When salt dissolves in water, it comes apart into ions with positive
and negative electrical charge. That's actually what we mean by calling
a material a salt. Ordinary table salt forms Na+ and Cl-. These charged
particles are dragged opposite ways by an electrical field, making an
electrical current. So that's how salt water conducts.
In a thunderstorm, strong vertical air currents carry charged water
droplets up and down. Typically, the negative charges are on the bottom
of the thundercloud, and the positive charges are on the top. The
negatively-charged droplets on the bottom of the cloud create a strong
electric field nearby, repelling negative charges nearby. This means
the negative Cl- ions in the water get pushed away from under the
thundercloud, and positive Na+ ions get attracted. The area in which
the charge distribution of the surface of the water is affected can be
quite large -- at least as big as the thunderstorm cloud. The polarity
of the charge buildup should switch at some distance from the bottom of
the cloud. Very far away from the thunderstorm, there should be very
little charge buildup on the water.
When the lightning bolt hits, the electrical current should flow
mostly on the surface of the water. Salt water is a very good
conductor, and so the electrical potential within the bulk of the water
is nearly constant (if it weren't constant current would flow until it
was); the same is true with metals. I'd expect the current density to
fall off the farther you get from the bolt in inverse proportion to the
distance, since the current spreads out in all directions to neutralize
the charge buildup in a large area under the cloud.
The current will take the path of least resistance, and that means
through seawater. People are made up mostly of saltwater (the salinity
of blood is remarkably close to that of the ocean) and conduct rather
well too, but maybe not quite as well as seawater. I wouldn't risk it
however. What might be a bigger risk is the fact that if you are
standing up near the shore, you become a living lightning rod. The
water is good at staying below your level, so you may well be the
tallest thing around, electrically connected with the charged water
surface, and therefore a good target for a lightning strike.
Mike W. and Tom J.
(published on 08/26/06)