how to tell how far lightning is

To tell how far away lightning is, use the "flash-to-bang" method:

1. Right after seeing the lightning flash, start counting the seconds until you hear the thunder.
2. Divide the number of seconds counted by 5 to get the distance in miles, or divide by 3 to get the distance in kilometers.

For example, if you count 10 seconds between the flash and thunder, the lightning struck about 2 miles away, or roughly 3 km away. This method works because light travels almost instantly, but sound travels much slower—about 1 mile in 5 seconds or 1 kilometer in 3 seconds. The National Weather Service advises seeking shelter if this gap is 30 seconds or less, which means lightning is within 6 miles (10 km) and could be dangerous.