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When 
Lightning Strikes
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.

Animation How It Works
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.

Illustration by Dave Joly