What makes lightning, and why does thunder boom?

Inside a thunderstorm

A thunderstorm is any cloud that makes thunder and lightning. Thunderstorms grow from tall, puffy, dark clouds called cumulonimbus clouds. Their tops sometimes spread out flat, like the anvil a blacksmith uses.

To form, a thunderstorm needs three ingredients: moist air, an unstable atmosphere, and something to push the air upwards, such as a cold front or a sea breeze. Once warm, damp air starts rising, it keeps going until it reaches air far colder than freezing. Up there, the cloud fills with ice crystals and soft, tiny hailstones called graupel.

Inside the cloud, strong currents of rising air called updrafts toss all these icy bits around. That is where the electricity begins.

Bumping ice builds up charge

As graupel falls through the cloud, it keeps crashing into smaller ice crystals that are being carried upwards. Each time they bump and bounce apart, a little electric charge passes between them. One ends up with a positive charge and the other with a negative charge.

The light ice crystals are swept high into the cloud, while the heavier graupel stays lower down. In most storms this leaves the top of the cloud with a positive charge and the middle with a negative charge. It is a bit like the static electricity you get when you pull off a woolly jumper, but on a gigantic scale. Scientists are still working out some of the details, because it is very hard to measure what happens inside a storm.

Air is an insulator, which means electricity does not flow through it easily. So the charges keep building until the difference between them reaches several million volts. Then the air suddenly gives way and a giant spark leaps across. That spark is lightning. For a moment it evens out the charges, until they build up again.

Where lightning goes

Lightning can flash inside a cloud, jump between clouds, or strike from a cloud to the ground. Most, if not all, lightning flashes start inside the cloud.

When a flash heads for the ground, an invisible path of electric charge works its way down in quick steps. As it gets close, tall objects such as trees and buildings send up small sparks to meet it. When one connects, a huge current rushes along the path. Surprisingly, the brilliant flash you see actually travels from the ground back up to the cloud, all in a few thousandths of a second.

This is why tall, pointy objects get struck more often. The old saying that lightning never strikes the same place twice is wrong: the Empire State Building in New York is hit about 23 times a year on average.

Why thunder booms

A lightning bolt heats the air around it to as much as 30,000 °C, much hotter than the surface of the Sun, in a tiny fraction of a second. The superheated air expands so suddenly that it creates a shock wave, which spreads out as the sound we call thunder.

Close by, thunder sounds like a sharp crack or a tearing noise. Far away, it becomes a long, low rumble, because the air soaks up the high-pitched sounds as they travel. Thunder can be heard up to about 40 kilometres from the flash.

Light travels roughly a million times faster than sound, so you see the flash almost straight away but hear the thunder later. Sound takes about 3 seconds to travel 1 kilometre. Count the seconds between the flash and the thunder, divide by 3, and you have roughly how many kilometres away the lightning was.

In Australia, thunderstorms are most common across the northern half of the country and least common in south-east Tasmania. Most happen in the warmer months, from September to March.

Staying safe when storms roll in

Lightning is powerful, but staying safe is simple if you follow a few rules. The most important one: if you can hear thunder, you are close enough for lightning to reach you. That is the moment to head inside.

The safest places are a solid building or a hard-topped car with the windows up. Don't shelter under a tree, and get out of pools, lakes and the sea. Wait at least 30 minutes after the last rumble of thunder before going back outside.

Indoors, stay off corded phones, keep away from taps, baths and showers, and give windows some space. It is a great time to read a book! The Bureau of Meteorology issues warnings for severe thunderstorms, so grown-ups can check the latest before heading out.

📚 Words to know

Cumulonimbus

A tall, dark, towering cloud that can bring thunder, lightning and heavy rain.

Graupel

Soft, tiny hailstones made of ice, found inside storm clouds.

Electric charge

Something tiny particles can carry, either positive or negative. Opposite charges pull towards each other.

Insulator

A material that does not let electricity flow through it easily, such as air, rubber or plastic.

Updraft

A strong current of air rising up inside a cloud.

🧪 Try this: work out how far away a storm is

1. During a thunderstorm, stay safely indoors and away from windows. You will still notice the flashes light up the room.
2. When you see a flash, start counting seconds: one-thousand-and-one, one-thousand-and-two, and so on.
3. Stop counting when you hear the thunder. Divide your number by 3. That is roughly how many kilometres away the lightning was.
4. Do this for several flashes and write down your answers. Are the numbers getting smaller (the storm is coming closer) or bigger (it is moving away)?
5. No storm? On a clear day, stand at one end of a big sports oval while a friend at the other end bangs two wooden blocks together above their head. You will see the blocks hit a moment before you hear the bang, because light travels much faster than sound.