View source for Atmosphere of Earth

{{Short description|Gas layer surrounding Earth}}
{{Redirect|Air}}
{{Redirect-distinguish|Qualities of air|Air pollution{{!}}Air quality}}
{{pp-semi-indef}}

[[File:Top of Atmosphere.jpg|thumb|[[Rayleigh scattering|Blue light is scattered]] more than other wavelengths by the gases in the atmosphere, surrounding Earth in a visibly blue layer at the [[stratosphere]], above the clouds of the [[troposphere]], when seen from space on board the [[International Space Station|ISS]] at an altitude of {{cvt|335|km|mi}} (the [[Moon]] is visible as a crescent in the far background).<ref>{{cite web |url=https://eol.jsc.nasa.gov/SearchPhotos/photo.pl?mission=ISS013&roll=E&frame=54329 |title=Gateway to Astonaut Photos of Earth |publisher=NASA |access-date=2018-01-29}}</ref>|330x330px]]The '''atmosphere of Earth''' is the layer of [[gas]]es, known collectively as '''air''', retained by [[Gravity of Earth|Earth's gravity]] that surrounds the planet and forms its planetary [[atmosphere]]. The atmosphere of [[Earth]] creates [[Atmospheric pressure|pressure]], absorbs most [[meteoroid]]s and [[ultraviolet]] [[Solar irradiance|solar radiation]], warms the surface through heat retention ([[greenhouse effect]]), and reduces temperature extremes between [[Daytime|day]] and [[night]] (the [[diurnal temperature variation]]), maintaining conditions allowing [[life]] and [[Water#Liquid water|liquid water]] to exist on the Earth's [[Planetary surface|surface]].

As of 2023, by [[mole fraction]] (i.e., by number of molecules), dry air contains 78.08% [[nitrogen]], 20.95% [[oxygen]], 0.93% [[argon]], 0.04% [[Carbon dioxide in Earth's atmosphere|carbon dioxide]], and small amounts of other gases.{{refn |name=total% |Two recent reliable sources cited here have total atmospheric compositions, including trace molecules, that exceed 100%. They are ''Allen's Astrophysical Quantities''<ref name=Allen2000>{{citation |editor-last=Cox |editor-first=Arthur N. |title=Allen's Astrophysical Quantities |edition=Fourth |year=2000 |publisher=AIP Press |pages=258–259 |isbn=0-387-98746-0}}, which rounds N{{sub|2}} and O{{sub|2}} to four significant digits without affecting the total because 0.004% was removed from N{{sub|2}} and added to O{{sub|2}}. It includes 20 constituents.</ref> (2000, 100.001241343%) and ''CRC Handbook of Chemistry and Physics''<ref name=handbook>{{citation |editor-last=Haynes |editor-first= H. M. |title=CRC Handbook of Chemistry and Physics |publisher=CRC Press |edition=97th |year=2016–2017 |page=14{{hyphen}}3 |isbn=978-1-4987-5428-6}}, which cites ''Allen's Astrophysical Quantities'' but includes only ten of its largest constituents.</ref> (2016–2017, 100.004667%), which cites ''Allen's Astrophysical Quantities''. Both are used as references in this article. Both exceed 100% because their CO{{sub|2}} values were increased to 345&nbsp;ppmv, without changing their other constituents to compensate. This is made worse by the April 2019 {{CO2}} value, which is 413.32&nbsp;ppmv.<ref name=CO2>{{citation |title=Trends in Atmospheric Carbon Dioxide |url=https://www.esrl.noaa.gov/gmd/ccgg/trends/ |website=Global Greenhouse Gas Reference Network, NOAA |year=2019 |access-date=2019-05-31}}</ref> Although minor, the January 2019 value for {{CH4}} is 1866.1&nbsp;ppbv (parts per billion).<ref name=methane>{{citation |title=Trends in Atmospheric Methane |website=Global Greenhouse Gas Reference Network, NOAA |url=https://www.esrl.noaa.gov/gmd/ccgg/trends_ch4/ |year=2019 |access-date=2019-05-31}}</ref> Two older reliable sources have dry atmospheric compositions, including trace molecules, that total less than 100%: ''U.S. Standard Atmosphere, 1976''<ref name=standard>{{citation |author=National Aeronautics and Space Administration |title=U.S. Standard Atmosphere, 1976 |url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19770009539.pdf |page=3 |year=1976}}</ref> (99.9997147%); and ''Astrophysical Quantities''<ref name=Allen1976>{{citation |last=Allen |first=C. W. |title=Astrophysical Quantities |url=https://archive.org/details/AstrophysicalQuantities/page/n127 |edition=Third |year=1976 |publisher=Athlone Press |page=119 |isbn=0-485-11150-0}}</ref> (1976, 99.9999357%).}} <!-- end refn -->Air also contains a variable amount of [[Water vapor#Water vapor in Earth's atmosphere|water vapor]], on average around 1% at sea level, and 0.4% over the entire atmosphere. Air composition, temperature, and [[atmospheric pressure]] vary with altitude. Within the atmosphere, air suitable for use in [[photosynthesis]] by [[terrestrial plant]]s and [[breathing]] of [[terrestrial animal]]s is found only in Earth's [[troposphere]].{{citation needed|date=July 2021}}

Earth's early atmosphere consisted of gases in the [[solar nebula]], primarily hydrogen. The atmosphere changed significantly over time, affected by many factors such as [[volcanism]], [[life]], and [[weathering]]. Recently, human activity has also contributed to [[Human impact on the environment#Impacts on Climate|atmospheric changes]], such as [[Climate change|global warming]], [[ozone depletion]] and [[acid deposition]].

The atmosphere has a mass of about 5.15{{e|18}}&nbsp;kg,<ref>Lide, David R. ''Handbook of Chemistry and Physics''. Boca Raton, FL: CRC, 1996: 14–17</ref> three quarters of which is within about {{convert|11|km|mi ft|abbr=on}} of the surface. The atmosphere becomes thinner with increasing altitude, with no definite boundary between the atmosphere and [[outer space]]. The [[Kármán line]], at {{convert|100|km|mi|abbr=on}} or 1.57% of Earth's radius, is often used as the border between the atmosphere and outer space. Atmospheric effects become noticeable during [[Atmospheric entry|atmospheric reentry]] of spacecraft at an altitude of around {{convert|120|km|mi|abbr=on}}. Several [[#Stratification|layers]] can be distinguished in the atmosphere, based on characteristics such as temperature and composition.

The study of Earth's atmosphere and its processes is called [[atmospheric science]] (aerology), and includes multiple subfields, such as [[climatology]] and [[atmospheric physics]]. Early pioneers in the field include [[Léon Teisserenc de Bort]] and [[Richard Assmann]].<ref>{{cite book|author1=Vázquez, M.|author2=Hanslmeier, A.|title=Ultraviolet Radiation in the Solar System|chapter=Historical Introduction|series=Astrophysics and Space Science Library|volume=331|chapter-url=https://books.google.com/books?id=OlckxY7BA_0C&pg=PA17|year=2006|publisher=Springer Science & Business Media|isbn=978-1-4020-3730-6|page=17|bibcode=2005ASSL..331.....V|doi=10.1007/1-4020-3730-9_1}}</ref> The study of historic atmosphere is called [[paleoclimatology]].

==Composition==
{{Main|Atmospheric chemistry}}[[File:Atmosphere gas proportions.svg|thumb|Composition of Earth's atmosphere by molecular count, excluding water vapor. Lower pie represents trace gases that together compose about 0.0434% of the atmosphere (0.0442% at August 2021 concentrations<ref name="CO2" /><ref name="methane" />). Numbers are mainly from 2000, with {{CO2}} and methane from 2019, and do not represent any single source.<ref name="handbook" />]]

The three major constituents of Earth's atmosphere are [[nitrogen]], [[oxygen]], and [[argon]]. Water vapor accounts for roughly 0.25% of the atmosphere by mass. The concentration of water vapor (a greenhouse gas) varies significantly from around 10 ppm by mole fraction in the coldest portions of the atmosphere to as much as 5% by mole fraction in hot, humid air masses, and concentrations of other atmospheric gases are typically quoted in terms of dry air (without water vapor).<ref name="WallaceHobbs">Wallace, John M. and Peter V. Hobbs. [http://cup.aos.wisc.edu/453/2016/readings/Atmospheric_Science-Wallace_Hobbs.pdf ''Atmospheric Science: An Introductory Survey''] {{Webarchive|url=https://web.archive.org/web/20180728040037/http://cup.aos.wisc.edu/453/2016/readings/Atmospheric_Science-Wallace_Hobbs.pdf |date=2018-07-28 }}. Elsevier. Second Edition, 2006. {{ISBN|978-0-12-732951-2}}. Chapter 1</ref>{{rp|8}} The remaining gases are often referred to as trace gases,<ref>{{cite web |url=http://www.ace.mmu.ac.uk/eae/Atmosphere/Older/Trace_Gases.html |title=Trace Gases |publisher=Ace.mmu.ac.uk |access-date=2010-10-16 |archive-url=https://web.archive.org/web/20101009044345/http://www.ace.mmu.ac.uk/eae/atmosphere/older/Trace_Gases.html |archive-date=9 October 2010 |url-status=dead }}</ref> among which are other [[greenhouse gas]]es, principally carbon dioxide, methane, nitrous oxide, and ozone. Besides argon, already mentioned, other [[noble gas]]es, neon, helium, krypton, and xenon are also present. Filtered air includes trace amounts of many other [[chemical compound]]s. Many substances of natural origin may be present in locally and seasonally variable small amounts as [[aerosol]]s in an unfiltered air sample, including [[dust]] of mineral and organic composition, [[pollen]] and [[spores]], [[sea spray]], and [[volcanic ash]]. Various industrial [[pollutant]]s also may be present as gases or aerosols, such as [[chlorine]] (elemental or in compounds), [[fluorine]] compounds and elemental [[Mercury (element)|mercury]] vapor. Sulfur compounds such as [[hydrogen sulfide]] and [[sulfur dioxide]] (SO<sub>2</sub>) may be derived from natural sources or from industrial air pollution.

{| class="wikitable" style="text-align: center;"
|+'''Major constituents of air, by mole fraction'''<ref name=total%/>
! colspan=4 | Dry air
|-
! colspan=2 | Gas
! colspan=2 style="font-weight: normal;" | '''Mole fraction'''<sup>(A)</sup>
|-
! style="width: 100px;" | Name
! style="width:  50px;" | Formula
! style="width: 100px; font-weight: normal;" | '''in [[Parts-per notation|ppm]]'''<sup>(B)</sup>
! style="width: 100px;" | in [[Percentage|%]]
|-
| align=left | [[Nitrogen]]
| N<sub>2</sub>
| 780,840
| 78.084
|-
| align=left | [[Oxygen]]
| O<sub>2</sub>
| 209,460
| 20.946
|-
| align=left | [[Argon]]
| Ar
| 9,340
| 0.9340
|-
| align=left | [[Carbon&nbsp;dioxide]]<br />(April 2022)<sup>(C)</sup><ref>{{Cite web|date=April 2022|title=Vital signs: Carbon Dioxide|url=https://climate.nasa.gov/vital-signs/carbon-dioxide/|access-date=16 May 2022|website=NASA Climate}}</ref>
| {{CO2}}
| 417
| 0.0417
|-
| align=left | [[Neon]]
| Ne
| 18.18
| 0.001818
|-
| align=left | [[Helium]]
| He
| 5.24
| 0.000524
|-
| align=left | [[Methane]]<br />(2022)<sup>(C)</sup><ref>{{Cite web|date=April 2022|title=Vital signs: Methane|url=https://climate.nasa.gov/vital-signs/methane/|access-date=2 Feb 2024|website=NASA Climate}}</ref>
| CH<sub>4</sub>
| 1.91
| 0.000191
|-
| align=left | [[Krypton]]
| Kr
| 1.14
| 0.000114
|-
! colspan=4 | If air is not dry:
|-
| align=left | [[Water vapor]]<sup>(D)</sup>
| H<sub>2</sub>O
| 0–30,000<sup>(D)</sup>
| 0–3%<sup>(E)</sup>
|-
! colspan=4 style="font-size: 0.85em; padding: 5px 2px 5px 10px; text-align: left; font-weight: normal;" | '''notes:'''
*<sup>(A)</sup> [[Mole fraction]] is sometimes referred to as [[volume fraction]]; these are identical for an ideal gas only.
*<sup>(B)</sup> ppm: [[parts per million]] by molecular count
**The total ppm above adds up to more than 1 million (currently 83.43 above it) due to [[experimental error]].
*<sup>(C)</sup> The concentration of {{CO2}} has been [[Keeling Curve|increasing in recent decades]], as has that of {{CH4}}.
*<sup>(D)</sup> Water vapor is about 0.25% ''by mass'' over full atmosphere
*<sup>(E)</sup> Water vapor varies significantly locally<ref name="WallaceHobbs" />
|}

The average [[molecular weight]] of dry air, which can be used to calculate densities or to convert between mole fraction and mass fraction, is about 28.946<ref name="Möller2003">Detlev Möller: ''Luft: Chemie, Physik, Biologie, Reinhaltung, Recht.'' Walter de Gruyter, 2003, {{ISBN|3-11-016431-0}}, S. 173. [https://books.google.com/books?id=7ITJXWDijA0C&pg=PA173 (View in Google Books)].</ref> or 28.96<ref>{{cite book | author=Yunus Çengel| title= Termodinamica e trasmissione del calore  }}</ref><ref>{{Cite web|title=Air - Molecular Weight and Composition|url=https://www.engineeringtoolbox.com/molecular-mass-air-d_679.html|access-date=2021-04-27|website=www.engineeringtoolbox.com}}</ref>&nbsp;g/mol. This is decreased when the air is humid.

The relative concentration of gases remains constant until about {{convert|10000|m|ft|abbr=on|sigfig=2}}.<ref>{{cite web |url=http://www.engineeringtoolbox.com/air-composition-d_212.html |title=Air Composition |publisher=The Engineering ToolBox |quote=The composition of air is unchanged until elevation of approximately 10.000 m |access-date=2017-07-04}}</ref>

== Stratification ==
[[File:Earth's atmosphere.svg|thumb|Earth's atmosphere. Lower four layers of the atmosphere in three dimensions as seen diagonally from above the exobase. Layers drawn to scale, objects within the layers are not to scale. Aurorae shown here at the bottom of the thermosphere can actually form at any altitude in this atmospheric layer.]]

In general, air pressure and density decrease with altitude in the atmosphere. However, the temperature has a more complicated profile with altitude, and may remain relatively constant or even increase with altitude in some regions (see the [[#Temperature|temperature]] section, below). Because the general pattern of the temperature/altitude profile, or [[lapse rate]], is constant and measurable by means of instrumented [[Weather balloon|balloon soundings]], the temperature behavior provides a useful metric to distinguish atmospheric layers. In this way, Earth's atmosphere can be divided (called atmospheric stratification) into five main layers: troposphere, stratosphere, mesosphere, thermosphere, and exosphere.<ref>{{Cite news |url=http://www.nasa.gov/mission_pages/sunearth/science/mos-upper-atmosphere.html#.VHg5AzHF8vY |title=Earth's Upper Atmosphere |last=Zell |first=Holly |date=2015-03-02 |newspaper=NASA|access-date=2017-02-20 |language=en}}</ref> The altitudes of the five layers are as follows:
* Exosphere: 700 to 10,000&nbsp;km (440 to 6,200 miles)<ref name="UCAR-2011">{{cite web|title=Exosphere - overview|url=http://scied.ucar.edu/shortcontent/exosphere-overview|date=2011|publisher=UCAR|access-date=April 19, 2015|archive-url=https://web.archive.org/web/20170517071138/https://scied.ucar.edu/shortcontent/exosphere-overview|archive-date=17 May 2017|url-status=dead}}</ref>
* Thermosphere: 80 to 700&nbsp;km (50 to 440 miles)<ref name="thermosphere">{{cite web |author=Randy Russell |title=The Thermosphere |year=2008 |url=http://www.windows2universe.org/earth/Atmosphere/thermosphere.html |access-date=2013-10-18}}</ref>
* Mesosphere: 50 to 80&nbsp;km (31 to 50 miles)
* Stratosphere: 12 to 50&nbsp;km (7 to 31 miles)
* Troposphere: 0 to 12&nbsp;km (0 to 7 miles)<ref name=tropopauseheight>{{cite web |url=http://www-das.uwyo.edu/~geerts/cwx/notes/chap01/tropo.html |title=The height of the tropopause |publisher=Das.uwyo.edu |access-date=2012-04-18}}</ref>

===Exosphere===
{{main|Exosphere}}
The exosphere is the outermost layer of Earth's atmosphere (though it is so tenuous that some scientists consider it to be part of interplanetary space rather than part of the atmosphere). It extends from the [[thermopause]] (also known as the "exobase") at the top of the [[thermosphere]] to a poorly defined boundary with the [[solar wind]] and [[interplanetary medium]]. The altitude of the exobase varies from about {{convert|500|km|mi ft}} to about {{convert|1000|km}} in times of higher incoming solar radiation.<ref name="UCAR">{{cite web |title=Exosphere - overview |url=http://scied.ucar.edu/shortcontent/exosphere-overview |date=2011 |publisher=UCAR |access-date=April 19, 2015 |archive-url=https://web.archive.org/web/20170517071138/https://scied.ucar.edu/shortcontent/exosphere-overview |archive-date=17 May 2017 |url-status=live }}</ref>

The upper limit varies depending on the definition. Various authorities consider it to end at about {{convert|10000|km}}<ref>{{cite web |url=https://www.nasa.gov/mission_pages/sunearth/science/atmosphere-layers2.html |date=January 22, 2013 |title=Earth's Atmospheric Layers}}</ref> or about {{convert|190000|km|mi}}—about halfway to the moon, where the influence of Earth's gravity is about the same as [[radiation pressure]] from sunlight.<ref name="UCAR" /> The [[geocorona]] visible in the far ultraviolet (caused by neutral hydrogen) extends to at least {{convert|100000|km}}.<ref name="UCAR" />

This layer is mainly composed of extremely low densities of hydrogen, helium and several heavier molecules including nitrogen, oxygen and carbon dioxide closer to the exobase. The atoms and molecules are so far apart that they can travel hundreds of kilometres without colliding with one another. Thus, the exosphere no longer behaves like a gas, and the particles constantly [[atmospheric escape|escape into space]]. These free-moving particles follow [[ballistics|ballistic]] [[trajectory|trajectories]] and may migrate in and out of the [[magnetosphere]] or the solar wind. Every second, the Earth loses about 3&nbsp;kg of hydrogen, 50&nbsp;g of helium, and much smaller amounts of other constituents.<ref name="Catling200922">David C. Catling and Kevin J. Zahnle, [http://faculty.washington.edu/dcatling/Catling2009_SciAm.pdf The Planetary Air Leak], ''Scientific American,'' May 2009, p. 26 (accessed 25 July 2012)</ref>

The exosphere is too far above Earth for [[meteorology|meteorological]] phenomena to be possible. However, Earth's [[aurora]]s—the aurora borealis (northern lights) and aurora australis (southern lights)—sometimes occur in the lower part of the exosphere, where they overlap into the thermosphere. The exosphere contains many of the [[Satellite|artificial satellite]]s that [[orbit]] Earth.

===Thermosphere===
{{main|Thermosphere}}
The thermosphere is the second-highest layer of Earth's atmosphere. It extends from the mesopause (which separates it from the mesosphere) at an altitude of about {{convert|80|km|mi ft|abbr=on|sigfig=2}} up to the [[thermopause]] at an altitude range of {{convert|500|-|1,000|km|mi ft|abbr=on|sigfig=2|comma=5}}. The height of the thermopause varies considerably due to changes in solar activity.<ref name="thermosphere"/> Because the thermopause lies at the lower boundary of the exosphere, it is also referred to as the [[exobase]]. The lower part of the thermosphere, from {{convert|80|to|550|km}} above Earth's surface, contains the [[ionosphere]].

The temperature of the thermosphere gradually increases with height and can rise as high as {{convert|1,500|C|abbr=on|sigfig=2|comma=5}}, though the gas molecules are so far apart that its [[Kinetic theory of gases|temperature in the usual sense]] is not very meaningful. The air is so rarefied that an individual molecule (of [[oxygen]], for example) travels an average of {{convert|1|km|mi ft|comma=5}} between collisions with other molecules.<ref>Ahrens, C. Donald. ''Essentials of Meteorology''. Published by Thomson Brooks/Cole, 2005.</ref> Although the thermosphere has a high proportion of molecules with high energy, it would not feel hot to a human in direct contact, because its density is too low to conduct a significant amount of energy to or from the skin.

This layer is completely cloudless and free of water vapor. However, non-hydrometeorological phenomena such as the [[aurora borealis]] and [[aurora australis]] are occasionally seen in the thermosphere. The [[International Space Station]] orbits in this layer, between {{convert|350|and|420|km|mi|abbr=on|sigfig=2}}. It is this layer where many of the satellites orbiting the Earth are present.

===Mesosphere===
{{main|Mesosphere}}
[[File:ISS-46 Soyuz TMA-17M reentry.jpg|thumb|upright=1.25|[[Afterglow]] of the [[troposphere]] (orange), the [[stratosphere]] (blue) and the mesosphere (dark) at which [[atmospheric entry]] begins, leaving smoke trails, such as in this case of a [[spacecraft]] reentry]]
The mesosphere is the third highest layer of Earth's atmosphere, occupying the region above the stratosphere and below the thermosphere. It extends from the stratopause at an altitude of about {{convert|50|km|mi ft|abbr=on|sigfig=2}} to the mesopause at {{convert|80|-|85|km|mi ft|abbr=on|sigfig=2}} above sea level.

Temperatures drop with increasing altitude to the [[mesopause]] that marks the top of this middle layer of the atmosphere. It is the coldest place on Earth and has an average temperature around {{convert|-85|C|F K|abbr=on|lk=on|sigfig=2}}.<ref>{{Cite journal |last1=States |first1=Robert J. |last2=Gardner |first2=Chester S. |title=Thermal Structure of the Mesopause Region (80–105 km) at 40°N Latitude. Part I: Seasonal Variations |journal=Journal of the Atmospheric Sciences  |volume=57 |issue=1 |pages=66–77 |date=January 2000 |doi=10.1175/1520-0469(2000)057<0066:TSOTMR>2.0.CO;2 |bibcode=2000JAtS...57...66S|doi-access=free }}
</ref><ref>{{cite web |author=Joe Buchdahl |url=http://www.ace.mmu.ac.uk/eae/Atmosphere/Older/Mesosphere.html |title=Atmosphere, Climate & Environment Information Programme |publisher=Ace.mmu.ac.uk |access-date=2012-04-18 |url-status=dead |archive-url=https://web.archive.org/web/20100701030705/http://www.ace.mmu.ac.uk/eae/atmosphere/older/mesosphere.html |archive-date=2010-07-01 }}</ref>

Just below the mesopause, the air is so cold that even the very scarce water vapor at this altitude can condense into polar-mesospheric [[noctilucent cloud]]s of ice particles. These are the highest clouds in the atmosphere and may be visible to the naked eye if sunlight reflects off them about an hour or two after sunset or similarly before sunrise. They are most readily visible when the Sun is around 4 to 16 degrees below the horizon. Lightning-induced discharges known as [[transient luminous event]]s (TLEs) occasionally form in the mesosphere above tropospheric [[thunderclouds]]. The mesosphere is also the layer where most [[meteor]]s burn up upon atmospheric entrance. It is too high above Earth to be accessible to jet-powered aircraft and balloons, and too low to permit orbital spacecraft. The mesosphere is mainly accessed by [[sounding rocket]]s and [[rocket-powered aircraft]].

===Stratosphere===
{{main|Stratosphere}}

The stratosphere is the second-lowest layer of Earth's atmosphere. It lies above the troposphere and is separated from it by the [[tropopause]]. This layer extends from the top of the troposphere at roughly {{convert|12|km|mi ft|abbr=on}} above Earth's surface to the [[stratopause]] at an altitude of about {{convert|50|to|55|km|mi ft|abbr=on}}.

The atmospheric pressure at the top of the stratosphere is roughly 1/1000 the [[pressure at sea level]]. It contains the [[ozone layer]], which is the part of Earth's atmosphere that contains relatively high concentrations of that gas. The stratosphere defines a layer in which temperatures rise with increasing altitude. This rise in temperature is caused by the absorption of [[ultraviolet radiation]] (UV) from the Sun by the ozone layer, which restricts turbulence and mixing. Although the temperature may be {{convert|-60|C|F K|abbr=on|sigfig=2}} at the tropopause, the top of the stratosphere is much warmer, and may be near 0&nbsp;°C.<ref name="stratopause">{{cite web |author=Journal of the Atmospheric Sciences |title=stratopause |year=1993 |url=http://www.webref.org/chemistry/s/stratopause.htm |access-date=2013-10-18 |archive-date=2013-10-19 |archive-url=https://web.archive.org/web/20131019124644/http://www.webref.org/chemistry/s/stratopause.htm |url-status=dead }}</ref>

The stratospheric temperature profile creates very stable atmospheric conditions, so the stratosphere lacks the weather-producing air turbulence that is so prevalent in the troposphere. Consequently, the stratosphere is almost completely free of clouds and other forms of weather. However, polar stratospheric or [[nacreous cloud]]s are occasionally seen in the lower part of this layer of the atmosphere where the air is coldest. The stratosphere is the highest layer that can be accessed by [[jet-powered aircraft]].

===Troposphere===
{{main|Troposphere}}
[[File:ISS-47_Islands_In_The_Sky,_Indonesia.jpg|thumb|upright=1.25|A picture of Earth's troposphere with its different [[cloud types]] of low to [[high altitude]]s casting shadows. Sunlight is reflected off the ocean, after it was filtered into a redish light by passing through much of the troposphere at sunset. The above lying [[stratosphere]] can be seen at the [[horizon]] as a band of its characteristic glow of [[Rayleigh scattering|blue scattered]] sunlight.]]
The troposphere is the lowest layer of Earth's atmosphere. It extends from Earth's surface to an average height of about {{cvt|12|km|mi ft}}, although this [[altitude]] varies from about {{cvt|9|km|mi ft}} at the [[geographic pole]]s to {{cvt|17|km|mi ft}} at the [[Equator]],<ref name=tropopauseheight/> with some variation due to weather. The troposphere is bounded above by the [[tropopause]], a boundary marked in most places by a [[temperature inversion]] (i.e. a layer of relatively warm air above a colder one), and in others by a zone that is [[Isotherm (contour line)|isothermal]] with height.<ref>{{cite book |last1=Barry |first1=R.G. |last2=Chorley |first2=R.J. |year=1971 |title=Atmosphere, Weather and Climate |url=https://archive.org/details/atmosphereweathe0000barr_m3n2 |url-access=registration |location=London |publisher=Menthuen & Co Ltd. |page=[https://archive.org/details/atmosphereweathe0000barr_m3n2/page/65 65]|isbn=9780416079401 }}</ref><ref>{{cite book |last1=Tyson |first1=P.D. |last2=Preston-Whyte |first2=R.A. |year=2013 |title=The Weather and Climate of Southern Africa |edition=2nd |location=Oxford |publisher=[[Oxford University Press]] |page=4}}</ref>

Although variations do occur, the temperature usually declines with increasing altitude in the troposphere because the troposphere is mostly heated through energy transfer from the surface. Thus, the lowest part of the troposphere (i.e. Earth's surface) is typically the warmest section of the troposphere. This promotes vertical mixing (hence, the origin of its name in the Greek word τρόπος, ''tropos'', meaning "turn"). The troposphere contains roughly 80% of the [[mass]] of Earth's atmosphere.<ref>{{cite book |publisher=[[McGraw-Hill]] |title=Concise Encyclopedia of Science & Technology |year=1984 |section=Troposphere |quote=It contains about four-fifths of the mass of the whole atmosphere.}}</ref> The troposphere is denser than all its overlying layers because a larger atmospheric weight sits on top of the troposphere and causes it to be most severely compressed. Fifty percent of the total mass of the atmosphere is located in the lower {{cvt|5.6|km|mi ft}} of the troposphere.

Nearly all atmospheric water vapor or moisture is found in the troposphere, so it is the layer where most of Earth's weather takes place. It has basically all the weather-associated cloud genus types generated by active wind circulation, although very tall cumulonimbus thunder clouds can penetrate the tropopause from below and rise into the lower part of the stratosphere. Most conventional [[aviation]] activity takes place in the troposphere, and it is the only layer accessible by [[Propeller (aeronautics)|propeller-driven aircraft]].

===Other layers===
[[File:Msis_atmospheric_composition_by_height.svg|thumb|339x339px|The volume fraction of the main constituents of the Earth's atmosphere as a function of height, based on the [https://www.ukssdc.ac.uk/wdcc1/msise90.html MSIS-E-90 atmospheric model]; the model only works above 85km]]
Within the five principal layers above, which are largely determined by temperature, several secondary layers may be distinguished by other properties:
* The [[ozone layer]] is contained within the stratosphere. In this layer [[ozone]] 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 Earth's atmosphere is contained in the stratosphere.
* The [[ionosphere]] is a region of the atmosphere that is ionized by solar radiation. It is responsible for [[Aurora (astronomy)|auroras]]. During daytime hours, it stretches from {{convert|50|to|1000|km|mi ft|abbr=on}} and includes the mesosphere, thermosphere, and parts of the exosphere. However, ionization in the mesosphere largely ceases during the night, so auroras are normally seen only in the thermosphere and lower exosphere. The ionosphere forms the inner edge of the [[magnetosphere]]. It has practical importance because it influences, for example, radio propagation on Earth.
* The [[homosphere]] and [[heterosphere]] are defined by whether the atmospheric gases are well mixed. The surface-based homosphere includes the troposphere, stratosphere, mesosphere, and the lowest part of the thermosphere, where the chemical composition of the atmosphere does not depend on molecular weight because the gases are mixed by turbulence.<ref>{{cite web |url=http://amsglossary.allenpress.com/glossary/search?id=homosphere1 |title=''homosphere'' – AMS Glossary |publisher=Amsglossary.allenpress.com |access-date=2010-10-16 |archive-url=https://web.archive.org/web/20100914045832/http://amsglossary.allenpress.com/glossary/search?id=homosphere1 |archive-date=14 September 2010  |url-status=live}}</ref> This relatively homogeneous layer ends at the ''[[turbopause]]'' found at about {{convert|100|km|mi ft|abbr=on}}, the very [[Kármán line|edge of space]] itself as accepted by the [[Federation Aeronautique Internationale|FAI]], which places it about {{convert|20|km|mi ft|abbr=on}} above the mesopause.
:Above this altitude lies the heterosphere, which includes the exosphere and most of the thermosphere. Here, the chemical composition varies with altitude. This is because the [[mean free path|distance that particles can move without colliding with one another]] 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.<ref name="thought">{{cite web |url=https://www.thoughtco.com/most-abundant-gases-in-earths-atmosphere-607594 |title=The 4 Most Abundant Gases in Earth's Atmosphere |author=Anne Marie Helmenstine, PhD |date=June 16, 2018}}</ref>
* The [[planetary boundary layer]] is the part of the troposphere that is closest to Earth's surface and is directly affected by it, mainly through [[turbulence|turbulent diffusion]]. During the day the planetary boundary layer usually is well-mixed, whereas at night it becomes stably stratified with weak or intermittent mixing. The depth of the planetary boundary layer ranges from as little as about {{convert|100|m|ft}} on clear, calm nights to {{convert |3000|m|ft|abbr=on}} or more during the afternoon in dry regions.

The average temperature of the atmosphere at Earth's surface 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 |url-status=dead |archive-url=https://web.archive.org/web/20090614054213/http://www.bambooweb.com/articles/e/a/Earth%27s_atmosphere.html |archive-date=2009-06-14 }}</ref> or {{convert|15|C|F K|abbr=on}},<ref>{{cite web |url=http://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html |title=NASA&nbsp;– Earth Fact Sheet |publisher=Nssdc.gsfc.nasa.gov |access-date=2010-10-16 |archive-url=https://web.archive.org/web/20101030234253/http://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html |archive-date=30 October 2010  |url-status=live}}</ref> depending on the reference.<ref>{{cite web |url=http://www.ncdc.noaa.gov/oa/climate/research/anomalies/index.php |archive-url=https://web.archive.org/web/20090303233131/http://www.ncdc.noaa.gov/oa/climate/research/anomalies/index.php |url-status=dead |archive-date=2009-03-03 |title=Global Surface Temperature Anomalies }}</ref><ref>{{cite web |url=http://oceanworld.tamu.edu/resources/oceanography-book/radiationbalance.htm |archive-url=https://web.archive.org/web/20050303202001/http://oceanworld.tamu.edu/resources/oceanography-book/radiationbalance.htm |url-status=dead |archive-date=2005-03-03 |title=Earth's Radiation Balance and Oceanic Heat Fluxes }}</ref><ref>{{cite web |url=http://www-pcmdi.llnl.gov/projects/cmip/overview_ms/control_tseries.pdf |title=Coupled Model Intercomparison Project Control Run |url-status=dead |archive-url=https://web.archive.org/web/20080528143343/http://www-pcmdi.llnl.gov/projects/cmip/overview_ms/control_tseries.pdf |archive-date=2008-05-28 }}</ref>

==Physical properties==
{{Comparison US standard atmosphere 1962.svg}}

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

The average atmospheric pressure at sea level is defined by the [[International Standard Atmosphere]] as {{convert|101325|Pa|Torr psi mmHg|lk=on|comma=off}}. This is sometimes referred to as a unit of [[Atmosphere (unit)|standard atmospheres (atm)]]. Total atmospheric mass is 5.1480×10<sup>18</sup> kg (1.135×10<sup>19</sup> lb),<ref>{{Cite journal |title=The Mass of the Atmosphere: A Constraint on Global Analyses |journal=Journal of Climate |volume=18 |issue=6 |pages=864 |date=1970-01-01 |doi=10.1175/JCLI-3299.1 |bibcode = 2005JCli...18..864T|last1 = Trenberth|first1 = Kevin E.|last2=Smith |first2=Lesley |s2cid=16754900 |citeseerx=10.1.1.727.6573 }}</ref> about 2.5% less than would be inferred from the average sea level pressure and Earth's area of 51007.2 megahectares, this portion being displaced by 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 [[low-pressure area|weather]].

If the entire mass of the atmosphere had a uniform density equal to sea level density (about 1.2&nbsp;kg per m<sup>3</sup>) from sea level upwards, it would terminate abruptly at an altitude of {{convert|8.50|km|ft|abbr=on}}.

Air pressure actually decreases exponentially with altitude, dropping by half every {{convert|5.6|km|ft|abbr=on}} or by a factor of 1/[[e (mathematical constant)|e]] (0.368) every {{convert|7.64|km|ft|abbr=on}}, (this is called the [[scale height]]) -- for altitudes out to around {{convert|70|km|mi ft|abbr=on}}. However, the atmosphere is more accurately modeled with a customized equation for each layer that takes gradients of temperature, molecular composition, solar radiation and gravity into account. At heights over 100&nbsp;km, an atmosphere may no longer be well mixed. Then each chemical species has its own scale height.

In summary, the mass of Earth's atmosphere is distributed approximately as follows:<ref>Lutgens, Frederick K. and Edward J. Tarbuck (1995) ''The Atmosphere'', Prentice Hall, 6th ed., pp. 14–17, {{ISBN|0-13-350612-6}}</ref>
* 50% is below {{convert|5.6|km|ft|abbr=on}}.
* 90% is below {{convert|16|km|ft|abbr=on}}.
* 99.99997% is below {{convert|100|km|mi ft|abbr=on}}, the [[Kármán line]]. By international convention, this marks the beginning of space where human travelers are considered [[astronaut]]s.

By comparison, the summit of [[Mount Everest]] is at {{convert|8848|m|ft|abbr=on}}; commercial [[airliners]] typically cruise between {{convert|10|and|13|km|ft|abbr=on}} where the lower density and temperature of the air improve fuel economy; [[weather balloons]] reach {{convert|30.4|km|ft|abbr=on}} and above; and the highest [[North American X-15|X-15]] flight in 1963 reached {{convert|108.0|km|ft|abbr=on}}.

Even above the Kármán line, significant atmospheric effects such as [[aurora (astronomy)|auroras]] still occur. [[Meteors]] begin to glow in this region, though the larger ones may not burn up until they penetrate more deeply. The various layers of Earth's [[ionosphere]], important to [[HF radio]] propagation, begin below 100&nbsp;km and extend beyond 500&nbsp;km. By comparison, the [[International Space Station]] and [[Space Shuttle]] typically orbit at 350–400&nbsp;km, within the [[F-layer]] of the ionosphere where they encounter enough [[atmospheric drag]] to require reboosts every few months, otherwise, [[orbital decay]] will occur resulting in a return to Earth. Depending on solar activity, satellites can experience noticeable atmospheric drag at altitudes as high as 700–800&nbsp;km.

===Temperature===
{{Main|Atmospheric temperature}}
[[File:Atmospheric Temperature Trend.jpg|thumb|Temperature trends in two thick layers of the atmosphere as measured between January 1979 and December 2005 by [[microwave sounding unit]]s and [[advanced microwave sounding unit]]s on [[NOAA]] weather satellites. The instruments record microwaves emitted from oxygen molecules in the atmosphere. Source:<ref>{{cite web |url=http://earthobservatory.nasa.gov/IOTD/view.php?id=7839 |title=Atmospheric Temperature Trends, 1979–2005 : Image of the Day |publisher=Earthobservatory.nasa.gov |date=2000-01-01 |access-date=2014-06-10}}</ref>]]

The division of the atmosphere into layers mostly by reference to temperature is discussed above. Temperature decreases with altitude starting at sea level, but variations in this trend begin above 11&nbsp;km, where the temperature stabilizes over a large vertical distance through the rest of the troposphere. In the [[stratosphere]], starting above about 20&nbsp;km, the temperature increases with height, due to heating within the ozone layer caused by the capture of significant [[ultraviolet]] radiation from the [[Sun]] by the dioxygen and ozone gas in this region. Still another region of increasing temperature with altitude occurs at very high altitudes, in the aptly-named [[thermosphere]] above 90&nbsp;km.

==== Speed of sound ====
{{Main|Speed of sound}}
Because in an [[ideal gas]] of constant composition the [[speed of sound]] depends only on temperature and not on pressure or density, the speed of sound in the atmosphere with altitude takes on the form of the complicated temperature profile (see illustration to the right), and does not mirror altitudinal changes in density or pressure.

===Density and mass===
[[File:Atmosphere model.png|thumb|Temperature and mass density against altitude from the [[NRLMSISE-00]] [[International Standard Atmosphere|standard atmosphere]] model (the eight dotted lines in each "decade" are at the eight cubes 8, 27, 64, ..., 729)]]

{{Main|Density of air}}

The density of air at sea level is about 1.2&nbsp;kg/m<sup>3</sup> (1.2&nbsp;g/L, 0.0012 g/cm<sup>3</sup>). 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 [[ideal gas law]]). Atmospheric density decreases as the altitude increases. This variation can be approximately modeled using the [[barometric formula]]. More sophisticated models are used to predict the orbital decay of satellites.

The average mass of the atmosphere is about 5 quadrillion (5{{e|15}}) [[tonne]]s or 1/1,200,000 the mass of Earth. According to the American [[National Center for Atmospheric Research]], "The total mean mass of the atmosphere is 5.1480{{E|18}}&nbsp;kg with an annual range due to water vapor of 1.2 or 1.5{{E|15}}&nbsp;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}}&nbsp;kg and the dry air mass as 5.1352 ±0.0003{{E|18}}&nbsp;kg."

=== Tabulated properties ===
Table of physical and thermal properties of air at atmospheric pressure:<ref>{{Cite book |last=Holman |first=Jack P. |url=https://www.worldcat.org/oclc/46959719 |title=Heat transfer |publisher=McGraw-Hill Companies, Inc. |year=2002 |isbn=9780072406559 |edition=9th |location=New York, NY |pages=600–606 |language=English |oclc=46959719}}</ref><ref>{{Cite book |first1=Theodore L. |last1=Bergman |first2=Adrienne S. |last2=Lavine |first3=Frank P. |last3=Incropera |first4=David P. |last4=DeWitt  |url=https://www.worldcat.org/oclc/62532755 |title=Fundamentals of heat and mass transfer. |publisher=John Wiley and Sons, Inc. |year=2007 |isbn=9780471457282 |edition=6th |location=Hoboken, NJ |pages=941–950 |language=English |oclc=62532755}}</ref>
{|class="wikitable mw-collapsible"
|Temperature (K)
|Density (kg/m^3 )
|Specific heat (J/kg °C)
|Dynamic viscosity (kg/m s)
|Kinematic viscosity (m^2/s)
|Thermal conductivity (W/m °C)
|Thermal diffusivity (m^2/s)
|Prandtl Number
|Bulk modulus (K^-1 )
|-
|100
|3.601
|1026.6
|6.92E-06
|1.92E-06
|0.000925
|2.50E-06
|0.77
|0.01
|-
|150
|2.3675
|1009.9
|1.03E-05
|4.34E-06
|0.013735
|5.75E-06
|0.753
|0.006667
|-
|200
|1.7684
|1006.1
|1.33E-05
|7.49E-06
|0.01809
|1.02E-05
|0.738
|0.005
|-
|250
|1.4128
|1005.3
|1.60E-05
|1.13E-05
|0.02227
|1.57E-05
|0.722
|0.004
|-
|300
|1.1774
|1005.7
|1.85E-05
|1.57E-05
|0.02624
|2.22E-05
|0.708
|0.003333
|-
|350
|0.998
|1009
|2.08E-05
|2.08E-05
|0.03003
|2.98E-05
|0.697
|0.002857
|-
|400
|0.8826
|1014
|2.29E-05
|2.59E-05
|0.03365
|3.76E-05
|0.689
|0.0025
|-
|450
|0.7833
|1020.7
|2.48E-05
|3.17E-05
|0.03707
|4.22E-05
|0.683
|0.002222
|-
|500
|0.7048
|1029.5
|2.67E-05
|3.79E-05
|0.04038
|5.56E-05
|0.68
|0.002
|-
|550
|0.6423
|1039.2
|2.85E-05
|4.43E-05
|0.0436
|6.53E-05
|0.68
|0.001818
|-
|600
|0.5879
|1055.1
|3.02E-05
|5.13E-05
|0.04659
|7.51E-05
|0.68
|0.001667
|-
|650
|0.543
|1063.5
|3.18E-05
|5.85E-05
|0.04953
|8.58E-05
|0.682
|0.001538
|-
|700
|0.503
|1075.2
|3.33E-05
|6.63E-05
|0.0523
|9.67E-05
|0.684
|0.001429
|-
|750
|0.4709
|1085.6
|3.48E-05
|7.39E-05
|0.05509
|1.08E-04
|0.686
|0.001333
|-
|800
|0.4405
|1097.8
|3.63E-05
|8.23E-05
|0.05779
|1.20E-04
|0.689
|0.00125
|-
|850
|0.4149
|1109.5
|3.77E-05
|9.08E-05
|0.06028
|1.31E-04
|0.692
|0.001176
|-
|900
|0.3925
|1121.2
|3.90E-05
|9.93E-05
|0.06279
|1.43E-04
|0.696
|0.001111
|-
|950
|0.3716
|1132.1
|4.02E-05
|1.08E-04
|0.06525
|1.55E-04
|0.699
|0.001053
|-
|1000
|0.3524
|1141.7
|4.15E-05
|1.18E-04
|0.06753
|1.68E-04
|0.702
|0.001
|-
|1100
|0.3204
|1160
|4.44E-05
|1.39E-04
|0.0732
|1.97E-04
|0.704
|0.000909
|-
|1200
|0.2947
|1179
|4.69E-05
|1.59E-04
|0.0782
|2.25E-04
|0.707
|0.000833
|-
|1300
|0.2707
|1197
|4.93E-05
|1.82E-04
|0.0837
|2.58E-04
|0.705
|0.000769
|-
|1400
|0.2515
|1214
|5.17E-05
|2.06E-04
|0.0891
|2.92E-04
|0.705
|0.000714
|-
|1500
|0.2355
|1230
|0.000054
|2.29E-04
|0.0946
|3.26E-04
|0.705
|0.000667
|-
|1600
|0.2211
|1248
|5.63E-05
|2.55E-04
|0.1
|3.61E-04
|0.705
|0.000625
|-
|1700
|0.2082
|1267
|5.85E-05
|2.81E-04
|0.105
|3.98E-04
|0.705
|0.000588
|-
|1800
|0.197
|1287
|6.07E-05
|3.08E-04
|0.111
|4.38E-04
|0.704
|0.000556
|-
|1900
|0.1858
|1309
|6.29E-05
|3.39E-04
|0.117
|4.81E-04
|0.704
|0.000526
|-
|2000
|0.1762
|1338
|0.000065
|3.69E-04
|0.124
|5.26E-04
|0.702
|0.0005
|-
|2100
|0.1682
|1372
|6.72E-05
|4.00E-04
|0.131
|5.72E-04
|0.7
|0.000476
|-
|2200
|0.1602
|1419
|6.93E-05
|4.33E-04
|0.139
|6.12E-04
|0.707
|0.000455
|-
|2300
|0.1538
|1482
|7.14E-05
|4.64E-04
|0.149
|6.54E-04
|0.71
|0.000435
|-
|2400
|0.1458
|1574
|7.35E-05
|5.04E-04
|0.161
|7.02E-04
|0.718
|0.000417
|-
|2500
|0.1394
|1688
|7.57E-05
|5.44E-04
|0.175
|7.44E-04
|0.73
|0.0004
|-
|3000
|0.1135
|2.726
|9.55E-05
|8.41E-04
|0.486
|1.57E-03
|0.536
|0.0003333333333
|}

==Optical properties==
{{See also|Sunlight}}
Solar [[radiation]] (or sunlight) is the energy Earth receives from the [[Sun]]. Earth also emits radiation back into space, but at longer wavelengths that humans cannot see. Part of the incoming and emitted radiation is absorbed or reflected by the atmosphere.<ref>{{Cite web |title=Absorption / reflection of sunlight |url=https://ugc.berkeley.edu/background-content/reflection-absorption-sunlight/ |access-date=2023-06-13 |website=Understanding Global Change}}</ref><ref>{{Cite web |title=The Atmospheric Window |url=https://www.noaa.gov/jetstream/satellites/absorb |access-date=2023-06-13 |website=National Oceanic and Atmospheric Administration}}</ref> In May 2017, glints of light, seen as twinkling from an orbiting satellite a million miles away, were found to be [[Reflection (physics)|reflected light]] from [[ice crystals]] in the atmosphere.<ref name="NYT-20170519">{{cite news |last=St. Fleur |first=Nicholas |title=Spotting Mysterious Twinkles on Earth From a Million Miles Away |url=https://www.nytimes.com/2017/05/19/science/dscovr-satellite-ice-glints-earth-atmosphere.html |date=19 May 2017 |work=[[The New York Times]] |access-date=20 May 2017}}</ref><ref name="GRL-201760515">{{cite journal |last1=Marshak |first1=Alexander |last2=Várnai |first2=Tamás |last3=Kostinski |first3=Alexander |title=Terrestrial glint seen from deep space: oriented ice crystals detected from the Lagrangian point |date=15 May 2017 |journal=[[Geophysical Research Letters]] |volume=44 |issue=10 |page=5197 |doi=10.1002/2017GL073248 |bibcode=2017GeoRL..44.5197M|s2cid=109930589 |url=https://zenodo.org/record/1229066 |hdl=11603/13118 |hdl-access=free }}</ref>

===Scattering===
{{Main|Atmospheric scattering}}
When light passes through Earth's atmosphere, [[photon]]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 [[overcast]] 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 [[Rayleigh scattering]], 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 sunsets are red. Because the Sun is close to the horizon, the Sun's rays pass through more atmosphere than normal before reaching your eye. Much of the blue light has been scattered out, leaving the red light in a sunset.

===Absorption===
{{Main|Absorption (electromagnetic radiation)}}
[[File:Atmospheric electromagnetic opacity.svg|thumb|Rough plot of Earth's atmospheric [[transmittance]] (or opacity) to various wavelengths of electromagnetic radiation, including [[visible light]]]]

Different molecules absorb different wavelengths of radiation. For example, O<sub>2</sub> and O<sub>3</sub> absorb almost all radiation with wavelengths shorter than 300 [[nanometer|nanometre]]s. Water (H<sub>2</sub>O) absorbs at many wavelengths above 700&nbsp;nm. When a molecule absorbs a photon, it increases the energy of the molecule. This heats the atmosphere, but the atmosphere also cools by emitting radiation, as discussed below.

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

===Emission===
{{Further|Emission spectrum}}

''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 "[[black body]]" 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. 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 Earth's surface cools down faster than on cloudy 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 [[greenhouse effect]] is directly related to this absorption and emission effect. Some gases in the atmosphere absorb and emit infrared radiation, but do not interact with sunlight in the visible spectrum. Common examples of these are {{CO2}} and H<sub>2</sub>O.

===Refractive index===
[[Image:SB DouglasPreserve SunAtmosphericEffects 2017 3 cropped.jpg|thumb|right|Distortive effect of [[atmospheric refraction]] upon the shape of the sun at the horizon]]
{{main|Atmospheric refraction}}
{{See also|Scintillation (astronomy)}}

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

The refractive index of air depends on temperature,<ref name="Edlén">{{cite journal |first=Bengt |last=Edlén |date=1966 |journal=Metrologia |volume=2 |issue=2 |title=The refractive index of air |pages=71–80 |doi=10.1088/0026-1394/2/2/002 |bibcode=1966Metro...2...71E }}</ref> giving rise to refraction effects when the temperature gradient is large. An example of such effects is the [[mirage]].

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

[[File:AtmosphCirc2.png|thumb|right|An idealised view of three pairs of large circulation cells]]
''Atmospheric circulation'' is the large-scale movement of air through the troposphere, and the means (with [[ocean circulation]]) by which heat is distributed around Earth. The large-scale structure of the atmospheric circulation varies from year to year, but the basic structure remains fairly constant because it is determined by Earth's rotation rate and the difference in solar radiation between the equator and poles.

=={{anchor|History of Earth's atmosphere|Atmosphere of early Earth}}Evolution of Earth's atmosphere==
{{See also|History of Earth|Paleoclimatology}}

==={{anchor|First atmosphere}}Earliest atmosphere===
The first atmosphere consisted of gases in the [[Solar nebula#Formation of planets|solar nebula]], primarily [[hydrogen]]. There were probably simple [[hydride]]s such as those now found in the [[gas giant]]s ([[Jupiter]] and [[Saturn]]), notably water vapor, [[methane]] and [[ammonia]].<ref name=Zahnle>{{Cite journal |last1=Zahnle |first1=K. |last2=Schaefer |first2=L.|author2-link=Laura K. Schaefer |last3=Fegley |first3=B. |doi=10.1101/cshperspect.a004895 |title=Earth's Earliest Atmospheres |journal=Cold Spring Harbor Perspectives in Biology |volume=2 |issue=10 |pages=a004895 |year=2010 |pmid=20573713 |pmc=2944365}}</ref>

===Second atmosphere===
{{See also|Prebiotic atmosphere}}
[[Outgassing]] from [[volcanism]], supplemented by gases produced during the [[late heavy bombardment]] of Earth by huge [[asteroids]], produced the next atmosphere, consisting largely of [[nitrogen]] plus [[carbon dioxide]] and inert gases.<ref name=Zahnle/> A major part of carbon-dioxide emissions dissolved in water and reacted with metals such as calcium and magnesium during weathering of crustal rocks to form carbonates that were deposited as sediments. Water-related sediments have been found that date 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 formed the major part of the then stable "second atmosphere". The influence of life has to be taken into account rather soon in the history of the atmosphere because hints of early life-forms appear 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> How Earth at that time maintained a climate warm enough for liquid water and life, if the early Sun put out 30% lower solar radiance than today, is a puzzle known as the "[[faint young Sun paradox]]".

The geological record however shows a continuous relatively warm surface during the complete early [[temperature record]] of Earth – with the exception of one cold glacial phase about 2.4 billion years ago. In the late [[Archean]] [[Eon (geology)|Eon]] an oxygen-containing atmosphere began to develop, apparently produced by photosynthesizing [[cyanobacteria]] (see [[Great Oxygenation Event]]), which have been found as [[stromatolite]] fossils from 2.7 billion years ago. The early basic carbon isotopy ([[isotope ratio]] proportions) strongly suggests conditions similar to the current, and that the fundamental features of the [[carbon cycle]] became established as early as 4 billion years ago.

[[Proxy (climate)#Lake and ocean sediments|Ancient sediments]] in the [[Gabon]] dating from between about 2.15 and 2.08 billion years ago provide a record of Earth's dynamic oxygenation evolution. These fluctuations in oxygenation were likely driven by the Lomagundi carbon isotope excursion.<ref>
{{Cite journal |year=2014 |author=Timothy W. Lyons, Christopher T. Reinhard & Noah J. Planavsky |title=Atmospheric oxygenation three billion years ago |journal=Nature |doi=10.1038/nature13068 |pmid=24553238 |bibcode=2014Natur.506..307L |volume=506 |issue=7488 |pages=307–15|s2cid=4443958 }}
</ref>

===Third atmosphere===
[[File:Sauerstoffgehalt-1000mj2.png|thumb|Oxygen content of the atmosphere over the last billion years<ref>{{cite journal |last1=Martin |first1=Daniel |last2=McKenna |first2=Helen |last3=Livina |first3=Valerie |title=The human physiological impact of global deoxygenation |journal=The Journal of Physiological Sciences |volume=67 |issue=1 |pages=97–106 |doi=10.1007/s12576-016-0501-0 |pmid=27848144 |pmc=5138252 |issn=1880-6546 |year=2016 }}</ref><ref>[https://www.nap.edu/openbook/0309100615/gifmid/30.gif Graph: Atmospheric Oxygen and CO2 vs Time]</ref>]]

The constant re-arrangement of continents by [[plate tectonics]] influences the long-term evolution of the atmosphere by transferring carbon dioxide to and from large continental carbonate stores. Free oxygen did not exist in the atmosphere until about 2.4 billion years ago during the [[Great Oxygenation Event]] and its appearance is indicated by the end of the [[banded iron formations]].

Before this time, any oxygen produced by photosynthesis was consumed by the [[Redox|oxidation]] of reduced materials, notably iron. Free oxygen molecules did not start to accumulate in the atmosphere until the rate of production of oxygen began to exceed the availability of [[Reducing agent|reducing materials]] that removed oxygen. This point signifies a shift from a [[reducing atmosphere]] to an [[oxidizing]] atmosphere. O<sub>2</sub> showed major variations until reaching a steady state of more than 15% by the end of the Precambrian.<ref>Christopher R. Scotese, [http://www.scotese.com/precamb_chart.htm Back to Earth History : Summary Chart for the Precambrian], Paleomar Project</ref> The following time span from 539 million years ago to the present day is the [[Phanerozoic]] Eon, during the earliest period of which, the [[Cambrian]], oxygen-requiring [[metazoan]] life forms began to appear.

The amount of oxygen in the atmosphere has fluctuated over the last 600 million years, reaching a peak of about 30% around 280 million years ago, significantly higher than today's 21%. Two main processes govern changes in the atmosphere: Plants [[photosynthesis|using carbon dioxide from the atmosphere]] and releasing oxygen, and then plants using some oxygen at night by the process of [[photorespiration]] while the remaining oxygen is used to break down organic material. Breakdown of [[pyrite]] and [[volcanic eruption]]s release [[sulfur]] into the atmosphere, which reacts with oxygen and hence reduces its amount in the atmosphere. However, volcanic eruptions also release carbon dioxide, which plants can convert to oxygen. The cause of the variation of the amount of oxygen in the atmosphere is not known. Periods with much oxygen in the atmosphere are associated with the rapid development of animals.

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

{{em|Air pollution}} is the introduction into the atmosphere of [[chemical]]s, [[Atmospheric particulate matter|particulate matter]] or [[Organic matter|biological materials]] that cause harm or discomfort to organisms.<ref>Starting from [http://www.merriam-webster.com/dictionary/pollution] Pollution – Definition from the Merriam-Webster Online Dictionary</ref> [[Stratosphere|Stratospheric]] [[ozone depletion]] is caused by air pollution, chiefly from [[chlorofluorocarbons]] and other ozone-depleting substances.

Since 1750, human activity has increased the concentrations of various greenhouse gases, most importantly carbon dioxide, methane and nitrous oxide. This increase has caused an [[Climate change|observed rise in global temperatures]]. Global average surface temperatures were {{Val|1.1|u=°C}} higher in the 2011–2020 decade than they were in 1850.<ref>{{Cite book|author=IPCC|title=IPCC AR6 WG1|year=2021|pages=4–5|chapter=Summary for Policymakers|ref={{harvid|IPCC AR6 WG1 Summary for Policymakers|2021}}|author-link=IPCC|chapter-url=https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_SPM.pdf|access-date=2021-11-20|archive-date=2021-08-11|archive-url=https://web.archive.org/web/20210811205522/https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_SPM.pdf|url-status=dead}}</ref>

[[File:Watching the Earth Breathe.ogv|thumb|Animation shows the buildup of tropospheric {{CO2}} in the Northern Hemisphere with a maximum around May. The maximum in the vegetation cycle follows in the late summer. Following the peak in vegetation, the drawdown of atmospheric {{CO2}} due to photosynthesis is apparent, particularly over the [[boreal forests]].]]

==Images from space==
{{main|Weather satellite}}

On October 19, 2015, NASA started a website containing daily images of the full sunlit side of Earth at https://epic.gsfc.nasa.gov/. The images are taken from the [[Deep Space Climate Observatory]] (DSCOVR) and show Earth as it rotates during a day.<ref>{{cite web |title=Daily Views of Earth Available on New NASA Website |url=http://www.nasa.gov/press-release/daily-views-of-earth-available-on-new-nasa-website |website=NASA |access-date=2015-10-21 |first=Karen |last=Northon|date=2015-10-19 }}</ref>

{{Gallery
|title=
|width=160
|height=170
|align=left
|File:Antarctic_aurora_ESA313457.jpg|Earth's atmosphere seen from space. From the bottom, [[afterglow]] illuminates the troposphere in orange with silhouettes of clouds, and the stratosphere in white and blue. Next the mesosphere (pink) and the pink line of [[airglow]] of the lower thermosphere (dark), which hosts green and red [[aurora]]e over several hundred kilometers.
|File:Sunset from the ISS.JPG|Limb view of Earth's atmosphere. Colors roughly denote the layers of the atmosphere.
|File:Moon Limb & Troposphere.JPG|This image shows the Moon at the centre, with the limb of Earth near the bottom transitioning into the orange-colored troposphere. The troposphere ends abruptly at the tropopause, which appears in the image as the sharp boundary between the orange- and blue-colored atmosphere. The silvery-blue [[noctilucent cloud]]s extend far above Earth's troposphere.
|File:Earth Eclipses Sun-ap12-s80-37406.jpg|Earth's atmosphere backlit by the Sun in an [[eclipse]] observed from [[Cislunar space|deep space]] onboard [[Apollo 12]] in 1969}}
{{clear}}

== See also ==
{{portal|Environment|Science|World}}
{{Div col|colwidth=25em}}
* [[Aerial perspective]]
* [[Air (classical element)]]
* [[Biosphere]]
* [[Hydrosphere]]
* [[Lithosphere]]
* [[Airglow]]
* [[Airshed]]
* [[Atmospheric dispersion modeling]]
* [[Atmospheric electricity]]
* [[Climate system]]
* [[COSPAR international reference atmosphere]] (CIRA)
* [[Environmental effects of aviation]]
* [[Global dimming]]
* [[Instrumental temperature record]]
* [[Reference atmospheric model]]
{{div col end}}

==References==
{{Reflist}}

==External links==
{{Commons category|Earth's atmosphere}}
{{Wikiquote|Air}}
* {{cite EB9 |wstitle= Atmosphere |volume= III |last= Buchan |first= Alexander |author-link=Alexander Buchan (meteorologist)|pages=28-36 |short=1}}
* [https://earth.nullschool.net/ Interactive global map of current atmospheric and ocean surface conditions.]

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