lifetime, you're much likelier to be killed (for instance) in a fall.

That said, you can see why most people tend to err on the cautious side in this area. A lightning strike certainly can cause a lot of electric current to pass through water – not for miles and miles, but shorter distances, sure. Responding to a question sent to USA Today about whether it's safer to swim in salt or fresh water during a lightning storm, Greg Forbes of the Weather Channel chose the correct answer (none of the above), then cautioned, "The lightning current may spread out in all directions and dissipate within 20 feet or so, but don't bet your life on how close the strike will be."

And swimmers do, in fact, sometimes get struck by lightning. In 2005, for instance, three people were struck while swimming in the ocean near Tampa, and four more were hit (two of them seriously injured) in waters off Chiba Prefecture, Japan. Swimming pools aren't necessarily safer. In July 2006 a 50-year-old Briton was dangling his feet in the pool at a rented villa in Italy when lightning struck the water, killing him and injuring a friend. In fact, experts recommend staying out of indoor pools during an electrical storm, as well as showers and tubs, as current from lightning has been known to travel through plumbing.

As you guessed, a big part of the risk here has to do with lightning's tendency to strike the highest point around – i.e., you don't want this to be your head. Being on an open boat in a lightning storm is probably even more dangerous than swimming, suggests Mary Ann Cooper of the University of Illinois Lightning Injury Research Program, as "increasing your height by any amount increases your chances of being hit by a calculable amount. … Avoid being the highest object anywhere, be it a beach, small open boat, pier, meadow, or ridge." According to the National Weather Service,

The vast majority of lightning injuries and deaths on boats occur on small boats with NO cabin. It is crucial to listen to the weather on a small aquatic vessel without a cabin. If thunderstorms are forecast, don't go out. If you are out on the water and skies are threatening, get back to land and find a safe building or vehicle.

The NWS goes on to say that boats with cabins are safer, particularly if they've been fitted with lightning protection, but one should avoid using the radio in a storm unless there's an emergency.

Looking at government data (.pdf) collected between 1959 and 2005, we see that incidents involving boats and water account for 13 percent of all lightning fatalities nationwide (among cases where circumstances are known), coming in behind instances where victims were out in the open (28 percent) or under a tree (17 percent). In Florida, which ranks first among the states in lightning casualties, boating and other water-related incidents make up 25 percent of lightning deaths.

Why don't fish get killed by lightning strikes? Well, actually, they do. I'd suggest you check out a 1941 article in Copeia (a scientific journal about fishes, amphibians, and reptiles) called "Mortality at Fish Hatchery Caused by Lightning," but really the title pretty much says it all. A 2005 episode of NOVA documents an instance of fish in a koi pond being injured by lightning; my guess would be that fish electrocution by lightning is a pretty underreported phenomenon. But fish typically don't get killed in large quantities by lightning because, as you guessed once again, they tend to swim deeper than humans do, while according to Florida physics professor Joseph Dwyer "most of the current from the lightning flows over the surface of the water."

The question about the bikes reflects a widely believed myth, namely that metal, being a good conductor of electricity, attracts lightning. But it doesn't; the chances of an object's being struck have practically nothing to do with what it's made out of. The NWS explains, "The three factors that dominate where lightning strikes near a thunderstorm are height, isolation, and shape – a tall pointy object alone in a large open area is the most likely point to get struck by lightning." So the bad idea is being out on a mountain ridge in a thunderstorm at all; it really doesn't matter what kind of bike you're sitting on. If you're determined to take the hit for your biking buddies, Darth, forget about your frame and invest in a tall, pointy helmet.

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