Atmosphere

The atmosphere is an envelope of gases surrounding the solid earth. This mixture of gases provides the oxygen we need to live and is responsible for the global diversity of weather phenomena. In addition, the atmosphere protects life on the earth's [Page 81]surface from the harmful effects of high-energy solar radiation and keeps the average temperature of the earth at a life-sustaining level through the natural greenhouse effect.

Although the atmosphere is a fluid, it is prevented from escaping out to space by the force of gravity. This downward gravitational pull compresses the lower atmosphere, meaning that atmospheric density is highest near sea level and decreases as one ascends. As a result, most of the atmosphere is held very close to the earth's surface; approximately 90 ...