"Jon O'Rourke" wrote in message


http://www.wrh.noaa.gov/Flagstaff/science/lightnin.htm

I think this bit has been superseded by some recent research that shows
electrons are dispersed far too quickly for electricity to build up
enough of a charge in one place in order to produce lightning.

"The formation of ice in a cloud appears to be very important in the
development of this charge separation and ultimately of lightning.

Inside a thunderstorm, these ice particles vary in size, from small ice
crystals to larger hailstones. Owing to the rising and sinking air
associated with thunderstorms, these particles collide frequently inside
the cloud. These collisions within the thunderstorm cause these
particles to build up electric charge.

Due to the different rates of
rising and falling within a thunderstorm, a separation of electrical
charge takes place. As the thunderstorm grows, intense electrical fields
can develop within it.

A large positive charge forms in the frozen upper
part of the cloud and two charge regions - a large, negatively charged
region and a smaller positively charged region - form in the lower
portion of the cloud.

The ground normally maintains a small negative
charge with respect to the atmosphere, but when a thunderstorm drifts
overhead, the negative charge at the cloud base induces a positive
charge on the ground below the storm. The positive ground current
follows the movement of the cloud like a shadow and concentrates on
elevated objects, such as trees, buildings, and higher portions of
terrain, in an attempt to establish a current to equalize the charges
between cloud base and ground.

Air, however is a good insulator, and the
electrical potential between cloud and ground must build up to levels of
tens to hundreds of millions of volts before the insulating properties
of the air break down and an ionized conductive channel is established
for the current to flow between the two charges.

If you have ever had
your hair stand on end while under a thunderstorm, you were in this
positive ground current, and could have become a lightning target."


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