Skip to content

lightning-distance

How far away was that lightning? Count the seconds between the flash and the thunder, and divide — sound takes roughly 5 seconds per mile (3 seconds per kilometer). If the gap is 30 seconds or less, the storm is close enough to be dangerous: go inside and wait 30 minutes after the last thunder.

lightning-distance is a pure-Rust, zero-dependency crate that does both the distance estimate and the 30-second safety check. It is the math behind the EarlyThunder storm tracker, a free tool for judging lightning safety outdoors.

Why the rule works

Light travels near-instantly; sound is slow (~343 m/s). So the delay between seeing the flash and hearing the thunder is essentially the sound's travel time:

Delay (seconds) Miles Kilometers Verdict
5 1.0 1.7 dangerous
10 2.0 3.3 dangerous
15 3.0 5.0 dangerous
30 6.0 10.0 dangerous (cutoff)
45 9.0 15.0 safer, still watch
60 12.0 20.0 likely safe

The 30-second rule: if thunder follows the flash within 30 seconds, the strike was within ~6 miles — close enough that the next strike could reach you. The standard advice is to shelter for 30 minutes after the last thunder you hear.

Install

[dependencies]
lightning-distance = "0.1"

Quick start

use lightning_distance::{miles, kilometers, is_dangerous};

let delay = 10.0; // seconds between flash and thunder

let mi = miles(delay);        // ≈ 2.0
let km = kilometers(delay);   // ≈ 3.33
let danger = is_dangerous(delay); // true (≤ 30 s)

println!("{mi:.1} mi / {km:.1} km — danger: {danger}");

Track a whole storm

The crate gives you the per-flash math. For a live storm timeline with approach/retreat detection, use the EarlyThunder storm tracker.

Features

  • Pure Rust, no deps — f64 arithmetic, single file, no features.
  • Miles & kilometers — from the flash-to-thunder delay.
  • 30-second rule — is_dangerous boolean safety check.
  • Tunable constants — SECS_PER_MILE, SECS_PER_KM, DANGER_THRESHOLD_SECS.
  • MIT licensed — embed anywhere (wearables, weather apps, IoT).