Thunder and Lightning

What is a thunderstorm?

A thunderstorm is a storm with lightning and thunder. It usually comes from a tall, dark storm cloud called a cumulonimbus, which also brings heavy rain, gusty wind and sometimes hail. You can learn more about these towering clouds in our lesson on types of clouds.

Thunderstorms can look and sound frightening, but once you understand what is happening, they make sense. The flash and the bang are two parts of the same event: a huge burst of electricity in the sky.

How a storm cloud builds up electricity

Inside a tall storm cloud, air is rushing up and down very fast. The cloud is full of water droplets, ice crystals and small lumps of soft hail. As all these bits crash and rub against each other, they pass tiny amounts of electric charge between them — a bit like the way you can build up static electricity by rubbing a balloon on your hair.

The lighter, positive charge tends to collect near the top of the cloud. The heavier, negative charge gathers near the bottom. Over time, a huge difference in charge builds up. The air normally stops electricity from flowing, but eventually the charge becomes so strong that it forces its way through in a sudden, dazzling rush. That rush is lightning.

What lightning actually is

Lightning is a giant spark of electricity. It can jump from one part of a cloud to another, from cloud to cloud, or from the cloud down to the ground. A bolt that reaches the ground usually strikes the highest point it can find, such as a tall tree, a hill or a tall building. That is why the safest place is low and indoors.

When lightning flashes, it heats the air around it to about 30,000 degrees Celsius in an instant — about five times hotter than the surface of the Sun. This is why a lightning bolt glows so brightly.

Why thunder follows lightning

So where does the thunder come from? When lightning heats the air so suddenly, the air expands — it pushes outward extremely fast, like a tiny explosion. That violent push of air creates a powerful sound wave. The sound is thunder.

The lightning and the thunder happen at almost the same moment. But you always see the flash first and hear the bang later. Why?

Because light travels much faster than sound. Light reaches your eyes almost instantly. Sound is far slower — it travels about 1 kilometre every 3 seconds. So the further away the storm is, the bigger the gap between the flash and the rumble. When a storm is right overhead, the flash and bang come almost together. When the gap is long, the storm is far away.

Staying safe in a storm

Thunderstorms are powerful, so it is important to know what to do:

• Go indoors. A solid building or a car with a metal roof is the safest place.
• Stay away from tall objects like single trees, flagpoles and pylons — lightning often strikes the tallest thing around.
• Get off open ground, hilltops and water. Swimmers and people in open fields are at higher risk.
• Avoid water indoors during the storm — no baths, showers or washing up, because electricity can travel through pipes.
• Wait it out. Stay inside for about 30 minutes after the last thunder before going back outside.

Thunderstorms are one of nature's powerful weather events. To see how storms fit alongside other big natural events, read about floods, droughts and storms.

Activity: how far away is the storm?

Here is a real method that meteorologists use. You can try it safely from inside during a storm.

1. When you see a flash of lightning, start counting the seconds: "one-and, two-and, three-and..." until you hear the thunder.
2. Write down the number of seconds.
3. Divide the seconds by 3 to find roughly how many kilometres away the storm is. For example, 6 seconds ÷ 3 = about 2 kilometres away. (If you prefer miles, divide the seconds by 5.)
4. Do this a few times during the storm and keep a list.
5. Watch your numbers. If the gap is getting smaller, the storm is coming closer. If the gap is getting bigger, the storm is moving away.

This little experiment turns a scary storm into a science investigation — and it shows you that thunder and lightning are simply light and sound from the very same flash.