Actually, there is no such thing as heat lightning, making it one of the more misused weather terms each summer.
Many people think heat lightning to be flashes of lightning produced by hot and humid weather, with no rain or thunder accompanying it.
In reality, the so-called heat lightning is actual lightning from a distant but actual thunderstorm, one that is far enough away to be seen but not heard.
Lightning can be seen from a thunderstorm that is 50 to 100 miles away. But in most cases, thunder can only be heard from a storm that is 10 to 15 miles from your location as sound does not travel as well through the atmosphere.
A good thing about this type of lightning is that it is safe to be outside to view it, since it is too far away to strike.
When the thunderstorm approaches and thunder accompanies the lightning, it is then time to move safely indoors, hence the safety tip "when thunder roars, go indoors."