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.
The cloud bottom carries a negative charge. 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.