This section is from the "The New Student's Reference Work Volume 5: How And Why Stories" by Elinor Atkinson.
That is the order in which we speak of them. But really we should say lightning and thunder. They are both made the same instant, by two electric currents in storm clouds coming violently together. This produces both light and heat. Heat expands the air. The expansion starts a great wave or billow of air to rolling, and makes the crashing sound we call thunder. But as light waves travel faster than sound, we see the flash before we hear the crash. If the two come very close together we know the sound did not have to travel far, and the storm causing both is very near. As a storm center moves away from us, the time between the flash and the roll lengthens. In that way you can tell when a storm will soon he over.