Earth's atmosphere

As the atmosphere has no abrupt cut-off, but rather thins gradually with increasing altitude, there is no definite boundary between the atmosphere and outer space. In the United States, persons who travel above an altitude of 50 miles (80 kilometres) are designated as astronauts. 400,000 feet (75 miles or 120 kilometres) marks the boundary where atmospheric effects become noticeable during re-entry. The altitude of 100 kilometres or 62 miles is also frequently used as the boundary between atmosphere and space.

Temperature and the atmospheric layers

• troposphere - 0 - 7/17 km, temperature decreasing with height.
• stratosphere - 7/17 - 50 km, temperature increasing with height.
• mesosphere - 50 - 80/85 km, temperature decreasing with height.
• thermosphere - 80/85 - 640+ km, temperature increasing with height.

The boundaries between these regions are named the tropopause, stratopause and mesopause.

The average temperature of the atmosphere at the surface of earth is 14 °C.

Pressure

Density and mass

Various atmospheric regions

• ionosphere - the region containing ions: approximately the mesosphere and thermosphere up to 550 km.
• exosphere - above the ionosphere, where the atmosphere thins out into space.
• ozone layer - or ozonosphere, approximately 10 - 50 km, where stratospheric ozone is found.
• magnetosphere - the region where the Earth's magnetic field interacts with the solar wind from the Sun. It extends for tens of thousands of kilometers, with a long tail away from the Sun.
• Van Allen radiation belts - regions where particles from the Sun become concentrated.

The "Evolution" of the Earth's atmosphere

The modern atmosphere is sometimes referred to as its "third atmosphere", in order to distinguish the current chemical composition from two notably different compositions. The original atmosphere was primarily helium and hydrogen; heat (from the still molten crust, and the sun) dissipated this atmopshere.

About 3.5 billion years ago, the surface had cooled enough to form a crust, still heavily populated with volcanoes which released steam, carbon dioxide, and ammonia. This led to the "second atmosphere"; which was, primarily, carbon dioxide and water vapor, with some nitrogen but virtually no oxygen. This second atmosphere had ~100 times as much gas as the current atmosphere. It is generally believed that the greenhouse effect, caused by high levels of carbon dioxide, kept the Earth from freezing.

During the next few billion years, water vapor condensed to form rain and oceans, which began to dissolve carbon dioxide. Approximately 50% of the carbon dioxide would be absorbed into the oceans. Photosynthesizing plants would evolve and convert carbon dioxide into oxygen. Over time, excess carbon became locked in fossil fuels, sedimentary rocks (notably limestone), and animal shells. As oxygen was released, it reacted with ammonia to create nitrogen; in addition, bacteria would also convert ammonia into nitrogen.

As more plants appeared, the levels of oxygen increased significantly (while carbon dioxide levels dropped). At first it combined with various elements (such as iron), but eventually oxygen accumulated in the atmosphere — resulting in mass extinctions and further evolution. With the appearance of an ozone layer (a compound of oxygen atoms) lifeforms were better protected from ultraviolet radiation. This oxygen-nitrogen atmosphere is the "third atmosphere".

Global warming

See Global Warming, Historical temperature record, IPCC and Anthropogenic Climate Change.

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