As we all know, lightning is a powerful electrical discharge. It occurs between areas that have acquired different electrical potentials (voltages) generated by friction between cloud water particles when shoved past each other by wind. Thunderstorms create lots of both water particles and high wind gusts, and so also lightning. The huge electrical current in a lightning bolt is nature’s way of transferring electrical charge to equalize that between the two regions.
And we don’t want to get in the way!
By far the greatest number of lightning strikes occurs from cloud to cloud, many at very high altitude. None of those pose any hazard to people on the ground. But the strikes that do go from cloud to ground pose lots of hazard to people, buildings and electrical power systems.
In a storm, it would be very useful to recognize your level of danger so you can behave accordingly.
As it turns out, there’s lots of information available for you to tell which type of lightning strike just occurred, and how far away it was, which helps to assess the chance of others like it.
Two principles apply:
1. Sound travels at a speed of about two city blocks per second, so takes about 5 seconds to travel a mile.
2. As sound travels through the air, the high-pitched portions of it die off more rapidly than the low pitches. That’s why ship’s horns are all very low in pitch – so they can be heard far away.
Now as to lightning: A ground strike travels more or less vertically. Consequently, all of its sound reaches you at the same time. When you see the flash, start counting seconds to determine how far away it was (5 seconds per mile). If it was close, you will hear a loud sharp “crack” with very little time between flash and the sound. The farther away it was, the more muffled the sound because the high-pitched portions of the sound have been diminished.
Now if the strike was from cloud-to-cloud, it can have traveled several miles more or less horizontally. Now not all of the sound reaches you simultaneously. First you hear that from where the strike passed closest to you, and then is followed by sound that has traveled greater and greater distances. Furthermore, cloud-to-cloud strikes tend to be pretty far away from you. So the high pitches have been lost, and the sound is reaching you over many seconds – the result is low pitched “rumbling”.
Big cumulus thunderheads can reach 5 or 6 miles up into the stratosphere. So when one contains many cloud-to-cloud strikes high up, the light filtering through the clouds has spread out and just lights up the whole sky in a continuous series of flashes.
As is widely known, lightning rods can protect buildings, but how they work is misunderstood. They are useless if lightning strikes your house! Their purpose is to keep it from happening. They are connected to the Earth by heavy cables, and are topped by sharp tips. When a cloud carrying a big potential difference with the ground passes nearby, the sharp point concentrates the resultant electrical field causing electrons to stream off toward the cloud, and so reduce the charge difference. The idea is to let the lightning strike somebody’s house in the next block, not yours.