Interesting article on estimating the distance to a thunderstorm:
http://www.livescience.com/37734-how-far-away-is-lightning-distance.html
She starts with the usual 5 seconds per mile, but she does include a metric
version. I'm not sure I want to multiply by 340 m/s in my head, however. It
might be easier to just divide by 3 for an approximate (and slightly
conservative) distance in kilometers.
She mentions, but perhaps doesn't adequately explain, the variation of the
speed of sound with temperature. That may be why she gives two values, only
one sentence apart. The lower figure is correct at 0 °C, the higher figure
near 20 °C (remember it is cooler aloft, so the sound is arriving via multiple
paths with differing, generally unknown, temperature profiles).
