|MadSci Network: Earth Sciences|
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 amazon.com 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.
Try the links in the MadSci Library for more information on Earth Sciences.