Heat lightning is a term used to describe the faint flashes of lightning that can be seen on the horizon. The important feature of these flashes is that there is no thunder sound accompanying the lightning flash. Heat lightning was named because it is usually seen on warm summer nights and noticeably lacks thunder or rain. This effect is simply caused by the distance between the lightning and the point at which it is being observed and is in fact no different from normal lightning. There is no sound because the sound waves have dissipated over the distance travelled.
Heat lightning is simply a misnomer. What the observer is seeing is normal lightning but from such a distance that the usual characteristics of a storm do not apply. This is very similar to dry lightning, which is simply the presence of lightning and thunder without rain. The observer is simply viewing the lightning flash from outside the storm and from such a distance that the sound of thunder cannot be heard. So really there is no true cause of heat lightning because heat lightning doesn’t exist.
There are a few reasons why the lightning may be visible from great distances. One of these is the curvature of the Earth. As the light from the flash is scattered in the upper atmosphere and therefore visible to observers that are great distances away. There are also reasons why the thunder may not be audible from a normal distance. The sound of thunder reflects off of the Earth’s surface and the sound is also refracted through the troposphere. This combination can cause pockets where the sound is inaudible due to the reflection and refraction.
Also thunder can be silenced by any airborne objects such as sand or snow. This can effectively muffle the sound to a point where the observer can no longer hear it, even from very close to the storm.
Heat lightning is a common sight in Florida and is usually viewed over the water at night and is used as a way of predicting incoming storms. Yet the truth remains that heat lightning doesn’t exist; however if the term is taken to describe lightning without accompanying thunder, then the true causes could be anything from atmospherically reflected light to airborne matter muffling the sound. The real cause of heat lightning must however be attributed to one thing; real lightning, because that is simply what it is. All that has changed is the distance from which the event is observed.
Sources:
http://www.theweatherprediction.com/habyhints/274/
http://www.islandnet.com/~see/weather/whys/heatlightning.htm
http://www.weather.com/
http://www.buzzle.com/articles/heat-lightning-facts.html
