How Long to Wait After Thunder Before Going Outside

The 30-minute rule

The US National Weather Service advises waiting 30 minutes after the last thunder before resuming outdoor activity. If you hear thunder again at minute 20, the count restarts from zero. In a slow or lingering storm, that can mean a long wait.

Thunder is a better signal than rain. Many people are struck because they go inside too late, or head back out while the storm is still in range.

• National Weather Service: lightning safety overview

Why 30 minutes

Thunder can usually be heard up to about 10 miles (16 km) away when it's quiet. Traffic, wind and rain shorten that range. The National Weather Service says the significant lightning threat reaches around 6 to 10 miles (10 to 16 km) from the base of a storm cloud.

Research quoted by the National Weather Service found that successive flashes in a storm are often around 9 km apart. A storm moving away needs time to carry its whole reach beyond you. Thirty minutes gives most storms that time, with a margin.

• National Weather Service: lightning myths and facts
• National Weather Service: lightning safety for sports

Count the seconds to judge distance

Light arrives almost instantly; sound takes about 3 seconds to travel 1 km, or 5 seconds for a mile. Count from the flash to the thunder and divide by 3 for kilometres. A 15-second gap means the flash was about 5 km away. Do the counting from inside.

Distance helps you understand the storm, but it doesn't change the rule. If you can hear thunder at all, you're within reach.

• Lightning distance calculator
• National Weather Service: the science of thunder

Sport, pools and outdoor events

For matches, swimming, school events and festivals, the National Weather Service recommends a written lightning plan. It should say who watches the weather, when play stops, where people go and when play restarts. Write the 30-minute rule into it, and decide in advance so nobody is tempted to finish the game.

Get swimmers out of the water and into a building, not under a poolside canopy. Changing rooms with wiring and plumbing count as shelter; sports dugouts and open rain shelters don't.

• National Weather Service: lightning safety for sports

Using a lightning map while you wait

The live map shows where lightning has been reported recently, which helps you see whether the storm is moving away. It can't tell you that the risk has passed. Satellite reports can arrive minutes after a flash, some lightning is missed, and a new cell can form nearby.

Use the map for context and thunder for the decision. When the map has gone quiet and you've heard nothing for 30 minutes, it's reasonable to head back out. If you hear thunder again, go back in.

• How to read a lightning map
• Check lightning near you

Frequently asked questions

How long after thunder is it safe to go outside?

Wait at least 30 minutes after the last thunder you hear. If you hear more thunder, start the 30 minutes again.

What is the 30-30 rule?

An older rule: shelter if the flash-to-thunder gap is under 30 seconds, then wait 30 minutes. The National Weather Service now advises going inside as soon as you hear any thunder, then waiting 30 minutes after the last.

Can lightning strike after the rain stops?

Yes. Lightning can strike miles from the rain, including from the back of a departing storm.

Does an empty lightning map mean it's safe to go out?

No. Reports can be delayed or missed. Use the 30-minute rule from the last thunder you heard.