You can tell how far away lightning is by using the time difference between seeing the lightning flash and hearing the thunder. This is because light travels almost instantaneously to you, while sound travels much slower. Here is the common method to estimate the distance:
- Count the number of seconds between the lightning flash and the thunder sound.
- Divide that number by 5 to get the distance in miles , or
- Divide that number by 3 to get the distance in kilometers.
For example, if you count 15 seconds between the flash and the thunder, the lightning is about 3 miles (or 5 kilometers) away
. This method is known as the "flash-to-bang" technique. It works because sound travels roughly 1 mile every 5 seconds (or 1 kilometer every 3 seconds), while light travels so fast it is seen almost immediately
. A safety tip: if the time between lightning and thunder is 30 seconds or less (about 6 miles or 10 kilometers), the storm is close enough to be dangerous and you should seek shelter
. In summary, the formula is:
Distance (miles)=Seconds between flash and thunder5\text{Distance (miles)}=\frac{\text{Seconds between flash and thunder}}{5}Distance (miles)=5Seconds between flash and thunder​
or
Distance (km)=Seconds between flash and thunder3\text{Distance (km)}=\frac{\text{Seconds between flash and thunder}}{3}Distance (km)=3Seconds between flash and thunder​
This quick calculation helps you estimate how far away the lightning strike occurred
