Do you realize that we live at the bottom of an ocean of gas? 
This ocean of gas is called the atmosphere. 
It is about 100 miles (160 kilometers) deep.
We can best survive in the lower layers of the atmosphere at or near the earth's surface.
This is because there is less and less air to breathe the farther you travel away from the earth's surface.
The earth is wrapped in layers of gases. 
If you could travel high up into the atmosphere, you would travel through these layers or bands of different gases.

What is the ozone layer?

The atmosphere becomes thinner as its altitude (height) above the earth increases.
In the troposphere are all the gases needed by living things on earth.
In the next layer, the stratosphere, is found the ozone layer.
Ozone is a gas formed when ultraviolet rays from the sun change some of the oxygen in the atmosphere to ozone.
Ozone makes a thin layer around the earth and blocks most of the sun's harmful ultraviolet rays, preventing them from reaching the earth.
Some people think ozone smells like garlic.
Others say it smells like seaweed.
Oxygen has no smell at all.
The temperature in the lower stratosphere is only about —67 °F (-55°C).   

Beyond the stratosphere

The stratosphere is about 20 miles (32 kilometers) thick.
The temperature rises steadily as you pass through the stratosphere.
But at the top, the temperature is still only 28 °F (-2°C).
Above the stratosphere lies the mesosphere, and then the ionosphere.
In this band of gases, the temperature becomes very hot.
Radio messages can be sent over long distances by bouncing them off the ionosphere.
The heavier gases like nitrogen tend to stay close to the earth.
So above 60 miles (100 kilometers), you would find more of the lighter gases such as helium and hydrogen.
If you traveled more then 250 miles (400 kilometers) up, you would find so little air that satellites orbiting the earth in this region encounter almost no air resistance.
The temperature is extremely hot—as much as 3600 °F (2000 °C).
In the uppermost part of the atmosphere, the exosphere, the earth's atmosphere gradually merges into outer space.