What should I do when cycling in a thunderstorm?

14 October 2007

Question

I was cycling on Frodsham marshes listening to your show on my ipod. The weather was disturbed - some clear sky and sunset, some rain and an intense thunder cloud. Having remembered your item on the chap who was struck by lightening wearing his ipod and realising I was in a very open exposed spot I wondered what the safest course of action would be. Would a cycle's tyres be sufficient to act as a Faraday cage assuming I didn't put my foot on the ground?

Answer

The way a car protects you from a lightning strike is not because the tyres are insulating so the lightning can't go through it. It's because you're surrounded by the steel body of the car which conducts electricity much better than you do. If lightning hits the car the current is going to want to go through the steel much more than it's going to want to go through you, so the current just goes around you and you're absolutely fine in the middle. This effect is called a Faraday Cage. If you're sitting on a bike and you're not surrounded by metal then the bike will probably make it worse because it will produce a very nice conductive path from your head, through your arms, through the bike and down to the ground very easily. Rubber is an insulator, but even though the tyres are rubber, They're only about 1 inch across and this means the voltage needed to put a spark across that isn't very large. The spark has already jumped about 500m from the clouds to you through the sky. Another inch to the ground isn't going to be too much of a problem.

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