What makes the wind blow?

Air on the move

Wind is air that is moving. Air is a mix of invisible gases, and it has weight. The weight of all the air above you pressing down is called air pressure. Air pressure is not the same everywhere. In some places the gases in the air are more squeezed together, so the pressure is higher. In other places they are more spread out, so the pressure is lower.

Air always tries to even things out. So it flows from places with high pressure to places with low pressure. That flow of air is the wind. The bigger the difference in pressure, the faster the wind blows. It is a bit like letting go of a blown-up balloon. The air rushes out from where it is squeezed together to where there is more room.

We name a wind after the direction it comes from. A south-westerly wind blows from the south-west. An easterly wind blows from the east.

The Sun gets the air moving

So what makes the pressure different from place to place? The answer is temperature. The Sun does not heat the Earth evenly. Land, sea, mountains and the curve of the Earth mean that some places soak up more of the Sun's heat than others, so the air above them warms by different amounts.

Warm air and cold air do different things. Warm air expands and becomes lighter, so it floats upwards, the same way a hot-air balloon rises. As it climbs, it leaves lower pressure near the ground. Cold air shrinks and becomes heavier, so it sinks and piles up near the ground, which makes higher pressure there. Then the wind blows from the high-pressure place to the low-pressure place.

This happens on a giant scale too. The Sun's rays strike the Earth most directly at the Equator, so warm air rises there. Cold air sinks near the poles. Air flows around the planet to balance the difference. Because the Earth spins, big winds do not travel in straight lines. In the Southern Hemisphere, where Australia is, they are bent to the left.

A breeze from the sea

Smaller, local winds work in the same way. Think about a day at the beach. The Sun heats the land faster than it heats the sea. The air over the hot land warms up and rises, so the pressure over the land goes down. The air over the cooler sea has higher pressure. So it flows in towards the land and takes the place of the rising air. That cool wind off the water is called a sea breeze.

Sea breezes are most common on hot, sunny summer days. When a sea breeze pushes inland, it can bring a sudden drop in temperature. At night, the land cools faster than the sea, and the breeze turns around. A coastal breeze at night that blows from the land to the sea is called a land breeze.

The Fremantle Doctor, and other Australian winds

Perth has very hot summers. Hot days there usually come with dry easterly winds, which blow in from the inland and start in the morning. Then, in the afternoon, a sea breeze arrives from the south-west and cools the city down. Perth people call it the Fremantle Doctor, because it is just the cure for a hot afternoon.

The Doctor does not always arrive on time. Hot easterly winds can delay the sea breeze, or even stop it from forming at all. When the Doctor is late or does not turn up, the temperature in Perth can climb above 40 degrees Celsius.

Other places have winds with names too. The southerly buster is a strong southerly wind change. It rushes up the New South Wales coast and drops the temperature on a hot day. The Ekka winds are dry, gusty westerlies. They arrive in Brisbane each August, around the time of the Royal Queensland Show, which people call the Ekka. These local winds happen again and again because the land, the sea and the hills line up in the same way.

How strong is the wind?

In 1805, a British naval officer named Sir Francis Beaufort made a scale. It helped sailors judge how strong the wind was just by looking at the sea. It is called the Beaufort scale, and it runs from 0, which is calm, up to 12, which is hurricane force. Forecasters still use it today. On land, you can judge the wind by what it does to trees and flags.

Here are the steps from 0 to 4, with the speeds in kilometres an hour. Force 0 is calm, and smoke rises straight up. Force 1 is light air, at about 2 to 6 kilometres an hour. Smoke drifts a little, and that shows which way the wind is blowing. Force 2 is a light breeze, at about 7 to 11 kilometres an hour. You can feel the wind on your face, and leaves rustle. Force 3 is a gentle breeze, at about 13 to 19 kilometres an hour. Leaves and small twigs move all the time, and a light flag stretches out. Force 4 is a moderate breeze, at about 20 to 30 kilometres an hour. Dust and loose paper lift off the ground, and small branches move.

Now the steps from 5 to 8. Force 5 is a fresh breeze, at about 31 to 39 kilometres an hour. Small trees in leaf begin to sway. Force 6 is a strong breeze, at about 41 to 50 kilometres an hour. Large branches move, and umbrellas are hard to use. Force 7 is a near gale, at about 52 to 61 kilometres an hour. Whole trees move, and it is hard to walk against the wind. Force 8 is a gale, at about 63 to 74 kilometres an hour. Twigs break off trees.

Weather stations also measure the wind with instruments. An anemometer has cups that spin in the wind. The faster the cups spin, the stronger the wind. A wind vane swings around to line up with the wind, and it shows which way the wind is blowing.

📚 Words to know

Air pressure

The weight of the air pressing down on the ground. Wind blows from high pressure towards low pressure.

Sea breeze

A wind that blows from the cool sea onto the warm land, usually on a hot, sunny day.

Land breeze

A wind that blows from the land out to the sea at night, when the land has cooled faster than the water.

Beaufort scale

A scale from 0 to 12 that tells you how strong the wind is by what you can see and feel.

Anemometer

An instrument that measures wind speed. A common kind has cups that spin faster as the wind gets stronger.

🧪 Try this: be a wind watcher

1. Feel a tiny wind. Ask a grown-up to help you blow up a balloon. Pinch the neck shut, hold the balloon near your face, then let go. The air rushes from high pressure inside the balloon to lower pressure outside, and you feel a small wind. Never put a balloon, or a piece of a burst balloon, in your mouth.
2. Make a wind streamer. Tape a strip of light ribbon or crepe paper, about 50 centimetres long, to the end of a blunt stick or a wooden spoon. Do not run with the stick.
3. Go outside at the same time each day for a week, and hold up your streamer. Rate the wind from 0 to 8 using the Beaufort steps in this article. Look at the trees, flags and leaves too. If the wind is strong enough to break twigs, watch from inside.
4. Find the wind direction. Turn around until the wind blows straight into your face. The way you are facing is the direction the wind comes from. If you are facing the south-west, it is a south-westerly. Use a compass, or the way the Sun rises in the east and sets in the west, to work out your directions. Record the speed and direction in a table.
5. Challenge: do you live near the coast? On a hot, sunny day, check the wind in the morning and again in the afternoon. Does it change, and start blowing in from the sea? You may have found a sea breeze. Write down what time it begins.