🌩️ How long until thunder reaches you? (…and canyon echoes!)

Learn more: how sound travel time works

When you see lightning and then hear thunder later, you’re watching physics in action. Light races to your eyes almost instantly, but sound has to push through the air molecule by molecule. That “push” travels at the speed of sound, which depends mostly on air temperature. Warmer air lets molecules jostle faster, so sound travels faster; colder air slows things down.

Classroom formula (nice and close)

A widely used classroom estimate for the speed of sound in air is:

c ≈ 331.3 + 0.606 × T(°C)   (meters per second)

At 20 °C this gives about 343 m/s. That turns into two handy rules of thumb: roughly 3 seconds per kilometer and roughly 5 seconds per mile. If you count 9 seconds between the flash and the boom on a mild day, the storm is around 3 km away (or ~1.8 mi).

Distance, time, and echoes

The relationships are simple:

time = distance ÷ c
distance = time × c
echo_time = 2 × distance ÷ c   (there and back again!)

For echoes, your voice goes out to a wall or cliff and returns to you, so the timing is a round trip. If a canyon wall is 300 m away and the air is mild, the echo comes back in well under two seconds. Big outdoor spaces (stadiums, valleys, canyons) make great “echo labs” because the geometry is large enough to hear a clear delay.

What else changes the number?

• Temperature: the biggest factor. Warmer → faster sound → shorter delay.
• Humidity: moist air is slightly faster than dry air, but the effect is small.
• Altitude & pressure: thinner air nudges the speed a bit; our estimate keeps it simple.
• Wind: can shave or add a fraction of a second depending on direction.
• Terrain & surfaces (echoes): soft, uneven surfaces absorb and scatter sound; hard, flat rock reflects crisply.

Why thunder sounds “rumbly”

Lightning is a long, branching path. Different parts of that path are different distances from you, so sound from some parts arrives earlier and some later. The first crack is usually from the nearest part of the bolt; the low rumble is all the slightly farther segments arriving afterward and bouncing around the landscape.

Safe, sensible use

This calculator is built for friendly estimates and classroom demonstrations. Treat the results as approximations, not professional safety advice. If the delay is small, the storm is close — head indoors and follow local guidance. For echoes, remember that not every location will produce a strong reflection even if the math says when it would return.

Try these fun experiments

• Count & compare: On a cool day vs. a hot day, count seconds after a distant slam (or a safe, known sound) and compare your calculated distances.
• Echo hunt: Clap near a large building, a gym, and a rocky outcrop. Which one gives the clearest echo? Measure the delay with your phone’s voice memo timeline.
• Thumb rules check: Does “3 s/km” or “5 s/mi” match your results at today’s temperature?

Behind the scenes, this page runs 100% in your browser and doesn’t store your inputs. It uses the temperature-based formula above to keep things simple, kid-friendly, and close to what you’ll observe outdoors.

Notes & safety

• Thunder & lightning safety: if the delay is small, the storm is close — go indoors.
• Wind, humidity, and altitude nudge the numbers; temperature is the big factor.
• Echo strength depends on the surface; some spots won’t echo even if the math says when it would arrive.
• No data leaves your device; everything is computed locally.