Tap when you see the flash. Tap again when you hear the thunder. Light arrives almost instantly, but sound takes about 3 seconds to cover a kilometre, so the gap tells you how far away the strike was.
| # | Flash to thunder | Distance | Compared with last |
|---|
The 30-30 rule. If the thunder comes less than 30 seconds after the flash, the storm is within about 10 km and close enough to strike you. Go inside a building or a hard-topped vehicle. Wait 30 minutes after the last thunder before going back out.
There's no safe distance outdoors. Lightning can strike more than 15 km from the storm, often from what looks like clear sky. On the open prairie you're often the tallest thing around. If you can hear thunder, you're close enough to be hit.
Your taps are timed to a tenth of a second. Human reaction time adds a little error, so treat each distance as roughly right.