#2027: Lightning Distance

XKCD comic, described below.
Transcript

[Cueball and Megan stand on either side of a window, observing a bolt of lightning in a dark sky.]

Cueball: What's that trick for telling how many miles away lightning is?

Megan: Just count the seconds between the visible flash and the radio wave burst, then multiply by 5 billion.


(Sourced from explainxkcd.com)

The index of radio refraction does have a lot of variation, which might throw off your calculations, so you can also look at the difference in brightness between the visible flash and more-attenuated UV and x-rays.

ExplainOriginal