What if you counted the seconds after lightning?

In a storm, the sky flashes. Then, a little later, you hear a rumble.

The flash and the rumble come from the same lightning. So why do we hear it later?

During a thunderstorm you see a flash of lightning, and then you wait. One, two, three... and then comes the rumble of thunder. Sometimes the wait is long; sometimes the crack comes almost at once.

The flash and the thunder are made at the same moment by the same lightning. So why do they reach you at different times, and what can the wait tell you? In this lab you can move a listener away from a lightning flash and time the thunder.

The National Weather Service explains that thunder is created when lightning heats the air in its channel very rapidly, to as much as 50,000 °F, making it expand and then contract: the sound wave we hear as thunder.

Light and sound leave the flash together but travel at very different speeds. This lab measures the delay between them for a listener at different distances and air temperatures, and tests the NWS rule for estimating how far away lightning is.

Sound needs time to travel. In one second, it goes about 343 meters.

Light is much, much faster. It gets to you right away.

Sound travels as a push passed from air particle to air particle. NASA explains that this passing-along depends on the gas and its temperature. In 20 °C air, our model's sound travels about 343 meters every second.

Start with a listener 340 meters from the flash. The thunder arrives after about 1 second. The light arrives so fast that, as the National Weather Service puts it, you see lightning immediately.

NASA's Glenn Research Center describes the speed of sound as the speed at which a small disturbance is transmitted through a gas by molecular collisions, depending on the gas and its temperature. The model uses c = 331.3√(1 + T/273.15) m/s for dry air: 343.2 m/s at 20 °C.

At d = 340 m the delay is 0.991 s. Light covers the same distance so quickly that, for a person counting, it arrives at once.

Now Olly moves farther from the lightning. The flash still comes right away.

But the thunder takes longer. One, two, three, four, five... boom!

Now move the listener to 1,700 meters, a little more than a mile. The thunder now takes about 5 seconds to arrive. That matches the National Weather Service rule: about 5 seconds for each mile.

Try 1,000 meters: the thunder takes about 3 seconds. So in kilometers, the model gives about 3 seconds for every kilometer. Try changing the air temperature too: in cold air at 0 °C the 1,700-meter thunder takes about 5.1 seconds, and in hot air at 30 °C about 4.9.

At d = 1,700 m and 20 °C, t = 1700/343.2 = 4.95 s. One mile (1,609 m) gives 4.69 s, so the NWS's 5 s per mile is a rounded rule. In metric units the model gives 2.91 s per kilometer.

Temperature matters only a little: at 1,700 m the delay is 5.13 s at 0 °C (331.3 m/s) and 4.87 s at 30 °C (349.0 m/s), a difference small next to the uncertainty of counting by hand.

Light zooms super fast. Sound is much slower.

So the flash wins the race. The thunder comes along later.

Lightning makes light and sound at the same moment. Light is so fast that it crosses a few kilometers in a tiny fraction of a second. Sound is much slower, so it falls behind, and the farther away the lightning is, the more it falls behind.

That's why counting works: each extra second of waiting means the thunder came from farther away. The National Weather Service rule is to count the seconds between the flash and the thunder, then divide by 5 to get the distance in miles.

For a source at distance d, the light delay is d/c_light and the sound delay d/c_sound; with light vastly faster than sound, the measured gap is effectively d/c_sound. Distance is then c_sound × t: 343 m per second of counting.

The model's limits: a straight path, no wind or temperature layers bending the sound, and a single arrival time. Real thunder rolls because, as the NWS explains, you first hear the nearest part of the long lightning channel and then parts farther and farther away.

If you hear thunder, go inside. A house or a car with a hard roof is a safe place.

The National Weather Service says: When thunder roars, go indoors!

The National Weather Service says thunder can usually be heard only about 10 miles from the lightning, and that if you can hear thunder, you are likely close enough for the storm to reach you. Its advice is simple: When Thunder Roars, Go Indoors.

The NWS reminds people to be in a safe place while counting. Sturdy buildings and cars with hard roofs are safe; rain shelters, small sheds and open vehicles are not. And once you are inside, wait 30 minutes after the last lightning or thunder before going back out.

The NWS notes that thunder is typically audible only within about 10 miles of the strike, and that hearing thunder means you are likely within striking distance. Its slogan is 'When Thunder Roars, Go Indoors'; substantial buildings and hard-topped vehicles are safe, while rain shelters, small sheds and open vehicles are not, and people should wait 30 minutes after the last lightning or thunder before going out.

The flash-to-bang count is therefore a way of understanding a storm, not a reason to stay outside: the NWS says to count from a safe place.