The rule in one sentence
Get inside as soon as lightning is close, and stay inside until 30 minutes have passed since the last lightning or thunder. The two 30s in the name are one number for each half of that sentence.
The first 30: when to go in
The original rule counted the seconds between a flash of lightning and its thunder. Sound travels about a mile every five seconds, so a count of 30 or less means the strike was within about six miles, and it was time to get to shelter.
The National Weather Service now teaches something simpler and safer: when thunder roars, go indoors. If you can hear thunder at all, you are likely within striking distance of the storm. There is no count to get wrong, and no waiting for a flash you might not see in daylight or a rumble you might not hear over a crowd.
The second 30: when to go back out
This half has not changed. The National Weather Service advises waiting 30 minutes after the last lightning or thunder before going back outside. Every new flash or rumble starts the 30 minutes over.
On a field, that means someone has to keep watching the whole time, and keep restarting the clock. It is the part of the rule most likely to be cut short when a game is waiting.
Where to shelter
The National Weather Service lists substantial buildings and hard topped vehicles as safe. Rain shelters, small sheds, and open vehicles are not. Inside, keep away from outside doors and windows.
How Sports Safe applies it
- Each field has a clear the field radius you choose, from 6 to 15 miles, 10 unless you change it. A strike inside it sends a clear the field alert.
- The all clear comes only after 30 minutes with no strikes inside the radius. Any new strike inside it restarts the 30 minutes, and the app shows the earliest time you can return.
- Sports Safe tracks lightning strikes; it cannot hear thunder. If you hear thunder before an alert arrives, go inside anyway.
See how a lightning alert works
Last reviewed October 14, 2026. This guide is general safety information, not a substitute for your organization's emergency action plan or the instructions of local authorities.