Curious Kids: how does thunder work? And why is it so loud?

A thunderstorm builds over the Karoo in South Africa. Sean Nel/Shutterstock

Curious Kids is a series for children in which we ask experts to answer questions from kids.

How does thunder work, and why is it so loud? (Savaar, 6, Johannesburg)

Thunder is the sound that lightning makes. So before I can explain how thunder works, I have to explain how lightning works, and clouds too – they all go together.


Not every cloud can make thunder and lightning. Thunderclouds are very tall – of course, from the earth you can’t always tell how tall the cloud is because you just see the bottom. But above that, it stretches up tall into the sky.

Clouds are made of tiny water droplets. It’s so cold high up in the sky that ice crystals start to form inside the clouds. Then the ice crystals move to the top of the cloud and the water droplets stay near the bottom of the cloud. When they move past each other and rub against each other, they make static electricity.

You can make static electricity by rubbing a balloon against your hair and then the static electricity makes your hair stand up. Sometimes, if you have socks on and you rub your feet on a carpet, then it makes a tiny shock when you touch somebody else. That is also static electricity.

The static electricity in the cloud makes the ice crystals positively charged and the water droplets negatively charged. If you have ever played with magnets, you will know that the positive side of a magnet is attracted to the negative side of another magnet – but it pushes away the positive side of another magnet. Opposites attract each other: those with the same charge (that is, positive or negative) push each other away. The same thing happens with the negatively charged water droplets near the bottom of the thunder cloud.

All the negative bits that collect near the bottom of the cloud are called electrons. Positive bits known as particles start to collect under the thunder cloud because they are attracted by the electrons near the bottom of the cloud.

The attraction of positive and negative bits is strong, so the electrons in the cloud start to make jagged fingers reaching down to the earth. As soon as the negative bits from the cloud connect with the positive bits from the earth, a huge current made of all those electrons flows to the earth – and that is the lightning flash you see.

The lightning flash heats the air around it so quickly that the air expands very fast. When you heat something, it gets bigger – it expands. The air around the lightning flash expands so fast that it makes a shock wave in the air. That shock wave is the thunder that you hear.

A big noise

Why is thunder so loud? It’s because the amount of electrical energy that flows from the cloud to the ground is so enormous: it’s like a very big waterfall of electricity.

The louder the sound that you hear, the closer you are to the lightning. Light travels through air much faster than sound. That’s why sometimes you see the lightning flash first and then you hear the thunder a few seconds later. If you see the lightning and hear the thunder immediately, then the lightning is very close to you.

Lightning is very dangerous.

Read more: Some truths about lightning: when thunder roars, go indoors

So, remember this important lesson: when thunder roars, go indoors. You can listen to the thunder getting louder as it gets closer, and softer as it moves away – and you’ll be safe from the lightning inside your house.

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