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 is called the atmosphere (AT muh
sfihr).
The atmosphere touches the ground everywhere. It is held to the earth by
the pull of the earth’s gravity. Close to the ground, where the pull of
gravity is strongest, the wrapper of air is thick. Farther away, where
the pull of gravity is not so strong, it gets thinner. Finally, many
miles from the earth’s surface, the atmosphere thins out and disappears.
Where it ends, outer space begins.
earth’s atmosphere
The bluish haze is the earth’s wrapper of air. The white ripples are
clouds. This photograph was taken from the Gemini 6 spacecraft.
Is air something?
Air doesn’t seem to be made of anything. It has no color, taste, or
smell. And you can see right through it. But it is made of 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
