Aviation Calcs

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Thunderstorm Distance Calculator

Count the seconds between a lightning flash and the thunder that follows, and this page turns that flash-to-bang time into a range. The conversion is just distance = speed of sound × time, with the speed of sound taken from the temperature you enter (standard-atmosphere sea-level 15 °C if you leave it blank). A five-seconds-per-statute-mile rule of thumb is shown beside the physical result. It is a line-of-sight estimate on the ground, not a cell-top range or a reason to fly closer to a storm.

Count from the flash you see to the first rumble you hear. Thunder is rarely useful beyond about 10–15 miles, and a storm you cannot yet hear can still be a factor for flight.

Result

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Distance equals the speed of sound times time

If the flash and the thunder come from the same stroke, the range is d = a t, where t is the delay you measured and a is the local speed of sound. In air, a = a0 √(T/T0) with the same sea-level constants used on this site’s atmosphere pages: a0 = 340.294 m/s at T0 = 288.15 K (15 °C). At 15 °C that is about 4.73 seconds per statute mile, or 5.44 seconds per nautical mile. The familiar “five seconds per mile” rule is therefore a slightly conservative (farther-storm) approximation in typical summer air.

Limits of the method

Thunder is a ground-observer tool. Sound is refracted, absorbed, and blocked by terrain; many strokes are never heard at 10–15 SM, and the first rumble may come from a different part of the cell than the flash you watched. The 30-mile “avoid” guidance in the Aeronautical Information Manual and the Pilot’s Handbook is about the storm, not about how loud the thunder is. In flight, use weather radar, lightning detection, PIREPs, and a wide berth — not a stopwatch. Related tools on this site: the weather go/no-go checker and the standard atmosphere (for the same speed-of-sound identity).

Sources

Frequently asked questions

Why is five seconds per mile a little long?

At 15 °C the speed of sound is about 1,126 ft/s, so a statute mile takes about 4.73 s. Rounding up to 5 s/SM puts the storm slightly farther away than it really is — conservative for a person standing outside, not a reason to fly closer.

Can I use this from the cockpit?

Not as a clearance to continue. In the air you cannot assume the flash and the first thunder share a path, and AIM guidance is to avoid thunderstorms by a wide margin with radar and other weather products.

What if I never hear the thunder?

A silent storm can still be close enough to matter. Treat a visual flash without thunder as “range unknown,” not as “far away.”