“They say for every second between the flash and sound is one mile,” is not accurate; in fact, it is off by 80%.

If one counts the seconds between the flash of lightning and when the thunder is heard, one can determine how far away the lightning struck.

The count starts because of the light transmitted to an observer, and light travels, well, at the speed of light. But the thunder, a sound, travels at the speed of sound. These are extremely different speeds.

The instant the lightning occurs is communicated to an observer at the speed of light. Light travels so fast that it can circle the Earth eight times in one second, so we consider events we see on Earth as instantaneous. However, thunder is communicated to you by sound. Sound travels at 1,087 feet per second; in aviation, it has a name, Mach 1. Remember that a mile is 5,280 feet. Add those facts up and you will discover that when the flash is seen, it takes fully five seconds for the sound to reach you if the lightning strike is one mile away. For example, if thunder is heard one second after the flash, it actually struck one-fifth of a mile away or about 1,000 feet, close to three football field lengths away, much closer than one mile. Either way, if the flash and the thunder are very close together, it can’t be argued that the strike was very close, and you are probably lucky to still be breathing.

The one-second delay equal to one-mile distance is an often-heard common knowledge thought; it’s the answer most everyone gives for this calculation. Unfortunately, it’s wrong.

John Salinas

San Juan Island