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The 30-Minute Lightning Rule: How Long to Wait After Thunder

By Noureddine, developer of Lightning Map · Published

After a thunderstorm, the hardest part is often knowing when it's safe to go back out. The rain has stopped, the sky is brightening, and everyone wants to restart the match, get back in the pool or finish the job.

The official answer is simple: wait 30 minutes after the last thunder you hear. This guide explains where that rule comes from, why it works, and how to apply it in real situations, with links to the sources.

The short version: when thunder roars, go indoors. Stay inside a safe building or a hard-topped car until 30 minutes have passed without any thunder. Every new rumble restarts the 30 minutes.

What the 30-minute rule says

The US National Weather Service puts it like this: "Stay inside a safe building or vehicle for at least 30 minutes after you hear the last thunder. While 30 minutes may seem like a long time, it is necessary to be safe" (NWS).

Two details matter:

  • The clock starts at the last thunder, not when the rain stops or when the storm looks like it's moving away.
  • The clock restarts with every new thunder or lightning. The US high school sports guidelines say: "Any subsequent thunder or lightning after the beginning of the 30-minute count will reset the clock and another 30-minute count should begin" (NFHS).

So if you hear thunder 20 minutes into your wait, you start again from zero.

Why 30 minutes?

Because a storm stays dangerous well after it seems to have passed:

  • Lightning "may strike as far as 10 miles from any rainfall" (NWS). The back edge of a storm moving away can still reach you under a clearing sky.
  • "Many lightning deaths occur ahead of storms before any rain arrives or after storms have seemingly passed and the rain has ended" (NWS).
  • The NWS also notes that "some people were struck because they went back outside too soon" (NWS).

Little League sums up the reason for the wait: "electrical charges can linger in clouds after a thunderstorm has passed", so "experts agree that people should wait at least 30 minutes after the storm before resuming activities" (Little League).

The old "30-30 rule", and why it changed

You may have learned the 30-30 rule. It was recommended in 1998 by the Lightning Safety Group, a panel of scientists, engineers and doctors (Beasley et al., American Meteorological Society). It had two parts:

  1. The first 30: if the time between a lightning flash and its thunder is 30 seconds or less, the storm is within about 6 miles, and you should already be in a safe place.
  2. The second 30: wait 30 minutes after the last thunder before going back out.

The second part is still the rule. The first part was replaced. As lightning safety expert William Roeder explains, "many people were remembering this as 'go to a safe place at this time', not 'be in a safe place'", so people started moving to shelter too late. The guidance became "When thunder roars, go indoors": head for shelter as soon as you hear any thunder at all (Roeder, Recent Changes in Lightning Safety).

Counting the seconds between flash and thunder is still a handy way to estimate distance (sound travels about 1 mile in 5 seconds, or 1 km in 3 seconds). Just don't use it to decide whether you can stay outside.

How to apply it

At home, in the park or on a walk

Go inside at the first rumble of thunder: a building with plumbing and electricity, or a hard-topped metal car with the windows closed. Note the time of the last thunder you hear, and don't go back out until 30 minutes have passed with no more thunder. Sheds, picnic shelters, tents and covered porches do not protect you (NWS).

Sports matches and training

The NFHS guidelines used by US high school sports set out a clear procedure (NFHS):

  • assign someone to monitor the weather before and during play;
  • decide in advance where everyone will shelter and how long it takes to get there;
  • when thunder is heard or lightning is seen, suspend play and move everyone to shelter immediately;
  • resume only after 30 minutes without thunder or lightning, restarting the count after each new one;
  • review the plan every year and tell athletes and parents about it.

The National Athletic Trainers' Association says activities "should be suspended until 30 minutes after the last lightning strike or sound of thunder", with a designated weather watcher and one named person who has the authority to stop play (NATA).

Swimming pools, lakes and beaches

Get out of the water and away from it at the first thunder (NWS). Canada's Lifesaving Society recommends that "swimming pools and waterfronts may reopen 30 minutes after the last sound of thunder" (Lifesaving Society).

Golf

Leave the course at the first thunder and wait in the clubhouse or a hard-topped car, not under a tree or in a golf cart (NWS myths). The same 30 minutes apply before you go back out.

Working outdoors

OSHA and NOAA advise that workers "remain in the shelter for at least 30 minutes after hearing the last sound of thunder", and that employers have a written plan saying when work stops and when it can restart (OSHA and NOAA).

What about lightning you see but can't hear?

At night you can sometimes see flashes from storms far away. The NFHS guidelines note that if no thunder can be heard and the flashes are low on the horizon, the storm may not pose a threat (NFHS). If you can hear thunder, or the flashes are getting closer, treat it as a storm in range.

Can a lightning map or app replace the rule?

A live lightning map is useful for seeing a storm coming and deciding whether to start an activity at all. But the NFHS guidelines are clear: lightning detectors and apps can help, but "hearing thunder or seeing lightning should always take precedence over information from a mobile app or lightning-detection device" (NFHS).

Every system has a delay and gaps. On Lightning Map, strikes in the Americas usually appear within about a minute, but in Europe and Africa the satellite data arrives in 10-minute batches (how our data works). That's fine for watching a storm's approach, but too slow to time a 30-minute countdown on its own. So use the map to plan, and use your ears for the 30 minutes.

For everything else about staying safe in a thunderstorm, see the complete lightning safety guide.

30-minute rule FAQ

What is the 30-minute lightning rule?

After a thunderstorm, stay inside a safe building or hard-topped car until 30 minutes have passed since the last thunder you heard. It is the rule recommended by the US National Weather Service and used in sports, swimming pools and outdoor work.

Does the 30 minutes restart if I hear thunder again?

Yes. Any new thunder or lightning during the wait restarts the count, and you wait another full 30 minutes from that moment.

What is the 30-30 lightning rule?

An older rule from 1998: be in a safe place if the gap between a flash and its thunder is 30 seconds or less, and wait 30 minutes after the last thunder. The first part has been replaced by "When thunder roars, go indoors", because people waited too long to seek shelter. The 30-minute wait is still the rule.

Can I go back outside when the rain stops?

No. Lightning can strike miles from the rain, including from the back of a storm that is moving away. Many lightning deaths happen after the rain has ended. Wait 30 minutes after the last thunder.

How long after lightning can you swim?

Pools and beaches should stay closed until 30 minutes after the last thunder. Canada's Lifesaving Society recommends reopening 30 minutes after the last sound of thunder, and every new rumble restarts the wait.

How long after thunder can sports resume?

At least 30 minutes after the last thunder or lightning, restarting the count after each new one. This is the guidance from the NFHS for US high school sports and from the National Athletic Trainers' Association.

Is the 30-minute rule the same in the UK?

The UK Met Office's thunderstorm advice doesn't give a specific waiting time. The 30-minute rule comes from US guidance and is also used by organisations such as Canada's Lifesaving Society. It's a sensible, widely used rule wherever you are.

Can I use a lightning app instead of listening for thunder?

Use it to help, not to decide alone. US high school sports guidelines say hearing thunder or seeing lightning should always take precedence over an app or detector, because every system has delays and gaps.

Sources

This guide is for general information. Always follow warnings from your national weather service and local authorities.