How the Rule Works
Count the Seconds
When you see a lightning flash, start counting seconds until you hear the thunder. Light travels essentially instantly; sound travels about 1 mile every 5 seconds.
The First 30
If the gap between flash and thunder is 30 seconds or less, the lightning struck within about 6 miles — close enough to threaten you directly. Seek shelter immediately.
The Second 30
Once sheltered, wait 30 minutes after the last flash or thunder you saw or heard before going back outside. Storms move and lightning can strike well after the rain has passed overhead.
Why "Blue Sky" Isn't Safe
Lightning has been documented striking more than 10 miles from the parent thunderstorm — often called a "bolt from the blue." That's why the simplest and most reliable rule from NOAA is: if you can hear thunder, you're close enough to be struck. Don't wait to see rain or dark clouds directly overhead before taking it seriously.
What Actually Counts as Shelter
Safe: Enclosed Buildings
A fully enclosed building with wiring and plumbing gives lightning a path to ground other than through you. Avoid corded phones, plugged-in electronics, and running water during the storm.
Safe: Hard-Topped Vehicle
A metal-roofed car with the windows up is safe — not because of the tires, but because the metal shell conducts current around the outside and into the ground. Convertibles, golf carts, and open-frame vehicles don't count.
Not Safe: Picnic Shelters, Sheds, Under Trees
Open-sided structures, small sheds, and standing under an isolated tree do not protect you — a tree is actually one of the most common places people are struck, since lightning can jump from the trunk to a person standing nearby (side flash).
Lightning Myths Worth Retiring
"Rubber tires protect you in a car"
False. It's the metal roof and frame of the car forming a conductive shell around you (a Faraday cage effect), not the rubber tires, that redirects the current to the ground.
"Lightning never strikes the same place twice"
False. Tall, isolated structures get struck repeatedly — the Empire State Building is struck roughly 20–25 times a year.
"A person struck by lightning is dangerous to touch"
False. The human body doesn't store an electrical charge. It's safe, and urgent, to begin CPR immediately on a lightning strike victim if they're unresponsive and not breathing.
The Scale of the Risk
Lightning kills around 20 people in the US in a typical year and injures hundreds more, according to NWS data — far fewer than heat or flooding, but the majority of strikes happen to people who were outdoors and could have reached shelter in time. Fishing, camping, golfing, and yard work are consistently among the activities most associated with lightning deaths, largely because people wait too long to head inside once a storm starts building.
How Lightning Actually Forms
Inside a thunderstorm, updrafts and downdrafts carry ice crystals and graupel (soft hail) past each other at different speeds, and the friction between them strips electrons off, leaving the upper part of the cloud positively charged and the lower part negatively charged. That separation builds an enormous voltage difference — often hundreds of millions of volts — between the cloud and the ground, or between different parts of the cloud itself. When the surrounding air can no longer insulate against that voltage, it breaks down and a channel of ionized air forms, carrying a strike that can reach temperatures around 50,000°F, roughly five times hotter than the surface of the sun, for a fraction of a second. That's also what causes thunder: the shockwave from the sudden, explosive heating and expansion of air along the lightning channel.
Frequently Asked Questions
Can lightning strike from a clear blue sky?
Yes — this is the "bolt from the blue" phenomenon, where lightning travels sideways from a distant thunderstorm and strikes an area that appears to have clear skies overhead. It's part of why the 30-30 rule uses hearing thunder, not seeing rain, as the trigger to seek shelter.
Is it safe to shower or do dishes during a thunderstorm?
No — plumbing can conduct electricity from a lightning strike to the house. NWS recommends avoiding contact with running water, corded phones, and plugged-in electronics until 30 minutes after the last thunder.
What should I do if I'm caught outside with no shelter nearby?
Get away from open fields, isolated trees, and high ground. Crouch low with feet together and minimal ground contact, and get as low and small as possible — but this is a last resort, not genuine protection; getting to real shelter is always the priority.
How many times can a single storm produce lightning?
A single active thunderstorm can produce dozens of strikes per minute at its peak — globally, lightning strikes the Earth roughly 8 million times a day.
Get Severe Storm Alerts on Your Phone
StormCast pushes Severe Thunderstorm Warnings the moment they're issued for your location, so you know a storm capable of dangerous lightning is close before you hear the first rumble.
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A note on this guide: This is general safety information, not medical or professional emergency advice, and specific criteria can vary by region. In an actual emergency, call 911 and follow instructions from local officials and the National Weather Service.