Lightning Safety and Lore

June 10, 2009

‘Tain’t what a man don’t know that hurts him; it’s what he knows that just ain’t so. — Kin Hubbard

It’s that time of year again: in the last couple of days we’ve had some pretty impressive thunderstorms here in the Washington DC area.  (One knocked down a tree on the White House lawn.)  Every year, too, questions come up about staying safe in thunderstorms, particularly from lightning.  Although, statistically speaking, auto accidents in the US kill many more people than lightning does, lightning is dangerous: sadly, a twelve-year-old boy was killed nearby by a lightning strike a few days ago.

A few years back, I was actively involved in the AIDS Rides, and was administrator of an e-mail list that was used to provide support and advice, especially to new people.  Lightning questions came up there, too, and I realized that many people really didn’t understand much about the actual risk.  One fairly common type of question was something like, “Is is safer to wear sneakers with rubber soles in a thunderstorm?”   Well, no.  Here is the explanation we put into the FAQ for the list:

Whether you ride with sneakers or clipless pedals, or the frame composition of your bike, probably makes very little difference. Remember that the electric current in lightning is travelling through air for a considerable distance, and air, even with some water droplets mixed in, is quite a good electrical insulator.

Perhaps more to the point, either the bike or the cyclist will conduct electricity considerably better than air, even air saturated with water vapor. The electrical potential difference between a thunderstorm cloud and the earth can reach ca. 100 million volts. Small amounts of insulating material (e.g., rubber-soled shoes) make very little difference.

Sometimes people think that being in a car is safer because the rubber tires insulate it from the ground. This is a correct conclusion for the wrong reason.  Mostly, the safety comes from the fact that you are (approximately) inside a closed metal (conductive) container. (This was called a “Faraday cage” in physics class — the net electric field inside a closed conductive surface is always zero, by Gauss’s Law.) Being inside a steel-framed skyscraper is also very safe for the same reason, as is being in a metal railway car (which of course has steel “tires”)

The mistaken belief falls in the category that I call lore. One of the mental goals that seems to be innate in people is the desire to make sense of the world.  We will work hard to construct a story about how and why things happen, and the lack of actual knowledge or understanding doesn’t seem to impede this process very much.  In this case, people know that rubber is an electrical insulator, and the idea that it would protect you from lightning seems plausible.  Getting in the car during a thunderstorm, if there’s no better shelter available, is a good idea.  But thinking you are safe because you’re wearing sneakers is a very bad idea.  As Kin Hubbard said, those things you know that just ain’t so can get you in big trouble.

The National Severe Storms Laboratory has a page of lightning safety advice.

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