30-30 Rule
The 30-30 Rule is an easy way to determine the threat of lightning in your area:
30 Seconds: Count the seconds between seeing lightning and hearing thunder. If this time is less than 30 seconds, lightning is a threat. Seek shelter immediately. Even if you can’t see the lightning, just hearing the thunder is a good backup rule.
30 Minutes: After hearing the last rumble of thunder, wait 30 minutes before leaving shelter. There is still danger after a storm is over, so stay in a safe area until you are sure the threat has passed.
