Lightning Distance Timer
Implements the 30/30 rule with thunder-speed correction, plus a visual risk dial.
Inputs
Result
A natural placement between the calculator and the explanation.
What this solves — and what it doesn't
Lightning kills about 20 people per year in the US and injures hundreds. The 30/30 rule says: take shelter if you count fewer than 30 seconds between flash and thunder; resume only 30 minutes after the last thunder. This tool implements that with a temperature-corrected speed of sound (warmer air is slightly faster).
How to read the result
Enter the seconds you count between flash and bang. The result is kilometers to the strike and an action line:
- <10 sec: dangerously close — shelter immediately.
- 10 to 30 sec: storm is within 10 km; take shelter.
- >30 sec: storm is far but monitor; the cell could move toward you.
The exposure modifier tells you what counts as ‘shelter’ in your location — a car, a building, a deep cave, or a stand of small trees well away from tall isolated trees.
Limitations
- Distance is to the most recent strike, not the next.
- Doesn’t model lightning type (cloud-to-ground vs cloud).
- Speed of sound correction is small; ignore unless very precise.
Frequently asked questions
What is the safe position if caught?
Crouch on the balls of your feet, heels together, head down, on a pad. Don’t lie flat. Avoid lone tall objects.
Is a cave safe?
Deep caves are safe. Shallow overhangs are NOT — current can jump the gap.
Should I leave a summit if a storm is coming?
Yes, immediately. Lightning tends to strike the highest point. Get below treeline before the cell arrives.