You see lightning the instant it occurs, but the sound it makes, thunder, takes about five seconds to travel 1 mile. If 30 seconds pass between seeing a lightning bolt and hearing its thunder, the lightning is about 6 miles away. From the ground, forked lightning can appear to travel from a cloud to earth, from cloud to cloud, or from the top or sides of a storm. A lightning stroke that flashes inside a cloud or that's hidden behind nearby clouds, so that a whole sheet of air appears to be illuminated. Any lightning in thunderstorms more than 10 miles away, or too far away to hear.
Heat lightning is seen most often on hot summer nights with clear skies.
The rarest type of lightning, balls are luminous sphere less than 3 feet wide that are associated with extremely violent thunderstorms. They exist for periods from several seconds to several minutes, vanishing after meandering slowly toward Earth. Named for the patron saint of sailors, St. Elmo's Fire is a greenish or bluish glow that gathers above pointed objects on the ground. These recently discovered flashes shoot from the top of clouds as lightning explodes within them. Benjamin Franklin was not the first, or last, to be intrigued by lightning.