MadSci Network: Earth Sciences

Re: Lightning cone of safety

Date: Sun Jan 3 13:43:51 1999
Posted By: William Beaty, Electrical Engineer / Physics explainer / K-6 science textbook content provider
Area of science: Earth Sciences
ID: 914115909.Es

Hi Doug!

You're close. The "cone of safety" applies to grounded lightning rods. Also, it's a cone of RELATIVE safety because it is not totally safe, it's just much safer than being out in the open.

A great book for this sort of thing is ALL ABOUT LIGHTNING, by Dr. Martin Uman of the U. of Florida. Here's an link to it.

Uman's book points out that, according to the US Lightning Code, the safety zone beneath a lightning rod of height H is a circle of diameter 2H. He also points out that, even though the Empire State Building has a huge lightning rod, lightning still strikes the building. The "cone of safety" is not 100% safe.

If I understand things correctly, the safety zone is created when an incoming lightning leader triggers an answering leader from the lightning rod. The tip of a lightning rod launchs an upwards-going lightning leader which intercepts the incoming leader from above. If the main leader that comes downwards from the sky should happen to come in from the side, then the lightning rod won't intercept it as easily.

Only a properly grounded lightning rod can protect you. For example, if you're outdoors during a thunderstorm, DO NOT run under a tall tree, since during a lightning strike, the electric currents in the ground around the tree can be lethal. The safest loction during a lightning storm is to be inside a large building or inside a metal car.

Lightning Info

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