Lightning Distance by Thunder Delay
Calculates distance in km and miles to a lightning strike from the seconds between flash and thunder using sound speed.
—
Lightning distance from thunder delay
Light reaches your eye almost instantly, while sound only manages about 343 m/s. So count the seconds between the flash and the thunder clap, divide by 3, and you have the distance in kilometers: d ≈ (t · v_sound) / 1000 ≈ t / 3 km. A 9 s gap puts the strike around 3 km away. A 3 s gap is roughly 1 km, which already means you're within striking range. Want miles instead? Divide by 5. In Brazil, the ELAT-INPE (Atmospheric Electricity Group) is the national reference for lightning monitoring.
Applications
It comes up in outdoor safety through the 30/30 rule: if thunder follows the flash within 30 s, head for shelter, and wait 30 min after the last clap before going back out. It also guides when to stop play in football, golf and beach activities as storms move in, helps with aviation ground operations, and works in weather education as a hands-on way to gauge how far a storm is without any instruments.
FAQ
Why divide by 3? At 343 m/s, sound takes about 3 seconds to cover 1 km. So every second of delay is roughly 340 m.
What if I don't hear thunder? Seeing the lightning but hearing nothing usually means the strike is more than 15–20 km off. Keep watching anyway, since storms can close in quickly.
Is 5 km a safe distance? Not really. Lightning can reach 10–15 km from the parent cloud, since positive flashes come off the anvil. During a storm, the only genuinely safe place is indoors.
Does temperature affect the calculation? A little. Sound runs about 3% faster at 30 °C than at 0 °C. For the distances you deal with in a storm, the t/3 rule of thumb holds up fine.
Related Tools
Lightning Distance by Thunder
Find out how far away lightning struck by counting the seconds between the flash and the thunder. Sound travels about 1 km every 3 seconds.
Pace ⇄ Time ⇄ Distance Calculator
Solve the triad pace (min/km), time (h:m:s) and distance (km). Provide any two and compute the third.
Speed, Distance & Time Calculator
Calculate speed, distance or time from the other two values. Supports km/h, m/s and mph. Ideal for sports, travel and physics.
Vehicle Braking Time
Computes braking time and distance from speed and deceleration.
Run Time from Distance
Computes total run time given distance in km and pace in min/km.
Shale Volume from Gamma Ray (Larionov)
Estimates the shale volume of a formation from the gamma ray log, the first step in any petrophysical well evaluation. The calculation normalizes the zone reading between the cleanest sand and the most radioactive shale in the interval, giving the gamma ray index IGR = (GR − GR min) ÷ (GR max − GR min), and then applies the non-linear Larionov curve, Vsh = 0.083 × (2^(3.7 × IGR) − 1). The result is the fraction of rock volume occupied by clay, as a percentage: intervals above 30 to 40% are usually discarded as reservoir, and the value later feeds the porosity and water saturation corrections for shaly sands. The Larionov curve for Tertiary, poorly consolidated rocks was adopted, which is the usual one in young sedimentary basins; for Mesozoic or older rocks the literature uses Vsh = 0.33 × (2^(2 × IGR) − 1), which returns far larger volumes for the same IGR. Enter the zone gamma ray reading, the minimum reading and the maximum reading of the interval.
The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.