The Composition and Structure of the Atmosphere

Several layers of air surround our planet. 
Together, these are known as the atmosphere. 

The layer nearest to the earth's surface is called the troposphere. It is about 6 miles (10 kilometers) thick at the poles and 10 miles (16 kilometers) thick at the equator.
About one-fifth of the troposphere is made up of oxygen. Nearly four-fifths is a gas called nitrogen, and the rest is made up of argon, carbon dioxide, and small amounts of other gases.

The top of the troposphere is called the tropopause. Here, the air does not have enough oxygen for living things to survive.
Above the tropopause lies the stratosphere.
This layer is about 21 miles (35 kilometers) thick. It contains a gas called ozone, which is related to oxygen. The ozone forms a layer which acts as a protective shield around the earth. Light from the sun contains powerful ultraviolet rays which can be harmful to living things. Fortunately, the ozone layer stops most of these rays from reaching the earth.

The layers of the atmosphere are rather like the blankets on a bed. If you lie under a lot of blankets, they feel heavy. The large mass of air in the atmosphere is very heavy and presses down hard on the earth. Scientists call this air pressure.
When you stand on the seashore, you are at sea-level. Here, the most air is pressing down on you.

The farther up you go from the sea, the less dense the air becomes. Its pressure becomes less and less.
In the top layer of the atmosphere, called the ionosphere, there is very little air pressure at all.
About 960 miles (1,600 kilometers) above the earth, the atmosphere fades into the airless emptiness of space.

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