What is 'Heat Lightning'?

As I was leaving the station Wednesday night I was able to see lightning from thunderstorms over Carroll and Whiteside counties.  Being that far away I couldn't hear any thunder, but it got me thinking about the times I've heard people refer to the term 'heat lightning'.


Often times people use the term 'heat lightning' on warm and muggy nights when lightning is observed, but the sound of thunder is absent.  Lightning will always be associated with a thunderstorm, but you may not always be able to hear thunder because the storm is too far away.  Light travels faster than sound and the movement of thunder (or sound of thunder) in the atmosphere depends on different atmospheric conditions, such as temperature and density.  Both temperature and density change with height and this causes the sound of thunder to be refracted through the troposphere.


The sound of thunder can also reflect off the earth's surface due to different elements on the surface, as well as the curvature of the earth.  Generally we can hear thunder up to 10 miles away, but after that the sound may become very faint or absent completely.


So the next time you're out on your back porch on a warm, muggy summer night and see lightning but don't hear thunder, just remember that it's a thunderstorm too far away to actually be able to hear the thunder!