Nobody ever thinks they're going to be struck by lightning — we even use this event as an expression of rarity with phrases such as “lightning never strikes twice”. Although being struck during a thunderstorm is likely to be a once-in-a-lifetime event unless you're extremely unlucky, it's certainly something you never want to experience. It's therefore wise to take reasonable precautions during summer storms to reduce this risk.

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One easy way to gauge the proximity of a lightning strike is often referred to as the “flash to bang” method. This technique involves counting the time in seconds between a visible lightning bolt (flash) and the audible thunder clap (bang). Since we know light travels faster than sound waves, you'll notice a delay between the two. This difference can be used to estimate roughly how far the lightning is from your current location.

Sound travels at about 1,088 feet per second, or about 0.2 miles per second (depending on air temperature and humidity). That means it'll take the sound of thunder about 5 seconds to travel 1 mile. So, the “flash to bang” distance can be calculated as follows: (seconds between flash and bang) divided by 5. Five seconds is one mile, 10 seconds is two miles, and so on.

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This may seem like little more than a cool piece of trivia, but it's actually a valuable early-warning system. The National Oceanic and Atmospheric Administration (NOAA) warns, “Lightning can strike as far as 10 to 15 miles from the area where it is raining. That's about the distance you can hear thunder. If you can hear thunder, you are within striking distance. Seek safe shelter immediately.”

So, there's always some risk when you can hear thunder, but we can calculate it further to better gauge your actual risk. A study by the National Severe Storms Laboratory (NSSL) found that 80% of lightning strikes in a thunderstorm are 2 to 3 miles from each other, and that more typically strikes occur within 6 miles of each other. NOAA says that “bolts from the blue” in the 10- to 15-mile radius are rare, but they're not entirely unheard of.

From this information, we can establish the following danger levels from the “flash to bang” method:

  • Less than 10 seconds: highest risk
  • 10 to 15 seconds: high risk
  • 15 to 30 seconds: moderate risk
  • 30 to 50 seconds: low risk
  • More than 50 seconds: minimal risk (thunder will be difficult to hear clearly at this range)

For more information on the “flash to bang” method, refer to the NOAA page “Understanding Lightning”. For additional tips on general safety in thunderstorms and other risk factors, check out this PDF from the National Weather Service.

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