When lightning flashes in the sky without a clap of thunder, it is called “heat lightning.” This, however, is a bit of a misnomer as it is not a special form of lightning and it is not generated by heat. In fact, it is just normal lightning that is too far away for the sound of thunder to be heard.

Lightning travels near the speed of light – 186,000 miles per second. Even at a distance of 10 miles, you will see it almost instantly. Thunder, on the other hand, travels at the speed of sound – about one mile in five seconds near the ground. It is often refracted through the atmosphere and reflected by the Earth’s surface, dissipating the sound. For this reason, thunder is rarely heard at a distance of more than 10 miles from where the lightning actually occurred.

The origins of the term, “heat lightning”, are probably linked to the fact that it is most often seen on hot, humid summer nights.