You may have heard that you can count the number of seconds between seeing lightning and hearing thunder to estimate the distance you are from the lightning strike. This is the “flash-to-bang” method of calculating distance and it works. Every 5 seconds between the flash and the rumble equates to about a mile of distance between you and the lightning.
Speeds of Light vs Speed of Sound
The reason it works is that light from lightning travels much faster through air than the sound of thunder. Air slightly slows down light, but only about 56 mi/s or 90 km/s. Its speed is still pretty close to 186,000 mph (~3 x 108 m/s), no matter where you are on Earth. The speed of sound is affected by air more than the speed of light. The speed of sound through air is not constant. Sound moves faster through warmer air than colder air (which is why sound travels more slowly at higher altitudes), ranging from roughly 760 mph (340 m/s) to 720 mph (320 m/s).
But, whichever number you use for the speed of sound, the flash from lightning reaches your eyes almost instantly, while the sound of thunder takes longer to reach your ears. Sound travels about 1 kilometer in 3 seconds or 1 mile in 5 seconds. It’s only a rough estimate, but it’s a good way to tell whether lightning is close enough to pose a threat.
Use Thunder to Estimate Lightning Strike Distance
Estimate by the distance to a lightning strike using a stop watch or counting the seconds. You can count “one Mississippi, two Mississippi..” or “one one-thousand, two one-thousand…” to gauge the time.
|If you hear thunder…||The lightning is…|
|5 seconds after the flash||1 mile away|
|10 seconds after the flash||2 miles away|
|15 seconds after the flash||3 miles away|
|20 seconds after the flash||4 miles away|
|25 seconds after the flash||5 miles away|
|30 seconds after the flash||6 miles away|
The 30/30 Rule
While counting seconds between thunder and lightning tells you how far you are from a lightning strike, it doesn’t tell you how far away the dangerous part of a storm is. Anyway, most people get struck by lightning before or after a thunderstorm and not during it. So, the 30/30 rule is a better use of the flash-to-bang method.
The first part of the 30/30 rule is that hearing thunder within 30 seconds of seeing lightning means the lightning is within 6 miles of your location. NOAA and the National Weather Service advise people seek shelter when lightning is this close. The second part of the 30/30 rule is that you should shelter for 30 minutes after hearing thunder 30 seconds after seeing lightning.