How close is that lightning, really?

If you can hear thunder, you are probably close enough to be struck. Counting the seconds between the flash and the thunder tells you roughly how far away the lightning was, but the National Weather Service no longer asks you to count before acting. Its national guidance is "When Thunder Roars, Go Indoors."

How do you tell how far away lightning is?

Light reaches you almost instantly, while sound travels much more slowly. The National Weather Service (NWS) says thunder takes about five seconds to travel a mile, so:

• count the seconds from the flash to the thunder;
• divide by five for miles, or by three for kilometres.

A 15-second gap puts the strike about 3 miles (5 km) away. Five seconds is about a mile. If the thunder arrives with the flash, the strike was very close.

The method is good enough for a rough distance and not much more. You may mistake which flash a rumble belongs to when several are going at once, and thunder from a distant strike is easy to miss under traffic or wind.

How far from a storm can lightning strike?

NWS says lightning can strike outward 10 miles (about 16 km) from a thunderstorm. That is also roughly the limit at which thunder can usually be heard, which is the reasoning behind the agency's shorthand: if you can hear it, you are likely within reach.

So a clear patch of sky overhead tells you little. A storm several miles away can still put a strike near you before any of its rain arrives.

Is the 30-30 rule still the advice?

The 30-30 rule said to head inside if you could not count to 30 between flash and thunder, then wait 30 minutes after the last thunder before going back out. Thirty seconds works out to about 6 miles, inside the 10 miles NWS gives for how far lightning can reach.

The national NWS lightning pages now leave out the count and say to go indoors when you hear thunder at all. The waiting half is unchanged: NWS advises staying in safe shelter for at least 30 minutes after the last sound of thunder. Some local NWS offices still describe the full 30-30 rule on their own pages, so you may see both.

What NWS counts as safe shelter

According to NWS, there is no safe place outside when thunderstorms are in the area. Its definition of safe shelter is narrow: a substantial building with electricity or plumbing, or an enclosed, metal-topped vehicle with the windows up.

Indoors, NWS also advises staying off corded phones and other equipment wired into the mains, keeping away from plumbing, and staying away from windows, doors and porches until the storm has passed. NWS puts lightning deaths in the United States at about 20 a year.

Where an app fits

A thunderstorm does not always come with an official warning. NWS issues a severe thunderstorm warning for hail of 1 inch (2.5 cm) or more, or wind gusts of 58 mph (93 km/h) or more. Lightning is not one of the criteria, so the watches and warnings in our guide to NWS alerts say nothing about how close the lightning is.

Ora Weather has a lightning overlay: wide-radius lightning alerts on the free tier, and the full lightning map with near alerts on OraPlus. Its severe weather alerts follow official NWS classifications. An alert can tell you lightning is in the area before you hear it. It cannot tell you that you are safe, and it does not replace the thunder itself, which NWS treats as the signal to go inside.