SAN ANTONIO — The close association of lightning and thunder isn't a mistake; it's essentially a cause-and-effect process. 

First comes the lightning – fives times hotter than the sun – moving at the speed of light. As a result of this extreme heat, we hear, you guessed...thunder. That intimidating crack is created from a rapid expansion of air. 

Thunder travels at the speed of sound, which is much slower than the speed of light. It travels at around 750 mph, but can travel even faster through warmer, less dense air. 

In fact, on a colder day, thunder takes about five seconds to travel every one mile. On a warmer day, it speeds up to about one mile every four-and-a-half seconds. 

Sometimes you can see the lightning, but not hear the thunder; this is sometimes known as heat lightning. It occurs when a person is too far away from the thunderstorm to hear the thunder. As observers, our line of sight doesn't compute the curvature of the earth's surface, so our line of sight can only see the illuminated tops of clouds. 

Once we're farther than 10 miles away from the lightning, the sound dissipates, and we can no longer register the sound.

A good rule of thumb is to go inside when you hear the roar of thunder, and stay inside until you don't hear it anymore. Keep away from windows and doors with glass, and avoid electrical equipment and corded telephones as well.