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] than other wavelengths by the gases in the atmosphere, giving the Earth a blue ] when seen from space]]

The '''atmosphere of Earth''' is a layer of ] surrounding the planet ] that is retained by Earth's ]. The ] protects ] by absorbing ] ], warming the surface through heat retention (]), and reducing ] extremes between ] and ]. Dry air contains roughly (by volume) 78% ], 21% ], 0.93% ], 0.038% ], and small amounts of other gases. Air also contains a variable amount of ], on average around 1%.

The atmosphere has a mass of about five quintillion (5{{e|18}} or 5,000,000,000,000,000,000) kg, three quarters of which is within about {{convert|11|km|mi ft|abbr=on}} of the surface. The atmosphere becomes thinner and thinner with increasing altitude, with no definite boundary between the atmosphere and ]. An altitude of {{convert|120|km|mi|abbr=on}} is where atmospheric effects become noticeable during ] of spacecraft. The ], at {{convert|100|km|mi|abbr=on}}, also is often regarded as the boundary between atmosphere and outer space.

== Composition ==
{{Main|Atmospheric chemistry}}

]

]

Air is mainly composed of nitrogen, oxygen, and argon, which together constitute the major gases of the atmosphere. The remaining gases often are referred to as trace gases,<ref>http://www.ace.mmu.ac.uk/eae/Atmosphere/Older/Trace_Gases.html</ref> among which are the ]es such as water vapor, carbon dioxide, methane, nitrous oxide, and ozone. Filtered air includes trace amounts of many other ]s. Many natural substances may be present in tiny amounts in an unfiltered air sample, including ], ] and ], ], ], and ]s. Various industrial ]s also may be present, such as ] (elementary or in compounds), ] (in compounds), elemental ], and ] (in compounds such as ] ).

{| class="wikitable"
|+'''Composition of dry atmosphere, by volume'''<ref>Source for figures: Carbon dioxide, , (updated 2007.01). Methane, ] , (updated to 1998). The NASA total was 17 ppmv over 100%, and CO<sub>2</sub> was increased here by 15 ppmv. To normalize, N<sub>2</sub> should be reduced by about 25 ppmv and O<sub>2</sub> by about 7 ppmv.</ref>
|colspan=2 style="font-size: 85%" |''ppmv: ] by volume (note: ] is equal to ] for ideal gas only, see ])''
|-
!align = "left" | Gas
!align = "left" |Volume
|-
| ] (N<sub>2</sub>) || 780,840 ppmv (78.084%)
|-
| ] (O<sub>2</sub>) || 209,460 ppmv (20.946%)
|-
| ] (Ar) || 9,340 ppmv (0.9340%)
|-
| ] (CO<sub>2</sub>) || 387 ppmv (0.0387%)
|-
| ] (Ne) || 18.18 ppmv (0.001818%)
|-
| ] (He) || 5.24 ppmv (0.000524%)
|-
| ] (CH<sub>4</sub>) || 1.79 ppmv (0.000179%)
|-
| ] (Kr) || 1.14 ppmv (0.000114%)
|-
| ] (H<sub>2</sub>) || 0.55 ppmv (0.000055%)
|-
| ] (N<sub>2</sub>O) || 0.3 ppmv (0.00003%)
|-
| ] (Xe) || 0.09 ppmv (9{{e|&minus;6}}%)
|-
| ] (O<sub>3</sub>) || 0.0 to 0.07 ppmv (0% to 7{{e|&minus;6}}%)
|-
| ] (NO<sub>2</sub>) || 0.02 ppmv (2{{e|&minus;6}}%)
|-
| ] (I) || 0.01 ppmv (1{{e|&minus;6}}%)
|-
| ] (CO) || 0.038 ppmv
|-
| ] (NH<sub>3</sub>) || trace
|-
| Colspan=2 |'''Not included in above dry atmosphere:'''
|-
| ] (H<sub>2</sub>O) || ~0.40% over full atmosphere, typically 1%-4% at surface
|}

== Structure of the atmosphere ==
====Principal layers====

]
Earth's atmosphere can be divided into five main layers. These layers are mainly determined by whether temperature increases or decreases with altitude. From highest to lowest, these layers are:

; ]: The outermost layer of Earth's atmosphere extends from the exobase upward. Here the particles are so far apart that they can travel hundreds of km without colliding with one another. Since the particles rarely collide, the atmosphere no longer behaves like a fluid. These free-moving particles follow ballistic trajectories and may migrate into and out of the ] or the ]. The exosphere is mainly composed of hydrogen and helium.

; ]: Temperature increases with height in the thermosphere from the mesopause up to the ], then is constant with height. The temperature of this layer can rise to {{convert|1500|C|abbr=on}}, though the gas molecules are so far apart that ] is not well defined. The ] orbits in this layer, between {{convert|320|and|380|km|mi|abbr=on}}. The top of the thermosphere is the bottom of the exosphere, called the ]. Its height varies with solar activity and ranges from about {{convert|350|-|800|km|mi ft|abbr=on}}.

; ]: The mesosphere extends from the stratopause to {{convert|80|-|85|km|mi ft|abbr=on}}. It is the layer where most ]s burn up upon entering the atmosphere. Temperature decreases with height in the mesosphere. The ], the temperature minimum that marks the top of the mesosphere, is the coldest place on Earth and has an average temperature around {{convert|-100|C|F K|abbr=on|lk=on}}.

; ]: The stratosphere extends from the tropopause to about {{convert|51|km|mi ft|abbr=on}}. Temperature increases with height, which restricts turbulence and mixing. The ], which is the boundary between the stratosphere and mesosphere, typically is at {{convert|50|to|55|km|mi ft|abbr=on}}. The pressure here is 1/1000th ].
; ]: The troposphere begins at the surface and extends to between {{convert|7|km|ft|abbr=on}} at the poles and {{convert|17|km|ft|abbr=on}} at the equator, with some variation due to weather. The troposphere is mostly heated by transfer of energy from the surface, so on average the lowest part of the troposphere is warmest and temperature decreases with altitude. This promotes vertical mixing (hence the origin of its name in the Greek word "τροπή", ''trope'', meaning turn or overturn). The troposphere contains roughly 80%{{Citation needed|date=November 2009}} of the mass of the atmosphere. The ] is the boundary between the troposphere and stratosphere.

====Other layers====

Within the five principal layers determined by temperature are several layers determined by other properties.

* The ] is contained within the stratosphere. In this layer ] concentrations are about 2 to 8 parts per million, which is much higher than in the lower atmosphere but still very small compared to the main components of the atmosphere. It is mainly located in the lower portion of the stratosphere from about {{convert|15|-|35|km|mi ft|abbr=on}}, though the thickness varies seasonally and geographically. About 90% of the ozone in our atmosphere is contained in the stratosphere.

* The ], the part of the atmosphere that is ionized by solar radiation, stretches from {{convert|50|to|1000|km|mi ft|abbr=on}} and typically overlaps both the exosphere and the thermosphere. It forms the inner edge of the magnetosphere. It has practical importance because it influences, for example, ] propagation on the Earth. It is responsible for ]s.

* The ] and ] are defined by whether the atmospheric gases are well mixed. In the ] the chemical composition of the atmosphere does not depend on molecular weight because the gases are mixed by turbulence.<ref></ref> The homosphere includes the troposphere, stratosphere, and mesosphere. Above the '']'' at about {{convert|100|km|mi ft|abbr=on}} (essentially corresponding to the mesopause), the composition varies with altitude. This is because the ] is large compared with the size of motions that cause mixing. This allows the gases to stratify by molecular weight, with the heavier ones such as oxygen and nitrogen present only near the bottom of the heterosphere. The upper part of the heterosphere is composed almost completely of hydrogen, the lightest element.

* The ] is the part of the troposphere that is nearest the Earth's surface and is directly affected by it, mainly through ]. During the day the planetary boundary layer usually is well-mixed, while at night it becomes stably stratified with weak or intermittent mixing. The depth of the planetary boundary layer ranges from as little as about 100 m on clear, calm nights to 3000 m or more during the afternoon in dry regions.

The average temperature of the atmosphere at the surface of Earth is {{convert|14|C|F K|abbr=on}}<ref>{{cite web|url=http://www.bambooweb.com/articles/e/a/Earth's_atmosphere.html| title=Earth's Atmosphere| first=|last=}}</ref> or {{convert|15|C|F K|abbr=on}}<ref></ref>, depending on the reference.<ref>{{cite web|url=http://www.ncdc.noaa.gov/oa/climate/research/anomalies/index.php| title=Global Surface Temperature Anomalies| first=|last=}}</ref>
<ref>{{cite web| url=http://oceanworld.tamu.edu/resources/oceanography-book/radiationbalance.htm| title=Earth's Radiation Balance and Oceanic Heat Fluxes| first=|last=}}</ref><ref>{{cite web| url=http://www-pcmdi.llnl.gov/projects/cmip/overview_ms/control_tseries.pdf
| title=Coupled Model Intercomparison Project Control Run| first=|last=}}</ref>

==Physical properties==
=== Pressure and thickness ===
{{Main|Atmospheric pressure}}

The average atmospheric pressure at ] is about 1 atmosphere (atm) = 101.3 kPa (kilopascals) = 14.7 psi (pounds per square inch) = 760 torr = 29.9&nbsp;inches of mercury (symbol Hg). Total atmospheric mass is 5.1480×10<sup>18</sup> kg (1.135×10<sup>19</sup> lb),<ref></ref> about 2.5% less than would be inferred naively from the average sea level pressure and the Earth's area of 51007.2 megahectares, this defect having been displaced by the Earth's mountainous terrain. Atmospheric pressure is the total weight of the air above unit area at the point where the pressure is measured. Thus air pressure varies with location and time, because the amount of air above the Earth's surface varies.

If atmospheric density were to remain constant with height the atmosphere would terminate abruptly at {{convert|8.50|km|ft|abbr=on}}. Instead, density decreases with height, dropping by 50% at an altitude of about {{convert|5.6|km|ft|abbr=on}}. As a result the pressure decrease is approximately exponential with height, so that pressure decreases by a factor of two approximately every {{convert|5.6|km|ft|abbr=on}} and by a factor of ''e'' = 2.718… approximately every {{convert|7.64|km|ft|abbr=on}}, the latter being the average ] of Earth's atmosphere below {{convert|70|km|mi ft|abbr=on}}. However, because of changes in temperature, average molecular weight, and gravity throughout the atmospheric column, the dependence of atmospheric pressure on altitude is modeled by separate equations for each of the layers listed above. Even in the exosphere, the atmosphere is still present. This can be seen by the effects of ] on ]s.

In summary, the equations of pressure by altitude in the above references can be used directly to estimate atmospheric thickness. However, the following published data are given for reference:<ref>Lutgens, Frederick K. and Edward J. Tarbuck (1995) ''The Atmosphere'', Prentice Hall, 6th ed., pp14-17, ISBN 0-13-350612-6</ref>
* 50% of the atmosphere by mass is below an altitude of {{convert|5.6|km|ft|abbr=on}}.
* 90% of the atmosphere by mass is below an altitude of {{convert|16|km|ft|abbr=on}}. The common altitude of commercial airliners is about {{convert|10|km|ft|abbr=on}} and Mt. Everest's summit is {{convert|8848|m|ft|abbr=on}} above sea level.
* 99.99997% of the atmosphere by mass is below {{convert|100|km|mi ft|abbr=on}}, although in the rarefied region above this there are ]s and other atmospheric effects. The highest ] plane flight in 1963 reached an altitude of {{convert|354300|ft|km|abbr=on}}.

=== Density and mass ===
] ] model]]

{{Main|Density of air}}

The density of air at sea level is about 1.2 &nbsp;kg/m<sup>3</sup> (1.2 g/L). Density is not measured directly but is calculated from measurements of temperature, pressure and humidity using the equation of state for air (a form of the ]). Atmospheric density decreases as the altitude increases. This variation can be approximately modeled using the ]. More sophisticated models are used to predict orbital decay of satellites.

The average mass of the atmosphere is about 5 quadrillion (5{{e|15}}) ]s or 1/1,200,000 the mass of Earth. According to the ], "The total mean mass of the atmosphere is 5.1480{{E|18}} kg with an annual range due to water vapor of 1.2 or 1.5{{E|15}} kg depending on whether surface pressure or water vapor data are used; somewhat smaller than the previous estimate. The mean mass of water vapor is estimated as 1.27{{E|16}} kg and the dry air mass as 5.1352 ±0.0003{{E|18}} kg."

== Optical properties ==
{{See also|Sunlight}}

Solar ] (or sunlight) is the energy the Earth receives from the ]. The Earth also emits radiation back into space, but at longer wavelengths that we cannot see. Part of the incoming and emitted radiation is absorbed or reflected by the atmosphere.

=== Scattering ===
{{Main|Scattering}}

When light passes through our atmosphere, ]s interact with it through ''scattering''. If the light does not interact with the atmosphere, it is called ''direct radiation'' and is what you see if you were to look directly at the Sun. ''Indirect radiation'' is light that has been scattered in the atmosphere. For example, on an ] day when you cannot see your shadow there is no direct radiation reaching you, it has all been scattered. As another example, due to a phenomenon called ], shorter (blue) wavelengths scatter more easily than longer (red) wavelengths. This is why the sky looks blue, you are seeing scattered blue light. This is also why ]s are red. Because the Sun is close to the horizon, the Sun's rays pass through more atmosphere than normal to reach your eye. Much of the blue light has been scattered out, leaving the red light in a sunset.

=== Absorption ===
{{Main|Absorption (electromagnetic radiation)}}
Different molecules absorb different wavelengths of radiation. For example, O<sub>2</sub> and O<sub>3</sub> absorb almost all wavelengths shorter than 300 ]s. Water (H<sub>2</sub>O) absorbs many wavelengths above 700&nbsp;nm. When a molecule absorbs a photon, it increases the energy of the molecule. We can think of this as heating the atmosphere, but the atmosphere also cools by emitting radiation, as discussed below.

] (or opacity) to various wavelengths of electromagnetic radiation, including ].]]

The combined ] of the gases in the atmosphere leave "windows" of low ], allowing the transmission of only certain bands of light. The ] runs from around 300&nbsp;nm (]-C) up into the range humans can see, the ] (commonly called ]), at roughly 400–700&nbsp;nm and continues to the ] to around 1100&nbsp;nm. There are also ] and ]s that transmit some infrared and ] at longer wavelengths. For example, the radio window runs from about one centimeter to about eleven-meter waves.

=== Emission ===
{{Main|Emission (electromagnetic radiation)}}
''Emission'' is the opposite of absorption, it is when an object emits radiation. Objects tend to emit amounts and wavelengths of radiation depending on their "]" emission curves, therefore hotter objects tend to emit more radiation, with shorter wavelengths. Colder objects emit less radiation, with longer wavelengths. For example, the Sun is approximately {{convert|6000|K|lk=on}}, its radiation peaks near 500&nbsp;nm, and is visible to the human eye. The Earth is approximately {{convert|290|K|abbr=on}}, so its radiation peaks near 10,000&nbsp;nm, and is much too long to be visible to humans.

Because of its temperature, the atmosphere emits infrared radiation. For example, on clear nights the Earth's surface cools down faster than on ]y nights. This is because clouds (H<sub>2</sub>O) are strong absorbers and emitters of infrared radiation. This is also why it becomes colder at night at higher elevations. The atmosphere acts as a "blanket" to limit the amount of radiation the Earth loses into space.

The ''greenhouse effect'' is directly related to this absorption and emission (or "blanket") effect. Some chemicals in the atmosphere absorb and emit infrared radiation, but do not interact with sunlight in the visible spectrum. Common examples of these chemicals are CO<sub>2</sub> and H<sub>2</sub>O. If there are too much of these ''greenhouse gases'', sunlight heats the Earth's surface, but the gases block the infrared radiation from exiting back to space. This imbalance causes the Earth to warm, and thus ].

=== Refractive index ===
The ] of air is close to, but just greater than 1. Systematic variations in refractive index can lead to the bending of light rays over long optical paths. One example is that, under some circumstances, observers onboard ships can see other vessels just over the ] because light is refracted in the same direction as the ] of the Earth's surface.

The refractive index of air depends on temperature, giving rise to refraction effects when the temperature gradient is large. An example of such effects is the ].

:''See also: ]''

== Circulation ==
{{Main|Atmospheric circulation}}

]
''Atmospheric circulation'' is the large-scale movement of air, and the means (with ]) by which ] is distributed around the Earth. The large-scale structure of the atmospheric circulation varies from year to year, but the basic structure remains fairly constant as it is determined by the Earth's rotation rate and the difference in solar radiation between the equator and poles.

== Evolution of Earth's atmosphere ==
{{See also|History of Earth|Gaia hypothesis|Paleoclimatology}}
=== Second atmosphere ===
Water related sediments have been found dating from as early as 3.8 billion years ago.<ref>B. Windley: ''The Evolving Continents.'' Wiley Press, New York 1984</ref> About 3.4 billion years ago, nitrogen was the major part of the then stable "second atmosphere." An influence of life has to be taken into account rather soon in the history of the atmosphere, since hints of early life forms are to be found as early as 3.5 billion years ago.<ref>J. Schopf: ''Earth’s Earliest Biosphere: Its Origin and Evolution.'' Princeton University Press, Princeton, N.J., 1983</ref> The fact that this is not perfectly in line with the - compared to today 30% lower - solar radiance of the early Sun has been described as the "]".

The geological record however shows a continually relatively warm surface during the complete early ] of the Earth with the exception of one cold glacial phase about 2.4 billion years ago. Sometime during the late ] era an oxygen-containing atmosphere began to develop, apparently from photosynthesizing algae which have been found as ] fossils from 2.7 billion years ago. The early basic carbon ] (isotope ratio proportions) is very much in line with what is found today,<ref name="CCD">Celestial climate driver: a perspective from 4 billion years of the carbon cycle Geoscience Canada, March, 2005 by Jan Veizer</ref> suggesting that the fundamental features of the carbon cycle were established as early as 4 billion years ago.

=== Third atmosphere ===
]

The accretion of continents about 3.5 billion years ago<ref>Veizer in B. F. Windley (ed.), The Early History of the Earth, John Wiley and Sons, London, p. 569., 1976</ref> added ], constantly rearranging the continents and also shaping long-term climate evolution by allowing the transfer of carbon dioxide to large land-based carbonate storages. Free oxygen did not exist until about 1.7 billion years ago and this can be seen with the development of the red beds and the end of the banded iron formations. This signifies a shift from a reducing atmosphere to an oxidising atmosphere. O<sub>2</sub> showed major ups and downs until reaching a steady state of more than 15%.<ref>Christopher R. Scotese, , Paleomar Project</ref> The following time span was the ] era, during which oxygen-breathing ] life forms began to appear.

Currently, ] greenhouse gases are increasing in the atmosphere. According to the Intergovernmental Panel on Climate Change, this increase is the main cause of ].<ref name="grida7">{{cite web | url= http://ipcc-wg1.ucar.edu/wg1/Report/AR4WG1_Print_SPM.pdf | format=] | title=Summary for Policymakers | work=Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change | date=5 February 2007 | publisher=]}}</ref>

=== Air pollution ===
{{Main|Air pollution}}

''Air pollution'' is the human introduction of ]s, ], or ]s that cause harm or discomfort to organisms into the atmosphere.<ref>Starting from Pollution - Definition from the Merriam-Webster Online Dictionary</ref> ] ] is believed to be caused by air pollution (chiefly from ]).{{Citation needed|date=October 2009}}

While ]s are often identified with air pollution, the greatest ] is actually mobile sources, principally the ].{{Citation needed|date=October 2009}}

== See also ==
{{portal|Atmosphere}}
* ]
* ]
* ]
* ] (for information on atmospheres in general)
* ]
* ]
* ]
* ] (ARM) (in the U.S.)
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]
* ]

== References ==
{{reflist|2}}

== External links ==
{{Commons category|Earth's atmosphere}}
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* Find out what the atmosphere contains.
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* Free video of Paul Crutzen Nobel Laureate for his work on decomposition of ozone talking to Harry Kroto Nobel Laureate by the Vega Science Trust.
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{{Earthsatmosphere}}
{{Atmospheres}}
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Revision as of 02:17, 8 March 2010

"Air" redirects here. For other uses, see Air (disambiguation).

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