Last night, I had my bedroom window open because even with the recent cooling, the room gets little circulation and stays very hot very late. So I was suddenly awaken at around 2:00 am, when a lightning storm rolled in. It was very cool with nothing between me and it. As I watched the storm and listened to thunder, half asleep, I remembered something I was told as a kid. “Every second between a lightning strike and the sound of its thunder represents a mile that the lightning is away from you.” So by this rule, if there is three seconds between the lightning and the thunder, the strike was three miles away.
That woke me up, because I knew it was wrong. Let’s go through this. A lightning strike causes sound and light to emanate from its location. The speed of light is so fast that it basically reaches our eyes instantaneously. But sound takes a little time. The amount of time it takes the thunder to reach us is the amount of time it takes sound to move from the origin of the lightning strike to us. That’s all very simple.
But that “one second equals mile rule” implies that sound travels at a speed of one second per one mile. And I certainly would have noticed that somewhere along the line. And I didn’t. What I remembered was that the speed of sound here at the surface of the earth is roughly 340 m/s. Why did I remember that? I have no idea. There are all kinds of bits of knowledge like that float around in my head—knowledge so useless you can’t even use it on Jeopardy! Anyway, that’s equal to roughly 0.2 miles per second.
So the rule is: for every 5 seconds, the strike was one mile away. Or, for my friends outside America: for every 3 seconds, the strike was one kilometer away. Except in China where the speed of sound is mandated to be much faster.