Lightning and thunder

Suddenly the sky darkens, a rumble is heard and a bright light crosses the clouds. Storms can be a little scary, but behind lightning and thunder there is a very interesting explanation.

How a storm is born

Storms form in very tall, huge clouds called cumulonimbus. Inside them, air currents rush up and down, and water droplets and bits of ice bump into each other nonstop.

Where the electricity comes from

With so many collisions, the ice and droplets pick up electric charge, just like when you rub a balloon on your hair and it sticks. In the cloud the charges separate: some gather at the top and others at the bottom. When the difference is huge, a giant spark jumps between parts of the cloud, between two clouds, or between the cloud and the ground. That is lightning.

Why thunder sounds

A lightning bolt heats the air around it to thousands of degrees in an instant. The air expands suddenly and makes a sound wave: thunder. If the lightning is close, you hear a sharp crack; if it is far away, a long, deep rumble.

First lightning, then thunder

Lightning and thunder happen at the same time, but light travels much faster than sound, so we see the flash before we hear the thunder. You can work out how far away the storm is: count the seconds between the flash and the thunder and divide by three. The result is roughly the number of kilometers.

How to stay safe

During a storm the safest place is inside a building or a closed car. Stay away from water, do not shelter under lone trees and avoid using appliances plugged into the mains. If you are outdoors, find shelter as soon as you can.

And afterwards, rain

Storms usually bring heavy rain and sometimes hail. Afterwards, the air is clean and fresh, and sometimes a rainbow appears.

💡 Every 3 seconds between lightning and thunder is about 1 km away.