What is Thunder?

Fersiwn Cymraeg.

Thunder is the sound that accompanies lightning during a thunderstorm. Sounds simple enough, but why does lightning even make a sound?


Any sound you hear is made up of vibrations. The vibrations travel as a sound wave through the air, until they reach your ear. Lightning is a huge discharge of electricity, and this electricity shoots through the air, causing vibrations to be formed in two ways:

1. The electricity passes through the air and causes air particles to vibrate. The vibrations are heard as sound.

2. The lightning is also very hot and heats up the air around it. Hot air expands, and in this case the air expands very quickly, pushing apart the air particles with force and creating more vibrations.

This is what we hear and call thunder – the rumbling of thunder is simply caused by the vibration or sound of the air affected by lightning. If you’re nearby to a lightning strike, you may have heard thunder as a really loud crack, almost like the sound of a whip being cracked. But, most of the time we hear thunder as a loud, long rumble.

In fact, the crack sound is the direct sound of the lightning near us, reaching our ears. The more common rumbling effect happens when thunder echoes off objects all around us. This happens a lot in towns and cities, where there are lots of buildings for the noise to bounce off. However even in flat areas of land, with no trees or other objects, there is quite often a rumble as the thunder simply bounces off the ground on its way to our ears. All this echoing transforms the original ‘crack’ sound into a longer ‘rumble’!

Here's an impressive photo of lightning that Abigail took in Bloemfontein, South Africa.

Here’s an impressive photo of lightning that Abigail took in Bloemfontein, South Africa.

Why is thunder not at the same time as the lightning?

We see the lightning before we hear the thunder because light travels faster than sound. The light from the lightning travels to our eyes much quicker than the sound from the lightning. so we hear it later than we see it. There is an old myth that counting seconds between a lightning flash and the accompanying thunder gives you the distance of how far away the storm is, in miles. However, from a mathematical point of view we know this isn’t true, as the speed of sound is roughly 330 metres per second. So it takes roughly 3 seconds for the thunder to travel one kilometre, and therefore about 5 seconds for thunder to travel a mile. So, a more scientific rule would be, count the number of seconds between the lightning flash and the thunder noise, and then divide that number by five, and that is how many miles away the thunderstorm is.

Extreme sounds like this are incredibly fascinating, especially to scientists like us who investigate how such things happen. Extreme sounds don’t just have to be loud, they can also be powerful enough to shatter a wine glass, or can even be used as a weapon in the animal kingdom. We’ve spent so much time finding out new ways that sound shapes and transforms our lives, that here at science made simple we even wrote a whole science show about extreme sounds, so we can tell as many people about them as we can.


Why not try it yourself?

Try out a light and sound experiment yourself. Get another person to stand along way away from you but so you can still see them. Clap your hands: get the other person to raise their left hand when they see you clap and their right when they hear you clap.

You might notice a very small gap, similar to that of lightning and thunder; except that lightning is even further away so the sound and light gap is usually bigger!



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Posted in Activity, Curriculum, Exploring Science, Physics, Primary