To estimate how far away lightning is, you use the time between seeing the flash and hearing the thunder.

Simple rule of thumb

• When you see the lightning, start counting seconds until you hear the thunder.
• Divide the number of seconds by:
  • 5 to get the distance in miles.

* 3 to get the distance in kilometers.

• Example:
  • 10 seconds between flash and thunder ≈ 2 miles (10 ÷ 5).

* 9 seconds between flash and thunder ≈ 3 km (9 ÷ 3).

Why this works

Light reaches you almost instantly, but sound travels much slower, about 1 mile in roughly 5 seconds, or 1 km in roughly 3 seconds. Counting the delay uses the speed of sound to estimate how far away the strike was.

Safety tip

If the time between flash and thunder is 30 seconds or less (about 6 miles or 10 km), you should get indoors or into a hard-topped vehicle because the storm is close enough to be dangerous.