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Hot air balloon floating over a river - hot air rising is what causes wind

Hot air balloon – hot air rising is what causes wind

Hot air is lighter than cold air

When something is hot, its molecules are farther apart than when it is cold. When air is hot, its molecules are further apart, so it takes up more room. That makes hot air lighter than cold air, because the cold air has more molecules in the same amount of room.

More about molecules
What is heat?

Hot air rises up above the cold air, then gets cold again

On Earth, air that is down near the ground is usually warmer than air further up in the sky, because the oceans warm up the air. Because the hot air is lighter than the cold air above it, it floats up through the cold air to lie on top of the cold air. Once the hot air gets high up, far from the ground, though, it’s nearer to space. It’s colder up there than it is down on the ground, just as it’s colder on top of a mountain. So the hot air cools down, and then gravity slowly pulls the cooler air back down to the ground again.

A hurricane in Cuba

A hurricane in Cuba

Tornadoes and hurricanes

This cycle of hot air rising, cooling off, and then sinking again is what causes wind and tornadoes and hurricanes on Earth, and on other planets as well probably. Sometimes when warmer air bumps into colder air it causes thunderstorms, too.

What are hurricanes?
What causes thunderstorms?

Global warming and climate change

As the oceans get warmer because of global warming, they warm up the air above them, and there’s more hot air to rise up and cause winds. We can expect more, and stronger, hurricanes in the future because of climate change.

What causes global warming?

Learn by doing – Hot air rises
Making a hot air balloon

Bibliography and further reading about the atmosphere:


Learn by Doing – Clouds
More about clouds
More about Thunderstorms
And more about Weather home