Lightning timer and storm distance
Tap Flash when you see lightning and Bang when you hear the thunder to see how far away it struck. Then use the 30-minute timer: weather safety guidance says to wait 30 minutes after the last thunder before going back outside.
How far away is the lightning?
30-minute wait after thunder
The flash-to-bang rule
Light arrives almost instantly, but sound travels about 343 meters per second. Count the seconds from the flash to the thunder: every 5 seconds is about 1 mile (every 3 seconds is about 1 km). Lightning can strike 10 miles or more from a storm, so if you can hear thunder at all, you are close enough to be struck.
The 30-minute rule
The US National Weather Service's advice is simple: "When thunder roars, go indoors." Shelter in a building with plumbing and wiring or in a hard-topped car, not under a tree, in a tent or in an open shelter. Stay there until 30 minutes after the last thunder; this timer restarts every time you tap that you heard thunder again.
Planning time outdoors? See sunrise and sunset times and the hike turnaround calculator.
Questions people ask
How do I tell how far away lightning is?
Count the seconds between the flash and the thunder, then divide by 5 for miles or by 3 for kilometers. 10 seconds is about 2 miles away.
How long should I wait after thunder to go outside?
At least 30 minutes after the last thunder you hear. If you hear thunder again, start the 30 minutes over.
How far can lightning strike from a storm?
Lightning can strike 10 miles or more from the storm. If you can hear thunder, you are close enough to be struck.