Heat lightning is distant lightning without thunder or rain. It usually occurs when the weather is hot and muggy. For this reason, it is also sometimes called summer lightning.
Heat lightning usually belongs to a storm which is too far away for the thunder to be heard. Thunderclouds are often very tall, so lightning in their highest parts can be seen from as much as 100 miles away even if the storm is beyond the horizon. This great distance is also why heat lightning is seen as sheet lightning rather than forks or strokes, which strike the ground from the base of the cloud.
As heat lightning becomes visible, the distant storm is coming closer. If you start to hear thunder, the storm is probably moving toward you.
Thunder is caused by the sudden expansion and contraction of the air surrounding the lightning bolt, which is hot enough to melt sand into glass. The sound travels at approximately one mile every five seconds, varying slightly depending on air quality and temperature. The light of the lightning flash travels so much faster that it might as well be instantaneous. It is therefore possible to know how far away the lightning struck by counting the seconds between seeing the flash and hearing the thunder. The further away the lightning, the longer it will take the sound of the thunder to arrive.
Because sound is a wave, it can be reflected and distorted by buildings or the earth's surface or different densities of air. This changes the sound of the thunder from a sharp crack to a low rumble. The further away the storm, the greater the distortion. Eventually the sound becomes too low-pitched and distorted to be recognizable as thunder. It is unusual to hear thunder with a storm that is more than ten miles away.
Heat lightning can occur outside a thunderstorm. Sometimes the positively-charged anvil part of a distant cumulonimbus cloud extends miles beyond its parent cloud. When the atmospheric conditions are right, anvil lightning can strike out of what seems to be a clear blue sky. This type of lightning is much more powerful than standard lightning, the stroke lasts nearly ten times as long, and it comes from much higher up. If it happens far away on a warm summer night, heat lightning will be seen.
Finally, what is called heat lightning may actually be sprites. These are streamer-like electrical discharges which occur up to 90 miles above the earth in conjunction with lightning strikes. They are much wider than lightning, and linger for a longer time.