This Week's Experiment - #236 Thunder Rumbles

a thunderstorm or a recording of thunder

If you are lucky enough to have a thunderstorm handy, get to a safe place (in your house or in a car) and listen to the sounds of the storm. If you don't have thunder right now, you can use your imagination and memory to remember how it sounds.

How does it sound? Does thunder just go "Boom!" and then stop? No. Instead, you usually hear a loud boom followed by several seconds of rumble. That rumble is what we are interested in today.

First, why does thunder rumble? You may remember that we talked about that in a past experiment. In case you don't remember or if you are new to the list, I will go quickly through that first. When you see the flash of lightning, it usually takes several seconds before you hear the thunder. That is because light travels much faster than sound. The light travels to you in a tiny fraction of a second, but it takes the sound about 5 seconds to travel each mile between the bolt of lightning and you. If you see the flash and count 10 seconds before you hear the boom, the lightning was 2 miles away.

Now picture a lightning bolt. They can be several miles long. If the nearest part of the bolt is one mile away, how many seconds will it take before you hear it? 5, right? Ok, now if the farthest part of the bolt is 3 miles away, how long will it take to hear that part of the bolt? 15 seconds. So you see the flash and 5 seconds later you hear the boom, followed by 10 seconds of rumble as you hear different parts of the lightning bolt. The end of the rumble is the farthest part of the bolt.

Now we want to take a closer look (or listen) at the rumble. Is it just a constant, steady rumble? Not usually. Often the thunder gets softer, then louder, then softer, and so on. It makes sense that as you hear parts of the lightning that are farther away that the sound would get softer, just as other sounds are louder when they are near and softer when the source of the sound is farther away.

But why would the rumble get louder? It has to do with the shape of a lightning bolt. Are lightning bolts straight? Not usually. Instead, they zig and zag, first one way and then another. Think about one segment of the bolt. Imagine that this segment is headed straight down towards the ground. Most of the sound is traveling to the sides, away from the bolt. The thunder would sound louder to someone standing beside the bolt than it would to someone directly under it. Don't worry. We are going to imagine that the bolt takes another zig, so it misses both of our imaginary observers.

A bolt that is heading directly towards you or away from you will not sound as loud as one that is traveling in another direction. As the lightning zigs and zags along its pathway, it sounds louder and softer depending on whether the segment you are hearing at that instant was traveling towards you or at some other angle.

Several years ago, scientists set up a set of microphones around a large field and used a computer to analyze the loudness of the bolt from different microphones. This allowed them to plot out a 3D model of the bolt's pathway.

Just be sure to remember lightning safety. Observe storms from a safe place, such as in your house or in a car. Safety always comes first with science.

From Robert Krampf's Science Education Company
PO Box 60982
Jacksonville, FL 32236-0982
904-388-6381
krampf@aol.com

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