Wind is simply air in motion. Usually in meteorology, when we are talking about the wind it is the horizontal speed and direction we are concerned about. For example, if you hear a report of a west wind at 15 mph (24 km/h) that means the horizontal winds will be coming FROM the west at that speed.

Although we cannot actually see the air moving we can measure its motion by the force that it applies on objects. We use a wind vane to indicate the wind's direction and an anemometer to measure the wind's speed. But even without those instruments we can determine the direction.

The Mystery of the Wind

Have you ever stood outside on a quiet day and suddenly felt a cool breeze brush across your face?

You may have wondered, Where did that come from?

There is no invisible fan hidden in the sky. There are no giant hands pushing the air around the Earth. Yet the air is constantly moving—from gentle breezes that rustle the leaves to powerful winds that can bend trees and send waves crashing against the shore.

So what makes the wind blow?

The answer begins with something we experience every day:

The Sun.

The Sun Starts the Investigation

The Sun warms the Earth, but it does not warm every part of our planet equally.

Think about standing on a beach on a sunny afternoon. The sand becomes hot beneath your feet, while the nearby ocean may remain considerably cooler.

Land and water absorb and release heat differently. Mountains, forests, cities, deserts, oceans, and ice-covered regions all respond differently to sunlight.

The result is a planet covered with places that are warmer or cooler than their surroundings.

And this difference in temperature sets the air in motion.

Warm Air Rises

Air may seem invisible and weightless, but it has mass.

When air is heated, its molecules move faster and spread farther apart. The warmed air becomes less dense than the cooler air around it.

That causes the warm air to rise.

Imagine a hot-air balloon. When the air inside the balloon is heated, it becomes less dense than the surrounding air, and the balloon rises.

Something similar happens naturally in Earth's atmosphere.

As warm air rises, cooler, denser air moves in to take its place.

And now we have air moving.

Moving air is wind.

The Great Atmospheric Conveyor Belt

The rising and sinking of air doesn't happen in just one small place.

Across the entire planet, the Sun continually heats Earth's surface. Near the equator, sunlight generally arrives more directly, making this region warmer than areas farther north and south.

Warm air near the equator rises high into the atmosphere.

As it rises, it spreads toward the north and south. Eventually, the air cools and sinks in other regions. Near Earth's surface, the cooler air flows back toward warmer areas.

This creates enormous circulation patterns in the atmosphere.

You can think of them as giant, invisible conveyor belts carrying heat around the planet.

But there is another important part of the mystery.

Earth Is Spinning

The Earth rotates on its axis once approximately every 24 hours.

Because the planet is spinning, moving air does not travel in a perfectly straight line over long distances.

Instead, Earth's rotation causes moving air to appear to curve.

This effect is called the Coriolis effect.

In the Northern Hemisphere, moving air is deflected toward the right. In the Southern Hemisphere, it is deflected toward the left.

The Coriolis effect helps shape enormous weather systems, including the paths taken by winds around the globe.

It also helps explain why hurricanes and other large rotating storms spin in opposite directions in the two hemispheres.

So the wind has another clue in its case file:

A moving atmosphere on a spinning planet.

Earth Is Spinning

The Earth rotates on its axis once approximately every 24 hours.

Because the planet is spinning, moving air does not travel in a perfectly straight line over long distances.

Instead, Earth's rotation causes moving air to appear to curve.

This effect is called the Coriolis effect.

In the Northern Hemisphere, moving air is deflected toward the right. In the Southern Hemisphere, it is deflected toward the left.

The Coriolis effect helps shape enormous weather systems, including the paths taken by winds around the globe.

It also helps explain why hurricanes and other large rotating storms spin in opposite directions in the two hemispheres.

So the wind has another clue in its case file:

A moving atmosphere on a spinning planet.

A Breeze by the Sea

We don't have to travel around the world to see wind being created.

We can watch it happen along a coastline.

During a sunny day, the land heats up faster than the water.

The air above the land becomes warmer and rises.

Cooler air over the water then moves toward the land to replace the rising air.

This creates a sea breeze.

At night, the situation can reverse.

The land cools more quickly than the water. The water may remain warmer, causing the air above it to rise. Cooler air from the land then moves toward the water.

This is called a land breeze.

A simple change in temperature can create a noticeable wind.

Mountains Have Their Own Clues

Mountains can also influence the movement of air.

When winds encounter a mountain range, the air may be forced upward.

As the air rises, it expands and cools. Moisture in the air can condense, forming clouds and sometimes producing rain or snow.

After crossing the mountain, the air may descend on the other side.

As it sinks, it can become warmer and drier.

This is one reason why two places located relatively close to one another can have very different climates.

The landscape itself can become part of the wind's story.

Not All Wind Is the Same

Wind comes in many forms.

A gentle breeze might barely move a curtain.

A strong wind can knock branches from trees.

A jet stream, high above Earth's surface, can race around the planet at tremendous speeds.

And during powerful storms, winds can become destructive forces capable of damaging buildings, uprooting trees, and changing coastlines.

Yet all of these winds have something in common:

They are moving air.

The difference lies in what causes the air to move, how much air is moving, how fast it is moving, and the larger atmospheric patterns surrounding it.

The Invisible World Around Us

The next time you feel the wind, stop for a moment.

Look at the trees.

Watch the clouds.

Notice how leaves move across the ground.

Listen to the sound of air passing through branches.

You are witnessing a process that began with sunlight striking Earth.

The Sun warms the surface.

The surface warms the air.

Warm air rises.

Cooler air moves in.

Earth's rotation changes the direction of moving air.

Mountains, oceans, forests, cities, and deserts influence its journey.

And all across the planet, enormous currents of air continually transport heat and moisture from one region to another.

The wind may be invisible, but its fingerprints are everywhere.

It shapes clouds.

It moves seeds.

It carries moisture.

It creates waves.

It spreads pollen.

It helps power sailing ships and wind turbines.

And sometimes, when the atmosphere becomes especially energetic, it becomes the powerful force we know as a storm.

So the next time a mysterious breeze suddenly brushes past you, remember:

You are feeling the movement of Earth's atmosphere—a gigantic, invisible system powered largely by the warmth of the Sun and shaped by a spinning planet.

The mystery of the wind isn't really a mystery at all.

It's one of Earth's greatest ongoing investigations.