How far away was that lightning?
Light reaches you almost instantly; the thunder crawls behind it at about 343 m/s. Count the seconds between the flash and the boom, multiply by the speed of sound, and you've measured the storm's distance without any instrument at all.
Delay Δt
Δt ∝ d
0 scount the seconds after the flash60 s
Watch the sound ring cross the gap — the animation is sped up, but the timing is honest: it always takes the same fraction of real time as Δt does.
Try
Sound travels at roughly 343 m/s in dry air at 20°C — a touch faster in warmer or more humid air, a touch slower in cold or dry air, but 343 m/s is the standard reference figure used here. That's close enough to the folk rule — "5 seconds per mile" (343 m/s × 5 s ≈ 1715 m, against a mile's 1609 m) or "3 seconds per kilometre" (343 m/s × 3 s ≈ 1029 m) — that either is a fine way to estimate a storm's distance while counting on your fingers. Treating the flash as instantaneous isn't a shortcut here, it's honest: light covers a kilometre in about 3.3 microseconds, genuinely negligible next to the seconds thunder takes. The one real approximation is the gauge below — the d = v·Δt formula keeps climbing past what you can actually hear, since atmospheric absorption and upward refraction usually mute thunder beyond roughly 15–20 km, well short of what a 60-second delay would imply. See Thunder and Speed of sound on Wikipedia.