Zap

Counting the gap

Three seconds per kilometre, near enough. Sound does about 343 m/s at sensible temperatures, light arrives while you are still deciding to count, so the gap between flash and thunder is entirely sound travel.

That gets you a distance, which is the number everyone quotes and the less useful one. What matters standing in a field with a tripod is the difference between consecutive gaps.

Between the second and third flash that cell covered four kilometres. It is not walking towards you, and the gear takes four minutes to pack.

I write the gaps down now, one line per flash, because counting in your head while also watching a histogram produces numbers you later cannot trust. Twice it has sent me back to the car earlier than I wanted to go. Once it did not send me early enough, and I spent eleven minutes sitting in a ditch with a carbon tripod lying flat some distance away, which is the correct place for a carbon tripod to be.