Science Questions

What should I do when cycling in a thunderstorm?

Sun, 14th Oct 2007

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Question

Andrew Dawson asked:

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 canít go through it.  Itís because youíre 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 itís going to want to go through you, so the current just goes around you and youíre absolutely fine in the middle.  This effect is called a Faraday Cage.  If youíre sitting on a bike and youíre 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, Theyíre only about 1 inch across and this means  the voltage needed to put a spark across that isnít very large.  The spark has already jumped about 500m from the clouds to you through the sky. Another inch to the ground isnít going to be too much of a problem.

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