How do lightning and thunder happen?

1. On a warm, humid day, warm air rises fast. It carries water high into the sky, where a huge, tall storm cloud grows.
2. Inside the cloud, small ice crystals are carried up while heavier, soft hail falls. When they bump, tiny electric charges rub off.
3. The top of the cloud fills with positive charge and the bottom with negative charge. The ground underneath becomes positive. Opposite charges pull toward each other.
4. When the charge gets too big, the air can't hold it back. A path zigzags down from the cloud, meets a spark rising from the ground, and a giant flash of lightning races along it.
5. Lightning heats the air to about 30,000 degrees Celsius, five times hotter than the surface of the sun. The air expands so fast that it makes a huge boom: thunder.
6. Light reaches you almost instantly, but sound is slower. Count the seconds from flash to boom and divide by 3 for kilometers. If you can hear thunder, go indoors.

Tap each part of the storm. Watch how the charge builds up and lets go!

• Updraft. Warm, wet air rushes up through the middle of the cloud. It carries water and ice high into the sky.
• Anvil top. When the rising air can't go any higher, it spreads out into a flat top called an anvil. The top of the cloud holds positive charge.
• Ice crystals. Small, light ice crystals get carried up by the rising air. When they bump into soft hail, they usually take positive charge.
• Soft hail (graupel). Soft hail, called graupel, is heavier, so it falls through the rising air. It bumps into ice crystals and picks up negative charge.
• Negative cloud base. Negative charge collects in the lower part of the cloud. It pulls on the positive charge in the ground below.
• Ground charge. The negative cloud base pushes negative charges in the ground away, leaving the ground below positive. Tall things like trees get the strongest pull.
• Lightning bolt. When enough charge builds up, it jumps as a giant spark. The bolt is only a few centimeters wide, but it can be kilometers long.
• Thunder. The bolt heats the air so much that it bursts outward. That push travels to your ears as the rumble of thunder.

Push air up to build charge until lightning flashes. Then compare when the flash is seen and when each listener hears the thunder.

Experiment: If you stand farther away from the lightning, what happens to the time between the flash and the thunder?

1. Stand close to the storm and make lightning. Count the seconds to the boom.
2. Move far away and make lightning again.
3. Compare the seconds. Watch the cow and the house too!

What we found: The farther away you are, the longer the thunder takes, because sound travels much more slowly than light. Sound goes about 1 kilometer every 3 seconds, so you can count the seconds and divide by 3 to find how far away the lightning was.

Turn on JavaScript to try it yourself.

Use the seconds between flash and boom to find how far away each strike is. Is the storm coming closer?

Have you ever touched a doorknob and felt a tiny zap? That is static electricity, the same kind of thing as lightning, just much, much smaller. Rubbing your feet on a carpet moves tiny electric charges, and when they jump to the doorknob you feel a spark.

A storm cloud does the same thing on a giant scale. Inside it, bits of ice and soft hail crash into each other millions of times. Each crash moves a little charge, so the top of the cloud becomes positive and the bottom becomes negative. When the difference gets big enough, the charge jumps as lightning.

A lightning bolt heats the air around it to about 30,000 degrees Celsius. The air expands so fast that it makes a sound wave: thunder. Light travels so fast that you see the flash almost at once, but sound needs about 3 seconds to travel 1 kilometer. Count the seconds and divide by 3 to find how far away the lightning was.

Lightning can strike about 15 kilometers away from a storm, which is about as far as you can hear thunder. So remember: when thunder roars, go indoors. A house or a car with a metal roof is a safe place. Wait until 30 minutes after the last thunder before you go back outside.

Key words

Electric charge

A property of tiny particles that can be positive or negative. Opposite charges pull toward each other.

Graupel

Soft hail: small balls of ice formed when supercooled droplets freeze onto ice crystals.

Static electricity

Electric charge that builds up in one place, like on a balloon rubbed on your hair.

Thunder

The sound made when lightning heats the air so fast that it bursts outward.

Speed of sound

How fast sound travels: about 1 kilometer in 3 seconds.

Check your thinking

1. What is the safest thing to do when you hear thunder?

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   Go inside a building or a car. Buildings and cars with metal roofs keep you safe. Trees attract lightning.

2. Why do you see lightning before you hear thunder?

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   Light travels much faster than sound. They happen at the same moment, but light zooms to you almost instantly while sound is much slower.

3. You count 9 seconds between a flash and its thunder. About how far away was the lightning?

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   About 3 km. Sound travels about 1 km every 3 seconds, so 9 ÷ 3 = 3 km.

4. What makes thunder?

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   Lightning heats the air so fast that it bursts outward. The air heats to about 30,000 °C and expands so fast that it makes a huge sound.

5. Inside a thundercloud, which particles usually end up carrying positive charge to the top?

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   Small ice crystals lifted by the updraft. In the main charging zone, small crystals gain positive charge in collisions with graupel and are carried upward.

6. The seconds between flash and boom go from 12, to 9, to 6. What is the storm doing?

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   Coming closer. Shorter delays mean shorter distances: the storm is getting closer. Time to go inside!