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A lightning flash can happen in half a second. In that instant, the
lightning
flash superheats the surrounding air to a temperature five times hotter
than
that on the surface of the sun. Nearby air expands and vibrates, forming
sound that we hear as thunder. Sound travels more slowly than light, so
it
seems that thunder occurs later.
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The cloud bottom carries a negative charge. Positive charges may collect
on the ground, buildings, boat masts, people, flagpoles,
mountaintops, or
trees.
A stepped leader—a negative electrical charge made of zig-zagging
segments,
or steps—comes partway down from the cloud. The steps are
invisible; each
one is about 150 feet long.
When the stepped leader gets within 150 feet of a positive charge, a
streamer
(surge of positive electricity) rises to meet it. The leader and the
streamer
make a channel.
An electrical current from an object on the ground surges upward through
the channel. It touches off a bright display called a return stroke.
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