how long after lightning before resuming activity
If thunderstorms develop, count the seconds between the flash of lightning and the bang of the thunder to estimate the distance between you and the lightning strike. Because sound travels at approximately 1 mile in 5 seconds, you can determine how far away the lightning is by using this flash-to-bang method. It is recommended that you seek shelter if the time between the lightning flash and the rumble of thunder is 30 seconds or less (6 miles). Once inside shelter, you should not resume activities until 30 minutes after the last audible thunder. This is known as the 30/30 Lightning Rule.