Lightning hands you a free measuring tape. The flash reaches your eyes essentially instantly; the thunder plods along at the speed of sound, so the count between them IS the distance, in disguise. Sound's speed in air follows the temperature: $v = 331.3 + 0.606\,T$ — read aloud: v equals three-thirty-one point three, plus zero point six-oh-six T), where $v$ is sound's speed in m/s and $T$ the air temperature in °C. At 0 °C sound runs 331.3 m/s, and every warmer degree buys about 0.6 m/s more — call it 340-and-a-bit on a pleasant evening.

Then the distance is last chapter's oldest friend: $d = v \cdot t$ — $d$ the metres out to the strike, $v$ that speed of sound, and $t$ the seconds you counted between flash and bang. At roughly 343 m/s, three seconds is very nearly a kilometre — the famous 3-seconds-per-kilometre counting rule, which this lesson lets you build honestly instead of just memorizing. And one rule that outranks the arithmetic: if the count ever reaches zero — flash and bang together — the storm is on top of you, and the lesson continues indoors.