Atmosphere

Composition of the Atmosphere

The composition of the atmosphere is dominated by three gases: nitrogen, oxygen, and argon. These gases are called “permanent gases” because their concentrations are nearly constant over time and space. Nitrogen (78 percent of the atmosphere) and argon (1 percent) are largely inert, meaning that they are used in very few geophysical or biological processes. Oxygen, which is crucial for nearly all life on earth, accounts for about 20 percent of dry air.

In addition to the permanent gases, there are several other gases known as “variable gases,” which show up in changing amounts over time and space. Two of the most important variable gases are carbon dioxide and methane, both of which are potent greenhouse gases. The third, extremely important variable gas is water vapor, which is a strong greenhouse gas and is necessary for the formation of clouds and precipitation.

While air pressure shows a simple pattern of decreasing with height, another important atmospheric variable—temperature—exhibits a more complex vertical pattern. Changes in temperature above sea level allow us to divide the atmosphere into four layers: the troposphere, stratosphere, mesosphere, and thermosphere.

The source of the energy that warms the earth, drives the circulation of the atmosphere and ocean, and makes life possible is the sun. Energy travels from the sun to the earth in the form of electromagnetic (EM) radiation. Solar radiation is often referred to as “shortwave radiation,” because the sun mainly emits radiation like visible light and ultraviolet (UV) radiation, which are characterized as having relatively short wavelengths. The atmosphere is largely transparent to these types of radiation, so most solar energy passes through the atmosphere and is absorbed by the earth's surface, which causes the surface to warm up.

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