What Causes Thunder

Question: I read your last post about what causes lightning. That got me thinking about what causes thunder. I know the two are somehow related, but could you please explain the connection? — EH, Oshkosh, WI

Answer: My pleasure. Thunder is much easier to understanding than lightning, and the cause of thunder was explained over a century ago. Here’s the answer provided by one M. Hirn, in the 1888 issue of Scientific American:

The sound which is known as thunder is due simply to the fact that the air traversed by an electric spark, that is, a flash of lightning, is suddenly raised to a very high temperature, and has its volume, moreover considerably increased. The column of gas thus suddenly heated and expanded is sometimes several miles long and as the duration of the flash is not even a millionth of a second, it follows that the noise bursts forth at once from the whole column, though for an observer in any one place it commences where lightning is at least distance.

My animation makes it even easier to understand. It’s in extreme slow motion. It also includes (for the first time ever in a Sky Lights animation) an audio track, so be sure your speakers are on. Here’s a step-by-step explanation:

  1. Electrical charge builds up in a cloud during a thunderstorm. For an explanation of how that happens, at least in our current theory, see last week’s post: What Causes Lightning.
  2. A trickle of electricity begins to move downward from the cloud, building up a higher and higher negative charge in the air above the ground. This trickle is known as a leader, and it often branches and turns, seeking the easiest electrically conducting path to the ground.
  3. As the negative leader nears the ground, a positive streamer starts upward building an opposite charge. These two electrical currents, being oppositely charged, will attract and eventually connect. Some observers have reported hearing these leaders and streamers emit a soft crackling/buzzing/electrical type of sound. If you ever hear that in a thunderstorm, duck and run! Lightning is about to strike nearby.
  4. When the leader meets the streamer, the air (normally an electrical insulator) is sufficiently ionized to conduct electricity, and the bulk of the charge flows from the cloud to the ground in a brief burst of high-energy electricity. This is the actual lightning bolt. Its large discharge (temporarily) neutralizes the charge in the cloud and ground.
  5. Lightning bolts are created by electrical potentials of up to 500,000 volts, with currents of up to 200,000 amperes. That’s enough electricity to power the entire United States for about 20 minutes.
  6. The bolt itself is only a few centimeters in diameter (judging from holes in buildings created by lightning strikes), but its temperature can reach 20,000 °C — three times hotter than the surface of the Sun.
  7. The bolt quickly heats the air around it, and that air expands explosively, generating a shock wave that propagates outward in all directions at the speed of sound. When it reaches your ear, you hear it as thunder.
  8. That shock wave can often reflect from large objects, like buildings and mountains, creating multiple echoes of the original sound delayed by several seconds.

So basically, the sound of thunder is the same type of thing caused by explosives, except that no combustion is involved. The explosion from lightning is caused purely by the rapid heating of air. And as you probably know, when something is heated it expands. That’s what starts the shock wave.

Since the speed of light is about a million times faster than the speed of sound, there’s a significant delay between when you see the flash and hear the thunder — unless the lightning strikes very near to you. You can estimate the distance to the lightning by counting seconds after the flash until the thunder arrives. The rule of thumb is:

1 second = 1/3 km = 1/5 mile  ⇒  3 seconds = 1 km  or 5 seconds = 1 mile

If the time keeps increasing, the storm is moving away from you. That can be a good thing to know.

Next Week in Sky Lights ⇒ How Much of the Earth Can Be Seen

What Causes Lightning
Q&A: How Much of the Earth Can Be Seen