Lightning safety and the 30-30 rule
Updated · 5 min read
If you hear thunder within 30 seconds of seeing lightning, the storm is within about 10 km and close enough to strike you, so go inside immediately. Wait a full 30 minutes after the last thunder before going back out, because strikes can reach well beyond the visible storm.
What is the 30-30 rule?[1]
The 30-30 rule has two halves. If thunder arrives within 30 seconds of the flash, the storm is roughly 10 km away and you should shelter immediately. After the storm passes, wait 30 minutes from the last thunder you hear before resuming outdoor activity.
The first 30 exists because 30 seconds corresponds to about 10 km, and lightning routinely strikes several kilometers from the storm core. If you can hear thunder at all, you are generally within reach of the storm.
The second 30 exists because the trailing edge of a storm remains electrically active after the rain stops. A significant share of lightning casualties occur after the visible storm appears to have ended.
How close does lightning have to be to be dangerous?[1]
Any storm close enough for you to hear thunder is close enough to be dangerous. Thunder is normally audible up to about 16 km, and lightning can strike more than 10 km from the storm core, so audible thunder and strike range overlap substantially.
Casualty studies bear this out. A large share of the people struck are hit before the rain arrives or after it has passed, when the sky overhead no longer looks threatening. A storm's electrical reach is wider than its rain shaft, and wider than most people's instinct for danger.
- If you can hear thunder
- you are within potential strike range, so go inside
- Substantial building
- the safest option. Avoid plumbing and corded electronics
- Hard-topped vehicle
- safe because the metal shell conducts current around you, not because of the tyres
- Open ground, water, high points
- the most dangerous places to be caught
- Isolated trees and shelters
- these attract strikes rather than protect from them
Can lightning strike when the sky is clear?
Yes. Positive lightning from a storm's upper anvil can travel more than 10 km horizontally and strike under apparently clear sky. These bolts from the blue give no local warning and carry peak currents up to ten times higher than typical negative strikes.
A live lightning map helps here in a way that watching the sky cannot. It shows strikes happening beyond your horizon and behind buildings, including the outlying positive strikes that give no visual warning.
Why did "When Thunder Roars, Go Indoors" replace counting?[1]
The US National Weather Service retired the counting half of the 30-30 rule in favour of a simpler instruction: if you can hear thunder at all, go indoors. People tended to keep counting instead of moving, and the first strike from a newly built storm arrives with no earlier thunder to count. The 30-minute all-clear survived unchanged.
The physics behind the count is still sound, and watching the flash-to-thunder gap shrink remains a genuinely good way to understand what a storm is doing. As a go-or-stay decision tool, though, it sets the threshold too late: a storm 10 km away can put its next strike on top of you.
The rule also fails silently at long range. Thunder becomes inaudible beyond about 16 km, while outlying positive strikes reach further than that from the storm core, so silence is not proof of safety. A live map showing strikes beyond earshot covers exactly that gap.
References
- Lightning SafetyUS National Weather Service, 2026
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