Why do we see lightning before hearing thunder?

One lightning bolt, two messengers

Light: the fastest runner in the Universe

When lightning flashes, it sends light in all directions. This light travels at about 300,000 kilometers per second. That is so fast that in a single second, it could circle the Earth almost 7.5 times!

For a storm a few kilometers away, light takes about one hundred-thousandth of a second to arrive. Your brain does not even have time to notice: for you, the lightning is seen at the exact moment it occurs.

Sound: a much slower traveler

Thunder is a sound. And sound does not travel like light. It needs air to move: the tiny particles of air push each other, a bit like people passing a message along a chain.

In air, sound travels at about 340 meters per second. That is already fast (over 1,200 km/h, much faster than a TGV!), but it is nearly a million times slower than light.

N.B.:
The speed of sound depends slightly on the temperature of the air: it is a little higher when it is warm and a little lower when it is cold. At 20 °C, it is about 343 meters per second.

How is thunder born?

A path of fire in the sky

Lightning is a huge electrical spark. In a storm cloud, small pieces of ice and droplets rub against each other and collide, and the cloud becomes electrically charged. When the charge becomes too strong, the electricity tries to escape: it carves a path through the air, toward another cloud or toward the ground.

Along its path, the air is heated to about 30,000 °C, nearly 5 times the temperature of the Sun's surface!

An explosion of air

When air is heated that fast and that hard, it expands all at once, like a balloon bursting. This sudden expansion pushes the air around it and creates a shock wave, which then becomes a sound wave: that is the thunder.

Why does thunder "rumble" instead of making a single "bang"?

A lightning bolt can be several kilometers long. Each part of the path makes its own noise, but those parts are not all at the same distance from you. The sound from the nearest part arrives first, then that from the more distant parts. The noise also echoes off hills, buildings, and clouds (that is the echo). Result: instead of a single "bang", you hear a long rumble.

The race: light versus sound

Let us imagine lightning striking the ground 3 kilometers away from you. Here is what happens.

Between the lightning and the thunder, you have time to count to nine. And if the storm is farther away, you count longer!

Your turn: calculate the distance of the storm

The seconds method

Sound travels about 1 kilometer in 3 seconds. So you can guess the distance of a storm with this recipe:

1. When you see the lightning, start counting the seconds: "one, two, three…" (or "one crocodile, two crocodiles…" to go at the right speed).
2. Stop counting when you hear the thunder.
3. Divide the number of seconds by 3: you get the distance in kilometers.

What if the storm is approaching?

If the time between lightning and thunder gets shorter and shorter, the storm is getting closer. If it gets longer and longer, it is moving away.

N.B.:
When thunder arrives less than 30 seconds after the lightning, the storm is less than 10 kilometers away: it is time to take shelter in a house or a car, and never under an isolated tree. Then wait about 30 minutes after the last clap of thunder before going back outside. This is the "30-30 rule".

Why do we sometimes see lightning but hear nothing?

Sound weakens as it travels, and air, terrain, and layers of air at different temperatures deflect it. In general, thunder is no longer heard when the storm is more than 15 or 20 kilometers away. But the light of the lightning can be seen from much farther, especially at night. This is sometimes called "heat lightning": a distant storm whose light you see, without hearing the noise.

The same race in everyday life

This difference between light and sound is seen everywhere:

• A fireworks display: you first see the colorful explosion, then you hear the "boom".
• A lumberjack in the distance: you see the axe hit the tree, then the "tock" arrives afterward.
• A football stadium: from the distant stands, you see the player kick the ball before hearing the kick.
• The Sun: its light takes about 8 minutes and 20 seconds to reach us. You see the Sun as it was more than 8 minutes ago!