Your turn
Each lab is one step from the video. Change things and see what happens.
Each lab isolates one part of how wind works. The models are simplified to show the idea, not to forecast real weather.
Slide the sliders and press the buttons.
Slide the wind from force 0 to force 12. What happens to the grass, the flag and the kite?
Beaufort force 0 (calm, under 1 knot) to 12 (hurricane force, 64+ knots), described by what the wind does.
Make the wind soft, then strong. Watch the kite!
Pump extra air into the left side, then open the gap. Which way does the air rush, and how fast?
Crowd molecules on one side and open the gap: net flow runs from high to low pressure, faster for a bigger difference.
Squish lots of air into one side. Then open the gap. Where does it go?
Molecules drawn huge and slow; real air molecules are far too small to see.
The Sun heats the land faster than the sea. Turn it up and down. When does the wind blow hardest?
Uneven heating makes a surface low over the warm land and a high over the cool water, which drives a circulation.
Make the Sun strong. Does the wind get stronger?
Slide through a day and a night. When does the breeze blow in from the sea, and when does it flip?
The afternoon sea breeze reverses into a weaker land breeze at night, when the land cools below the sea.
Try day, then night. Which way does the breeze blow?
Turn Earth’s spin off, then back on. How do the winds bend?
With no rotation, one cell per hemisphere and straight north–south surface winds. With rotation, the Coriolis effect gives three cells and the prevailing wind belts.
A simplified picture: real circulation shifts with the seasons and with land and sea.
Drag the H and the L. How close do they have to be to make a gale?
Bring the high and low together to steepen the gradient; the wind circulates clockwise out of the H and counterclockwise into the L.
Drag the H and the L close together. Watch the kite!
A toy map: the wind follows the pressure gradient, turned by the Coriolis effect.
You can't see the wind. But you can see what it does. It bends the grass, flaps flags and pulls kites up into the sky.
Air is made of tiny bits, much too small to see. They zoom around and bump into everything. When air moves from one place to another, we call it wind.
Sometimes lots of air gets squished into one place. Squished air always moves to where there's more room. Let go of a balloon, and the air rushes out. That's a tiny wind!
The Sun warms some places more than others. Warm air is light, so it floats up, like a hot air balloon. Cool air is heavy, so it sinks. Cool air slides in to fill the gap. That's wind!
On a sunny day, the sand gets hot fast, but the sea stays cool. So a cool breeze blows in from the sea. At night, it turns around.
Want to see the wildest wind of all? See how tornadoes form. Or find out what makes waves.
Wind is air moving from a place with higher air pressure to a place with lower air pressure.53 The pressure differences are made mostly by the Sun heating the Earth unevenly.7 Where the air gets warm, it spreads out and rises, leaving lower pressure near the ground. Where the air stays cool, it sinks and crowds together, making higher pressure. Air then flows from the high to the low, and we feel that flow as wind.3
Two more things shape the wind we actually feel. Earth's spin bends moving air over long distances (the Coriolis effect), and friction with the ground slows it down near the surface.1
Wind is simply air in motion. When weather forecasters talk about wind, they usually mean how fast the air is moving sideways across the ground and which direction it's coming from.1 We can't see air, but we can see what it pushes: flags, leaves, kites and waves.
Winds are named for where they come from. A "west wind" blows from the west toward the east.15 A wind vane shows the direction and an anemometer measures the speed.1
Air is made of countless tiny molecules zooming around in every direction. Each time one hits a surface it gives it a tiny push. Add up all those pushes and you get air pressure.2 At sea level, the air presses on every square inch with about 14.7 pounds of force.4
Where the molecules are packed closely, the pressure is high. Where they're more spread out, it's low.3 Air always moves from high pressure toward low pressure, because things in nature tend to even out, a bit like people on a bus spreading out into the empty seats.3
You can make a tiny wind yourself. Blow up a balloon, pinch the neck, then let a little air escape. The air rushes out, not in, because the pressure inside the balloon is higher than outside.5
bigger pressure difference → faster wind
The bigger the difference in pressure, the faster the air moves.3 Meteorologists call the change in pressure over a distance the pressure gradient, and wind speed goes up as the gradient goes up.1
The Sun doesn't heat the Earth evenly. Sunlight hits different places at different angles, and oceans, mountains, deserts and forests all warm up at different rates.310 So some pockets of air end up warmer than others.
When air warms up, its molecules move faster and spread out, so the air becomes lighter and rises, which is why steam goes upward.3 You might expect warm air to have higher pressure, but at the ground it's the opposite: the rising air leaves fewer molecules behind, so the pressure there drops.35 Cooler, heavier air sinks and crowds together, making higher pressure. Air slides in along the ground from the high toward the low, and that's the wind.34
| Warm air | Cool air | |
|---|---|---|
| Molecules | Faster, more spread out | Slower, closer together |
| Movement | Rises | Sinks |
| Pressure at the ground | Lower (L) | Higher (H) |
| Typical weather | Clouds, rain | Clear, fair |
Rising air cools and often forms clouds, which is why low pressure tends to bring cloudy, stormy weather. Sinking air warms and dries out clouds, so high pressure usually means fair weather.14
A sea breeze is the easiest way to see all of this at once. On a sunny day, sunshine only heats the top few inches of the ground, so land warms up quickly. In the sea, light reaches deep (in clear tropical water, down to 150–200 meters) and waves keep mixing the heat, so the water warms slowly.6
When a sea breeze arrives, the temperature can drop by as much as 15–20 °F (8–11 °C).6 Sea breezes also happen beside big lakes, like the Great Lakes.6
At night the loop flips. The land cools faster than the water, so the air over the land becomes the cooler, heavier air, and a land breeze blows out to sea. Land breezes are usually weaker than sea breezes.610
The same idea works for the whole planet. The Sun heats the tropics much more than the poles, and that imbalance keeps the atmosphere moving like a giant heat engine.118 If Earth didn't spin, warm air would rise at the equator, flow toward the poles high up, sink there, and flow straight back along the ground: one big loop in each hemisphere.912
But Earth spins, and that changes everything. Because Earth rotates, air moving over long distances curves: to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This is the Coriolis effect.125 It happens because the ground at the equator travels east at over 1,000 mph (1,600 km/h), while the ground at the poles barely moves. Air that travels north or south keeps its old eastward speed while the ground under it changes.1314
The spin breaks each hemisphere's circulation into three cells: the Hadley, Ferrel and Polar cells.98 Between them are belts of high pressure (around 30° north and south, and at the poles) and low pressure (at the equator and around 50–60°).9 That's why so many deserts sit near 30° latitude.9
| Wind belt | Where | Blows from |
|---|---|---|
| Trade winds | Tropics | The east7 |
| Doldrums | Near the equator | Very weak, often calm7 |
| Westerlies | Mid-latitudes (about 35°–60°) | The west79 |
| Polar easterlies | Near the poles | The east79 |
Sailors have relied on these steady winds for centuries.5 Far above them, around 30,000 feet, flow the jet streams: narrow rivers of air blowing west to east that can top 275 mph.13
Weather maps mark high-pressure centers with an H and low-pressure centers with an L, surrounded by lines of equal pressure called isobars.1 Wind blows out of the highs and into the lows. Because of Earth's spin, in the Northern Hemisphere it swirls clockwise around a high and counterclockwise around a low.14
A windy day happens when the pressure changes a lot over a short distance. On the map, the isobars are squeezed close together. The closer they are, the stronger the wind.1 Storms are the extreme case: tornado winds can blow at over 200 mph, and hurricanes have winds of at least 74 mph.514
The Beaufort scale rates wind from 0 to 12 by what it does.515
| Force | Name | Speed (knots) | What you see on land |
|---|---|---|---|
| 0 | Calm | under 1 | Smoke rises straight up |
| 2 | Light breeze | 4–6 | Wind felt on your face, leaves rustle |
| 4 | Moderate breeze | 11–16 | Dust and loose paper lifted |
| 6 | Strong breeze | 22–27 | Large branches moving |
| 8 | Gale | 34–40 | Twigs break off trees |
| 10 | Storm | 48–55 | Trees uprooted, serious damage |
| 12 | Hurricane force | 64+ | Sea completely white with spray |
Source: NOAA Storm Prediction Center.15
In a way, yes. Wind is caused by the Sun heating Earth's surface unevenly, so the energy a wind turbine captures started out as sunshine.10 Today, wind energy is mainly used to make electricity.10
Wind at its most extreme can be deadly: see how tornadoes form. And wind is what shapes the sea surface too: see what causes waves.
The Sun heating the Earth unevenly. Warm air rises and leaves lower pressure near the ground, cooler air sinks and makes higher pressure, and air flows from high pressure to low pressure. That flow is wind.
From differences in air pressure. Air always moves from where it's crowded (high pressure) to where it's spread out (low pressure). Those differences are made mostly by uneven heating from the Sun.
Not the winds we know. Almost all of Earth's wind is driven by temperature differences that sunlight creates. Without sunlight, the pressure differences that push air around would fade away.
A big difference in pressure over a short distance. On a weather map, that looks like a high and a low sitting close together, with the lines of equal pressure (isobars) packed tightly. The tighter they are, the stronger the wind.
Because air molecules spread out when they have room. Where they're packed tightly, they push harder, so air is shoved toward places where the molecules are more spread out, like air rushing out of an untied balloon.
Because Earth spins. Over long distances, moving air seems to curve to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This is called the Coriolis effect, and it's why winds swirl around highs and lows.
A wind vane shows which direction the wind is coming from, and an anemometer measures its speed. The Beaufort scale describes wind strength from 0 (calm) to 12 (hurricane force) by what it does to trees, smoke and the sea.