"But why does thunder make a noise?" asked Fred. "You told me the clouds didn't bang together."
"They don't," the Forecaster answered. "Thunder is caused by the electric discharge. You've heard Bob's big wireless outfit crackle, when he sends out a spark, haven't you?"
"Sure," said Fred, "you can hardly hear yourself talk, when Bob's got his wireless busy."
"And why does that crackle? Do you know, Bob?" he asked, turning to the wireless expert.
"No," answered the boy.
"You've often heard the crackling of a near-by thunder compared to an irregular volley of rifles, haven't you?"
"Yes."
"Naturally, because that's exactly what it is. A rifle shot is an explosion caused by the firing of a powder, which, in turn, means the expansion of the powder into gases, the force of that expansion driving forward the bullet. Sound, as you know, is a series of air vibrations. The explosion wave sets up a series of these vibrations, by compressing the air in front of it.
"Lightning does the same thing. When a lightning flash breaks down the resistance of the air, and passes through a channel of air, it heats the air suddenly to a temperature of two or three thousand degrees, causing a terrific expansion along the entire length of the flash and starting an explosion wave. This compresses the air on all sides and sets sound vibrations in action. As soon as the flash is discharged, the air rushes back to fill the partial vacuum that the heating by electricity has caused, adding force to the vibrations.