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Atmosphere is the general name for a layer of gases that may surround a material body of sufficient mass. The gases are attracted by the gravity of the body, and held fast if gravity is sufficient. Some planets consist mainly of various gases, and thus have very deep atmospheres.

On Earth, the atmosphere consists of nitrogen (78.1%) and oxygen(20.9%), with small amounts of argon (0.9%), carbon dioxide (variable, but around 0.035%), water vapour and other gases. The atmosphere protects life on the planet by absorbing ultraviolet radiation of the sun, and reducing the temperature difference between day and night.

The temperature of the Earth's atmosphere varies with height above sea level:

• 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.

Atmospheric regions are also named in other ways:

• 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 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

Originally the Earth's surface was molten, so that any atmosphere was very unstable. About 3.5 billion years ago the surface had cooled enough to form a crust, still heavily populated with volcanoes shooting steam, carbon dioxide and ammonia in to the air. At this time the atmosphere was mostly carbon dioxide with virtually no oxygen.

During the next one or two billion years the water vapour condensed to form photosynthesizing plants evolved over the surface of the earth converting carbon dioxide in to oxygen and locking the carbon in to fossil fuels. Carbon was also locked in sedimentary rocks. As more oxygen was released in to the atmosphere it reacted with the ammonia releasing nitrogen gas. Ammonia was also converted in to nitrogen gas by nitrifying bacteria.

As more photosynthesizing plants appeared the levels of oxygen increased dramatically - leading to mass extinction of species adapted to the reducing atmosphere, and encouraging the evolution of more complex organisms. The appearance of the ozone layer further favoured these new plants by protecting them from harmful ultraviolet radiation. At this time the carbon dioxide levels became very low - as it remains today.

In modern times, the burning of fossil fuels and subsequent increase in carbon dioxide in the atmosphere possibly causing an increase in global warming.

An atmosphere (abbreviation atm) is a unit of pressure, approximately equal to the air pressure at sea level and defined to be :1 atm = 101 325 Pascal