Lightning: Countdown to danger

In honor of Lightning Safety Awareness Week, I thought I would dust off the formula you can use to determine how close you are to lightning. A friend and I were debating that equation while watching a recent thunderstorm laden with lightning, and it turns out we were both wrong. Ah, well. At least we were safely indoors.

Here’s the formula, courtesy of the National Oceanic and Atmospheric Administration’s National Severe Storms Laboratory:

When you see the lightning flash, start counting the seconds until you hear the thunder. Every five seconds equals a mile. Divide the total time by five to learn how many miles you are from the lightning.

If it’s 30 seconds or less, suspend all outdoor activities and seek safe shelter. That means the lightning bolt is 6 miles away or less.

If it’s 15 seconds or less, a lightning strike could occur where you are. That means it’s no more than 3 miles away, and from that distance a bolt can arc and hit anywhere within that radius.

There’s also a “30-30 Rule” worth remembering, the NSSL reports. If you hear thunder by the time you reach 30, you are in danger and need to find safe shelter immediately. Wait at least 30 minutes after you see the last flash of lightning or hear the last roll of thunder before resuming outdoor activities.