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How Far Away Is the Lightning?
The oldest trick in storm safety, why it works, and the mistake people make with the answer.
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Count the seconds, divide by five
Light reaches you effectively instantly; sound takes about 5 seconds per mile, 3 seconds per kilometre. Count from the flash to the first crack of thunder and divide.
15 seconds means the strike was about 3 miles away. 5 seconds means one mile — close enough that the next strike could be yours, because storms do not fire in a neat line.
What the number does not tell you
It tells you where the last strike landed, not where the next one will. Lightning routinely jumps 10 miles from the cloud that produced it, and strikes ahead of the rain — the so-called bolt from the blue — are the ones that catch people who thought the storm was still far off.
So treat the count as a trend, not a safety margin. Three flashes going 20 seconds, 15, then 9 is a storm coming at you; that is your signal to move, not the moment it hits 5.
The 30/30 rule
If the gap is 30 seconds or less, go inside. Wait 30 minutes after the last thunder before going back out. The second half is the part everyone skips, and a meaningful share of casualties happen after the rain stops, when the sky looks like it has moved on.
Safe means a building with wiring and plumbing, or a hard-topped car — the metal shell carries the current around you. Not a shelter, not a porch, not a tree.
Storm risk by city
Live conditions and storm-day history for the cities that get the most storms.
Common questions
- How many seconds per mile for thunder?
- About 5 seconds per mile, or 3 seconds per kilometre. Count from the flash to the thunder and divide.
- If I hear thunder, am I in danger?
- Yes, in the sense that matters: thunder is rarely audible beyond about 10 miles, and lightning reaches that far from its parent storm. Hearing thunder at all means going indoors is the right call.
- How long should I wait after the storm passes?
- 30 minutes after the last thunder. Strikes after the rain has stopped are a well-documented cause of casualties.