The moving air

When the air seems calm, and you can't feel a wind, it doesn't mean that the wind has stopped blowing everywhere. For there isn't just one wind, there are many. In one place, there may be a soft breeze. In another place, there may be a fierce gale. In one place, the wind may blow from the north. In another place, it may blow from the south.

There may even be two different winds in the same place ! Near the ground, a wind may be pushing all the smoke from chimneys in one direction. But up in the sky, another wind may be making the clouds scurry in another direction.

Earth's air is always moving. It may move only a few feet (meters) - from a cool, shady park into a hot street. Or, it may move great distances - from the middle of the ocean to a place far inland. But wherever and whenever the air moves, there will be a wind.

High above the clouds, more than five miles (8 kilometers) up in the sky, are the fastest of all winds. These winds are called jet streams - long, narrow currents of air that sometimes move as fast as three hundred miles ( 483 kilometers) an hour.

Much of the time, jet streams are connected together. Then they form one great, rushing river of wind that circles the earth. This great wind moves from west to east. When a jet airplane takes off for a long trip from west to east, the pilot usually heads up into the jet stream. The strong wind of the jet stream gives the plane a powerful push. This great tail wind can sometimes cut the regular flight time almost in half

What makes wind blow ?

The wind moves over the land. It sways the tops of tall grass in meadows and sets leaves a-rustle in the woods. It scatters the smoke from chimneys. It lifts up limp flags and makes them flap. It touches your cheek with cool, quick fingers and impishly ruffles your hair. Wind is moving air. And it is the sun that makes air move.




The earth spins around like a big top. As it spins, each part of the earth, in turn, comes into the sunlight. The sun's light warms the earth. The heat makes the molecules of gas in the air move faster and spread apart. This warm air rises up, in a kind of big, invisible cloud. As the warm air rises, cooler air from other places flows in to replace the warm air. This moving, cooler air is the wind. When you feel the wind blow, you are feeling the movement of the cooler air pushing in to take the place of the warm air.

Air can push !

You're like a fish living at the bottom of the ocean. But you live at the bottom of an ocean of air ! Miles (kilometers) of water are piled up above the fish, and miles of air are piled up above you. All that air is heavy. It has weight. The weight of the air around you presses against you, even though you don't feel it.

Here's how you can tell that air is pressing on things. Fill a drinking glass with water. It must be a glass - a plastic or paper cup won't do. Fill the glass to the very top. Put a piece of stiff cardboard over the top of the glass. The cardboard must cover the whole top.

Now, hold the cardboard in place with your hand and turn the glass upside down. Do this over a sink in case you make a mistake. When the glass is upside down, take your hand off the cardboard.

You may think the water will push the cardboard away and spill out. But this won't happen ! The air beneath the cardboard weighs more than the water in the glass. So the air holds the cardboard in place because it pushes harder against the cardboard than the water does.

Where the air came from

Scientists think the earth's atmosphere has changed during the billions of years since the earth was formed. At first, the atmosphere was probably very different from what it is now. It began to change as some of the gases went off into space.

For many millions of years the earth was fiercely hot. Clouds of hot gases from inside the earth came out of cracks and volcanoes. As these clouds spread out, they became part of the atmosphere. These gases changed the atmosphere, but, you couldn't have breathed the air. It had no oxygen to keep you alive. It was made up mostly of steam, and gases called nitrogen and carbon dioxide.

But as the earth grew cooler, some of the steam broke up into the two gases steam is made of - and one of those gases is oxygen. So, some oxygen became part of the air. Much of the steam became water, and formed the first ocean.

After millions of years there were billions of tiny green plants in the ocean. These plants, called diatoms, made their own food out of water, sunlight, and carbon dioxide gas, just as green plants do today. When plants make food, they give off oxygen, so for millions of years the little diatoms put oxygen into the air, helping to make the kind of atmosphere we have now.

Why is the sky blue ?

I don't suppose you happen to know
Why the sky is blue ? It's because the snow
Takes out the white. That leaves it clean
For the trees and grass to take out the green.
Then pears and bananas start to mellow
And bit by bit they take out the yellow.
The sunsets, of course, take out the red
And pour it into the ocean bed
Or behind the mountains in the west.
You take all that out and the rest
Couldn't be anything else but blue.
--Look for yourself. You can see it's true.

WHY THE SKY IS BLUE
John Ciardi

This is a fun way to explain why the sky is blue, but it isn't true, of course. The sky really gets its color from the sunlight passing through the air.

Sunlight contains all the colors of the rainbow. These colors are scattered as they pass through the air. The color blue is scattered most of all. On a clear day, the molecules of gas in the air make the blue scatter. It is reflected into our eyes from all parts of the sky. So the sky looks bright blue.

But at sunrise or sunset, the sun's light comes to us through all the hot, dusty, smoky air close to the ground. The smoke and dust make other colors, such as yellow and red, scatter. Then, the sky seems pink or red, and the sun is a big orange ball.

Is air something ?

Air doesn't seem to be made of any thing. It has no color, taste, or smell. And you can see right through it. But it is made something. It is made up of many kinds of gases. And these gases are made up of tiny, tiny, things called molecules.

Actually, everything is made up of molecules - rocks, plants, animals, water, and gases. The molecules in a rock are packed close together and hardly move. The molecules in a liquid, such as water, are farther apart and move rather fast. But the molecules of a gas are quite far apart and zip about rapidly. That's why gases are so "thin" and invisible.

For us, the most important of all the gases in the air is the one called oxygen. The only reason we breathe is to get oxygen into our bodies. We couldn't live without it. Almost every kind of animal and plant in the world must have oxygen or it will die.

Only about one-fifth of the air is made up of oxygen. Most of the air - nearly four-fifths of it - is a gas called nitrogen. The rest of the air is made up of many different gases.

There is also a great deal of water vapor and bits of dust floating in the air. But these things are not really part of the air itself.

If air is just thin, floating gases, why doesn't it drift away into outer space ? What keeps the gases from just floating away until they are gone ?

The answer to that is gravity. Earth's gravity pulls at all the molecules of air, just as it pulls at you. The air can no more float off into space than you can !

A wrapper of air

Long ago, people believed that the sky was a shiny metal roof stretched over the earth ! Now, of course, we know what the sky really is. It's simply air - a covering of air around the earth. Air is wrapped around the earth somewhat like the skin of an orange is wrapped around the fruit inside. This wrapper of air around the earth is called the atmosphere.

The atmosphere touches the ground everywhere. Close to the ground, the air is thick. Farther away, the air gets thinner, just as smoke from a chimney gets thinner the higher it rises. Finally, the air just comes to an end. Where the air ends, outer space begins