Composition
The atmosphere is composed of a shell of gases that surrounds the Earth. This shell is remarkably thin, only about several hundred kilometers thick. While this may seem large, it is extremely small when you consider it in comparison with the size of the Earth.

The atmosphere contains many gases, but the 2 most abundant by far are Nitrogen and Oxygen. Together, these 2 gases make up approximately 99% of the air. The chart on page 11 of the Earth Science Reference Tables shows this. The chart refers to the "Troposphere", which is the bottom layer of the atmosphere.

Layers of the Atmosphere
The temperature of the atmosphere changes with altitude. It is not a simple direct or inverse relationship. If you were to begin at the Earth's surface and travel upwards, you would find that the temperature decreases with altitude up to a point, then begins to increase with altitude. This trend reverses itself two more times as you travel toward outer space. Scientists have divided the atmosphere into layers based on these temperature changes, as shown in the chart on page 14 of the Earth Science Reference Tables.

This chart can be very confusing! Notice that we are first concentrating on only the left side of the chart. This is the section that describes the temperature changes, and gives the names of the different layers.

	As you travel upward from sea level, the temperature steadily drops. This layer is called the Troposphere. Unless you fly in a plane at very high altitudes, you spend your entire life in this layer! The top of the Troposphere is called the Tropopause, and it marks the end of this layer and the beginning of the next layer: the Stratosphere.
In the Stratosphere, the temperature rises with altitude. This is caused mainly by the presence of the ozone layer, which absorbs ultraviolet radiation. The end of the Stratosphere is marked by the Stratopause, and this is also the beginning of the next layer: the Mesosphere.
	In the Mesosphere, the temperature again decreases with altitude. This continues until the end of the Mesosphere, which is called the Mesopause.
Following the Mesopause, the last layer of the atmosphere, the Thermosphere, is characterized by temperatures that rise dramatically. There is no "Thermopause", the Thermosphere simply blends gradually into outer space.

Other Properties of the Atmosphere
If we look at the right side of the chart, we see two other variables described: Atmospheric Pressure, and Water Vapor. These are relatively straight forward, as they both decrease with altitude (inverse relationships).

An important observation here is that pressure drops dramatically with altitude. Almost all of the mass of the atmosphere is located at the bottom, due to gravity pulling the gases down.

Another important thing to note is that all of the water vapor is contained in the Troposphere. This is where all weather also occurs.