[[image:Edge_of_Space.png|thumb|right|175px|Layers of Atmosphere (NOAA)]]
'''Earth's atmosphere''' is a layer of gases surrounding the planet [[Earth]] and retained by the Earth's gravity. It contains roughly 78% [[nitrogen]] and 21% [[oxygen]], with trace amounts of [[#Composition|other gases]]. The atmosphere protects [[life]] on [[Earth]] by absorbing [[ultraviolet]] solar [[radiation]] and reducing temperature extremes between [[day]] and [[night]].
The atmosphere has no abrupt cut-off. It slowly becomes thinner and fades away into space. There is no definite boundary between the atmosphere and [[outer space]]. Three-quarters of the atmosphere's mass is within 11 km of the [[planet|planetary]] surface. In the [[United States]], persons who travel above an [[altitude]] of 50.0 miles (80.5 km) are designated as [[Astronaut|astronauts]]. An altitude of 120 km (75 mi or 400,000 ft) marks the boundary where atmospheric effects become noticeable during re-entry. The [[Karman line]], at 100 km (62 mi), is also frequently used as the boundary between atmosphere and [[space]].
==Temperature and the atmospheric layers==
The [[temperature]] of the Earth's atmosphere varies with [[altitude]]; the [[mathematical relationship]] between temperature and altitude varies between the different atmospheric layers:
* [[troposphere]]: From the Greek word ''tropos'' meaning to turn or mix. The troposphere is the lowest layer of the atmosphere starting at the surface going up to between 7 km at the poles and 17 km 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 then rise to release [[latent heat]] as sensible heat that further buoys the air mass. This process continues until all water vapor is removed. In the troposphere, on average, temperature decreases with height due to [[Joule-Thomson effect|expansive cooling]].
* [[stratosphere]]: from that 7–17 km range to about 50 km, temperature increasing with height.
* [[mesosphere]]: from about 50 km to the range of 80 km to 85 km, temperature decreasing with height.
* [[thermosphere]]: from 80–85 km to 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.
===Various atmospheric regions===
Atmospheric regions are also named in other ways:
* [[ionosphere]] — the region containing [[ion]]s: approximately the mesosphere and thermosphere up to 550 km.
* [[exosphere]] — above the ionosphere, where the atmosphere thins out into [[outer space|space]].
* [[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.
* [[ozone layer]] — or ozonosphere, approximately 10 - 50 km, where [[stratosphere|stratospheric]] [[ozone]] is found. Note that even within this region, ozone is a minor constituent by volume.
* upper atmosphere — the region of the atmosphere above the [[mesopause]].
* [[Van Allen radiation belt]]s — regions where [[Solar wind|particles]] from the Sun become concentrated.
==Pressure==
:''Barometric Formula: (used for airplane flight) [[barometric formula]]''
:''Main article: [[Atmospheric pressure]]''
:''Nasa mathematical model: [[NRLMSISE-00]]''
Atmospheric pressure is a direct result of the weight of the air. This means that air pressure varies with location and time because the amount (and weight) of air above the earth varies with location and time. Atmospheric pressure drops by ~50% at an altitude of about 5 km (equivalently, about 50% of the total atmospheric mass is within the lowest 5 km). The average atmospheric pressure, at [[sea level]], is about 101.3 [[kilopascal]]s (about 14.7 pounds per square inch).
==Thickness of the atmosphere==
The atmosphere is present to heights of 1000 km. or more. But at this height it is so thin and at such low pressure that it's almost like it isn't there.
*57.8% of the atmosphere is below the summit of [[Mount Everest]].
*72% of the atmosphere is below the common flight height of airplanes, (about 10000 m or 32800 ft).
*99.99999% of the atmosphere is below the highest [[North American X-15|X-15]] plane flight on August 22, 1963 which reached an altitude of 354,300 ft or 108 km.
Therefore, most of the atmosphere is below 100 km (99.9999
although in the rarified region above this there are [[aurora]]s, and other atmospheric effects.
==Composition==
[[Image:Atmosphere_gas_proportions.gif|thumb|250px|Composition of Earth's atmosphere. The lower pie represents the least common gases that compose 0.038% of the atmosphere. Values normalized for illustration.]]
{| style="width:300px"
|+'''Percent composition of<br>dry atmosphere, by volume'''
|colspan=2 style="font-size: 85%" |''[[Parts per million|ppmv]]: parts per million by volume''
|-
!align = "left" | '''Gas'''
!style="width:100px" align = "left" |'''Volume'''
|-
|[[Nitrogen]] (N<sub>2</sub>) ||78.084%
|-
|[[Oxygen]] (O<sub>2</sub>)<td>20.946%
|-
|[[Argon]] (Ar) ||0.9340%
|-
|[[Carbon dioxide]] (CO<sub>2</sub>) ||365 ppmv
|-
|[[Neon]] (Ne) ||18.18 ppmv
|-
|[[Helium]] (He) ||5.24 ppmv
|-
|[[Methane]] (CH<sub>4</sub>) ||1.745 ppmv
|-
|[[Krypton]] (Kr) ||1.14 ppmv
|-
|[[Hydrogen]] (H<sub>2</sub>) ||0.55 ppmv
|-
|colspan=2 |<br>'''Not included in above dry atmosphere:'''
|-
|[[Water vapor]] (highly variable) ||typically 1%
|}
<br><font size=1>''Source for figures above: [
http://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html NASA]''<br>
''Carbon dioxide and methane updated (to 1998) by [[IPCC]] TAR table 6.1 [
http://www.grida.no/climate/ipcc_tar/wg1/221.htm]''</font><br>
<br>
'''Minor components of air not listed above include:'''
{| style="width:500px"
!align = "left" | '''Gas'''
!style="width:300px" align = "left" |'''Volume'''
|-
|[[nitrous oxide]] ||0.5 ppmv
|-
|[[xenon]] ||0.09 ppmv
|-
|[[ozone]] ||0.0 to 0.07 ppmv (0.0 to 0.02 ppmv in [[winter]])
|-
|[[nitrogen dioxide]] ||0.02 ppmv
|-
|[[iodine]] ||0.01 ppmv
|-
|[[carbon monoxide]] ||trace
|-
|[[ammonia]] ||trace
|}
*The mean molecular mass of air is 28.97 g/mol.
=== Heterosphere ===
Below an altitude of about 100 km, the Earth's atmosphere has a more-or-less uniform composition (apart from water vapor) as described above. However, above about 100 km, the Earth's atmosphere begins to have a composition which varies with altitude. This is essentially because, in the absence of mixing, the density of a gas falls off exponentially with increasing altitude, but at a rate which depends on the [[molecular mass]]. Thus higher mass constituents, such as oxygen and nitrogen, fall off more quickly than lighter constituents such as [[helium]], molecular [[hydrogen]], and atomic hydrogen. Thus there is a layer, called the '''heterosphere''', in which the earth's atmosphere has varying composition. As the altitude increases, the atmosphere is dominated successively by helium, molecular hydrogen, and atomic hydrogen. The precise altitude of the heterosphere and the layers it contains varies significantly with temperature.[
http://www.oma.be/BIRA-IASB/Public/Research/Thermo/Thermotxt.en.html]
==Density and mass==
The density of air at sea level is about 1.2 kg/m<sup>3</sup>. Natural variations of the [[barometric pressure]] occur at any one altitude as a consequence of [[weather]]. This variation is relatively small for inhabited altitudes but much more pronounced in the outer atmosphere and space due to variable solar radiation
The atmospheric density decreases as the altitude increases. This variation can be approximately modeled using the [[barometric formula]]. More sophisticated models are used by meteorologists and space agencies to predict weather and orbital decay of satellites.
The total mass of the atmosphere is about 5.1 × 10<sup>18</sup> kg, or about 0.9 [[Parts per million|ppm]] of the Earth's total mass.
The above composition percentages are done by volume. Assuming that the gases act like ideal gases, we can add the percentages p multiplied by their molar masses m, to get a total t = sum (p·m). Any element's percent by mass is then p·m/t. When we do this to the above percentages, we get that, by mass, the composition of the atmosphere is 75.523% N<sub>2</sub>, 23.133% O<sub>2</sub>, 1.288% Ar, 0.053% CO<sub>2</sub>, 0.001267% Ne, 0.00029% CH<sub>4</sub>, 0.00033% Kr, 0.000724% He, and 0.0000038 % H<sub>2</sub>.
[[Image:Atmosphere model.png|500px|center]] This graph is from the [[NRLMSISE-00]] atmosphere [[Mathematical_model|model]], which has as inputs: latitude, longitude, date, time of day, altitude, solar flux, and the earth's magnetic field daily index.
== The evolution of the Earth's atmosphere ==
[[Image:Atmosphere composition diagram.jpg|thumb|right|350px|Diagram of chemical and transport processes related to atmospheric composition.]]
The history of the Earth's atmosphere prior to one billion years ago is poorly understood, but the following presents a plausible sequence of events. This remains an active area of research.
The modern atmosphere is sometimes referred to as Earth's "third atmosphere", in order to distinguish the current [[chemical]] composition from two notably different previous compositions. The original atmosphere was primarily [[helium]] and [[hydrogen]]. [[Heat]] (from the still-molten crust, and the sun) dissipated this atmosphere.
About 3.5 billion years ago, the surface had cooled enough to form a [[crust]], still heavily populated with [[volcano]]es 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]] (though very recent simulations run at the University of Waterloo and University of Colorado in 2005 suggested that it may have had up to 40% hydrogen [
http://newsrelease.uwaterloo.ca/news.php?id=4348]). This second atmosphere had approximately 100 [[multiplication|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 [[condensation|condensed]] to form [[rain]] and [[ocean]]s, which began to dissolve carbon dioxide. Approximately 50% of the carbon dioxide would be absorbed into the oceans. One of the earliest types of bacteria were the [[cyanobacteria]]. Fossil evidence indicates that these bacteria existed approximately 3.3 billion years ago and were the first oxygen-producing evolving phototropic organisms. They were responsible for the initial conversion of the earth’s atmosphere from an anoxic state to an oxic state (that is, from a state without oxygen to a state with oxygen). Being the first to carry out oxygenic photosynthesis, they were able to convert carbon dioxide into oxygen, playing a major role in oxygenating the atmosphere.
[[photosynthesis|Photosynthesizing]] plants would later [[evolution|evolve]] and convert more carbon dioxide into oxygen. Over time, excess carbon became locked in [[fossil fuels]], [[sedimentary rock]]s (notably [[limestone]]), and [[animal shell]]s. As oxygen was released, it reacted with ammonia to create nitrogen; in addition, [[bacterium|bacteria]] would also convert ammonia into nitrogen.
As more plants appeared, the levels of oxygen increased significantly, while carbon dioxide levels dropped. At first the oxygen combined with various [[chemical element|element]]s (such as [[iron]]), but eventually oxygen accumulated in the atmosphere, resulting in [[mass extinction]]s and further evolution. With the appearance of an [[ozone layer]] (ozone is an [[allotrope]] of oxygen) [[life]]forms were better protected from [[ultraviolet]] [[radiation]]. This oxygen-nitrogen atmosphere is the "third atmosphere".
== References ==
*''[
http://www.oma.be/BIRA-IASB/Public/Research/Thermo/Thermotxt.en.html The thermosphere: a part of the heterosphere]'', by J. Vercheval (viewed 1 Apr 2005)
==See also==
*[[Air]]
*[[Air glow]]
*[[Atmospheric electricity]]
*[[Global warming]]
*[[Greenhouse effect]]
*[[Historical temperature record]]
*[[Intergovernmental Panel on Climate Change]] (IPCC)
== External links==
*[
http://nssdc.gsfc.nasa.gov/space/model/models_home.html#atmo NASA atmosphere models]
*[
http://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html NASA's Earth Fact Sheet]
*[
http://atmospheres.agu.org/ American Geophysical Union: Atmospheric Sciences]
*[
http://www.srh.noaa.gov/srh/jetstream/atmos/layers.htm Layers of the Atmosphere]
although in the rarified region above this there are [[aurora]]s, and other atmospheric effects.
)