It depends on how far you are from the lightning. The closer you are, the higher the pitch of the sound you hear; the farther away you are, the lower the rumble.
First, we have to remind ourselves of what thunder is.
A stroke of lightning is extremely fast; it occurs with what might be called lightning speed. Its sudden heat makes the surrounding air white hot, heated to tens of thousands of degrees. The air expands at tremendous speed, after which it rapidly cools and contracts back to its normal temperature and pressure. Air moving so suddenly makes huge vibrations, and that’s what sound waves are: shudders, or pressure waves, moving through the air. Hence, the noise of thunder.
It will not surprise you to learn that thunder travels at the speed of sound. But light travels almost a million times as fast as sound. Obviously, then, you’re going to see the lightning flash almost instantaneously, but you won’t hear the thunder until it travels from the lightning strike to your ears.
The next time you have the privilege of witnessing a bang-up thunderstorm, count the number of seconds between a lightning flash and the beginning of the associated thunderclap. Divide that number of seconds by 4 to find out roughly how many miles away the lightning was. Or multiply the number of seconds by 400 to get the approximate distance in yards. (But see the Nitpicker’s Corner.) You may be shocked, sorry, I mean surprised, to find how close many of the lightning strikes are. And while you’re at it, notice that the closer the lightning is, the higher-pitched “crack” you hear. Read on.
Sound doesn’t always travel at the same speed. It depends, for one thing, on what medium it is traveling through. The pressure waves can’t be transmitted from one place to another unless the transmitting substance has molecules that can collide with one another effectively and pass the energy on.
Suppose we have two trains on the same track, colliding head-on. (DO NOT TRY THIS AT HOME!) The impact energy will be transmitted, car by car, down the lengths of the trains, from their engines all the way to their cabooses (unless they derail, of course). Each car transmits its shock to the next car in line by colliding with it; that car transmits it to the next one in line by colliding with it, and so on, and the shock energy travels down the trains like a wave. That’s how the pressure waves of sound are transmitted through materials, but by collisions of molecules, rather than railroad cars.
You can see that if the railroad cars weren’t coupled very tightly together it would take more time for the shock wave to travel all the way to the cabooses, because time would be lost by each car’s having to move toward the next car before it could collide with it. In the same way, it takes more time for a sound wave to be transmitted through a substance if the molecules of that substance aren’t very close together.
In air, as in all gases, the molecules are very far apart, so sound travels relatively slowly through air: about 900 miles per hour (1,400 kilometers per hour) at sea level and room temperature. In water, the molecules are much closer together; sound travels through water at 3,300 miles per hour (5,300 kilometers per hour). In a dense solid such as steel, it travels at 13,000 miles per hour (21,000 kilometers per hour).
So much for how fast sound travels. Now let’s look at how it changes as it travels.
As you can imagine, the close-up sound of lightning is a sharp, high-pitched crackle, just what you’d expect from a huge spark. But by the time a distant thunderclap reaches you, it may be a low-frequency rumble. The conclusion we draw from that is that low-frequency sounds travel longer distances than high-frequency sounds, which tend to peter out with distance. Ever notice that when your idiot neighbor plays his stereo loud enough to peel the paint off the walls you hear primarily the bass notes? The treble notes just don’t carry as far and are also absorbed better by the walls. The reason is that the higher-frequency sounds are making the air and the walls vibrate more times per second, so they are using up their energy faster as they go.
That’s why the low frequencies of the thunderclap carry farther than the high-pitched pops and crackles, and the farther away you are from the actual electrical event the lower the sound pitch will be. That’s another way of comparing the nearness or farness (why isn’t that a word?) of lightning strikes. The farther away the strike is, the later and lower will be the sound.
You must have noticed that thunder isn’t simply high- or low-pitched, but is a mixture of high- and low-frequency sounds. That’s because the lightning itself happens at a mixture of distances from you. The bolt may be miles long, with huge branches spreading out from the main stroke, so various parts of it are various distances from you, and that spreads out the frequencies of the sounds you hear.
You have also noticed that thunder rumbles and rolls for an extended period of time. There are two reasons for that. One, the sound is traveling various distances from the various branches of the bolt, and two, it is echoing off the ground as it travels. Now you may crawl back under the bed.
Sound waves aren’t transmitted through air simply by making the air molecules collide with one another in a straight line, like a string of railroad cars in a crash. Sound energy converts “smooth air” into a series of zones that are alternately compressed and expanded.
That is, sound forces the air into alternating regions of high and low density. It is these density alternations that hit your eardrum at the rate of a certain number of compressions and expansions per second. The more of these compressions and expansions that hit your eardrum per second, the higher the frequency, or pitch, of the sound that you hear.
The speed of sound in air varies quite a bit depending on the air’s temperature and pressure. The rule of thumb I gave above for timing how far away a lightning bolt struck is only a rough guide, because we can’t know the temperature and pressure of the air where the bolt created most of its thunder noise or the air conditions between there and us.
I chose four seconds for each mile of sound delay, but you’ll see five seconds suggested in other books. Don’t sweat it. As mentioned above, lightning bolts are long, and they may create thunder all along their paths in air that has a variety of temperatures and pressures and is at various distances from you.
That’s why you may have trouble timing the thunder anyway; do you time from the flash to the beginning of the rumble, or the end? It’s far from an exact science, unless we know a lot more about the lightning bolt than we usually do.