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New page: '''Earth's atmosphere''' is a layer of gases surrounding the planet Earth and retained by the Earth's gravity. It contains roughly (by molar content/volume) 78% nitrogen, 2...
'''Earth's atmosphere''' is a layer of [[gas]]es surrounding the planet [[Earth]] and retained by the Earth's [[gravity]]. It contains roughly (by molar content/volume) 78% [[nitrogen]], 20.95% [[oxygen]], 0.93% [[argon]], 0.038% [[Carbon dioxide in the Earth's atmosphere|carbon dioxide]], trace amounts of [[#Composition|other gases]], and a variable amount (average around 1%) of [[water vapor]]. This mixture of gases is commonly known as '''air'''. The atmosphere protects [[organism|life on Earth]] by absorbing [[ultraviolet]] [[solar radiation]] and reducing [[temperature]] extremes between day and night.

There is no definite boundary between the atmosphere and [[outer space]]. It slowly becomes thinner and fades into space. Three quarters of the atmosphere's mass is within 11 [[Kilometre|km]] of the [[planet]]ary surface. In the [[United States]], people who travel above an [[altitude]] of 80.5 km (50 [[statute mile]]s) are designated [[astronaut]]s. An altitude of 120 km (400,000 ft) marks the boundary where atmospheric effects become noticeable during re-entry. The [[Kármán line]], at 100 km (328,000 ft), is also frequently regarded as the boundary between atmosphere and outer space.

==Temperature and layers==
The [[temperature]] of the Earth's atmosphere varies with [[altitude]]; the [[lapse rate|mathematical relationship]] between temperature and altitude varies among six different atmospheric layers:

# '''[[Troposphere]]''': From the [[Greek language|Greek]] word "τρέπω" meaning to turn or mix. The troposphere is the lowest layer of the atmosphere; it begins at the surface and extends to between 7 km (23,000 ft) at the poles and 17 km (60,000 ft) at the equator, with some variation due to weather factors. The troposphere has a great deal of vertical mixing due to solar heating at the surface. This heating warms air masses, which makes them less dense so they rise. When an air mass rises the pressure upon it decreases so it expands, doing work against the opposing pressure of the surrounding air. To do work is to expend [[energy]], so the temperature of the air mass decreases. As the temperature decreases, water vapor in the air mass may condense or solidify, releasing [[latent heat]] that further uplifts the air mass. This process determines the maximum rate of decline of temperature with height, called the [[adiabatic lapse rate]].
# '''[[Stratosphere]]''': From the [[Latin]] word "stratus" meaning a spreading out. The stratosphere extends from the troposphere's 7 to 17 km (23 – 60,000 ft) range to about 50 km (160,000 ft). Temperature increases with height. The stratosphere contains the [[ozone]] layer, the part of the Earth's atmosphere which contains relatively high concentrations of ozone. "Relatively high" means a few parts per million—much higher than the concentrations in the lower atmosphere but still small compared to the main components of the atmosphere. It is mainly located in the lower portion of the stratosphere from approximately 15 to 35 km (50 – 115,000 ft) above Earth's surface, though the thickness varies seasonally and geographically.
# '''[[Mesosphere]]''': From the Greek word "μέσος" meaning middle. The mesosphere extends from about 50 km (160,000 ft) to the range of 80 to 85 km (265 – 285,000 ft), temperature decreasing with height.
# '''[[Thermosphere]]''': from 80 – 85 km (265 – 285,000 ft) to 640+ km (400+ mi), temperature increasing with height.
# '''[[Ionosphere]]''': is the part of the atmosphere that is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere. It has practical importance because, among other functions, it influences radio propagation to distant places on the Earth. It is located in the thermosphere and is responsible for [[aurora (astronomy)|aurora]]s.
# '''[[Exosphere]]''': from 500 – 1000 km (300 – 600 mi) up to 10,000 km (6,000 mi), free-moving particles that may migrate into and out of the [[magnetosphere]] or the [[solar wind]].

The boundaries between these regions are named the [[tropopause]], [[stratopause]], [[mesopause]], [[thermopause]] and [[exobase]]. For more see: [http://en.wikipedia.org/wiki/Earth%27s_atmosphere]

[[Category: General Reference]]

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