Is it a Boom or Just a Rumble?

Rick posed a good question by e-mail last week:

You just mentioned on the air that lightning is no wider than the thickness of a pencil...

Well what causes some lightning to be SOOOOO incredibly booming and loud while others are just little "cracks and pops"?


-Rick

Great question, Rick. But to answer it, we should probably start with another question: "What causes thunder?"

In short, lightning causes thunder. Towering cumulonimbus clouds (thunderstorms) often include ice particles. And those ice particles help create an electrical charge within the cloud. In most cases, the charge separates itself out with the positive charge near the top of the cloud and the negative charge near the bottom of the cloud. The earth tends to have a positive charge. When the charge builds up enough, it finds a release. The cloud sends down a negatively-charged "leader" and the earth sends up a positively-charged "return stroke." When they meet, we see the lightning.

The lightning channel is super-heated. It can be pencil-thin or up to two inches wide. Estimates vary, but it's somewhere between 50,000 and 70,000° F (28,000 to 39,000° C). In any case, it's much hotter than the surface of the sun... but only for a few milliseconds. Still, that's enough time to super-heat the air around the lightning flash. When the air is heated it expands. When it expands that much that quickly (violently), it creates a shock wave. That shock wave propagates through the molecules in the air and we hear it as thunder.

From the National Lightning Safety Institute:

Less than 1% of lightning's energy is converted into sound and the rest released in the form of light. A sudden increase in pressure and temperature causes surrounding air to expand violently at a rate faster than the speed of sound, similar to a sonic boom. The shock wave extends outward for the first 30 feet (10 m), after which it becomes an ordinary sound wave called thunder.

When the lightning/thunder is very close to us, the sound waves don't have very far to go... so we hear it as an incredibly loud, boom. When the thunder is farther away, some of the sound wave dissipates in the air before it gets to us (loses some of its energy)... so we hear it only as a low rumble.

Here are some other good resources with info about lightning:

National Weather Service Lightning Safety Program

Wikipedia entry about Thunder