Thunder

If you hear the word “thunder” in isolation, a few things might come to mind. If you were old enough in 1990 to hear this AC/DC song on the radio, Thunderstruck’s opening minute or so is first up:

Thunder!

Similarly, you might recall another invocation of the word thunder, this time in an incredibly popular kids’ cartoon:

Thundercats, ho!

Thunder is everywhere in popular culture, and it has been fetishized by our ancestors since time immemorial in the form of deities and demigods. One such god who is still in the spotlight today gives us the name of one of our days of the week, Thursday.

Thursday is “Thor’s day”, the day the Norse named after the god of thunder.

All of this hearkens back to our wonder with thunder. It must have been daunting for our ancient ancestors to hear a crashing roar as the very land shook under their feet.

Thunder must have seemed far beyond our control, and well into the domain of the gods. It’s no wonder we’ve been obsessed with it for so long.

With a modern understanding, the magical spell of thunder is broken. Thunder is a byproduct of another natural process: lightning.

Here’s the National Weather Service:

Thunder is the sound caused by a nearby flash of lightning and can be heard for a distance of only about 10 miles from the lightning strike. The sound of thunder should serve as a warning to anyone outside that they are within striking distance of the storm and need to get to a safe place immediately!

Yes! That’s what I meant about our ancient ancestors. Hearing a loud, rumbling crack outside would have meant that it was no time to mess around, especially if you could get a sense of what direction the storm was moving in.

In fact, you can do exactly this: because thunder is caused by lightning, and because light travels virtually instantaneously, you see the flash before you hear the sound. Kids in the American South are taught to do this by counting Mississippis: saying “one Mississippi, two Mississippi”, and so on.

If you measure the time in between flash and sound every time you see lightning, you can take note of whether it’s taking more or fewer “Mississippis” to measure. The longer it takes, the further away the storm is, since sound takes a measurable amount of time to arrive and light doesn’t.

“Thunder is the sound caused by a nearby flash of lightning” does far too much heavy lifting. Let’s break this down just a bit.

Lightning itself is a topic that deserves its own piece, so I’ll make sure we don’t have to wait too long for a deeper explanation, but the main thing to know for today is that the charge in the air gets built up, and lightning is the way nature releases all that extra energy all at once.

Lightning heats the air to a temperature that is hotter than the surface of the Sun. The extreme heat causes the air surrounding the strike to expand rapidly, like an explosion. This causes a shockwave to emanate from the strike spot, leaving at the speed of sound just as the lightning heads to your eye at the speed of light.

If you hear lightning strike very close to you, it sounds really different, more like a very loud crash or a snap than a rumble. That’s because as the sound travels along the air, those higher frequencies are absorbed, leaving only the low, ominous sound we associate with thunder behind.

Humidity can play a role, too. In the Deep South, it’s very humid, and the thunder sounds even more rumbled and low due to the moisture in the air. Temperature variations in the air can change the way thunder sounds, too, because frequency is directly tied to the density of the medium (the air), and heat expands gas.

Did you count Mississipis as a kid?

Are there any other good thunder-related cultural references you know of?