Stormy nights lately. I was looking up some information about lightning for a post tomorrow, but I also clicked a few thunder links.

Lightning can be frightening and very dangerous, but thunder is the scary but safer result of the lightning.

Thunder is the shockwave radiating away from the lightning strike path. When the air heats up from the lightning, it expands rapidly, creating a compression wave in the form of a sound wave. That heat can be as much as 60,000 degrees – hotter than the sun.

I was surprised that thunder is not completely harmless. If you are close enough, you can feel the shockwave as it shakes the surroundings. The shockwave that produces the thunder from a lightning strike can actually damage structures and people, especially if you are close to the strike.

I was taught as a kid to count the time between the flash and the thunder to see if the storm was headed my way. Sound travels much slower than light, so we see the lightning flash before we hear the thunder. (Why is that still not true of movie and TV lightning storms?) In air, sound travels roughly 1 mile every 4.5 seconds. Light travels at 186,000 miles (299,000 kilometers) per second. That’s why the time between seeing a lightning flash and hearing the thunder it produces is a rough guide to how far away the lightning (and storm) is from you.

Normally, thunder can be heard up to 10 miles from its lightning. The light travels at 186,000 miles per second but sound, including thunder, travels about a mile in five seconds near the ground. That means that if 15 seconds pass between seeing the flash and hearing its thunder, the lightning was about three miles away.

Count the number of seconds until you hear thunder. If you don’t have a watch, say “One one thousand, two one thousand etc.” Every five (well, 4.7) seconds equals one mile, so if strike number two is a second less, the lightning is a fifth of a mile closer. If that lightning is less than three miles away, take shelter. If the time increases, the storm is moving away. (more detailed calculations)