View source for Atmosphere of Earth

{{Short description|Gas layer surrounding Earth}}
{{Redirect|Air}}
{{Pp-semi-indef}}
[[File:Top of Atmosphere.jpg|thumb|upright=1.3|Earth's atmosphere from space, showing a blue layer at the [[stratosphere]], above the clouds of the [[troposphere]]. The [[Moon]] is visible as a crescent in the background.<ref>{{cite web |url=https://eol.jsc.nasa.gov/SearchPhotos/photo.pl?mission=ISS013&roll=E&frame=54329 |title=Gateway to Astronaut Photos of Earth |publisher=NASA |access-date=2018-01-29}}</ref>]]

The '''atmosphere of Earth''' consists of a layer of mixed [[gas]] (commonly referred to as '''air''') that is retained by [[gravity of Earth|gravity]], surrounding the [[Earth]]'s [[planetary surface|surface]]. It contains variable quantities of suspended [[aerosol]]s and [[particulate]]s that create [[weather]] features such as [[cloud]]s and [[haze]]s. The atmosphere serves as a protective buffer between the Earth's surface and [[outer space]]. It shields the surface from most [[meteoroid]]s and ultraviolet [[solar irradiance|solar radiation]], reduces [[diurnal temperature variation]] – the temperature extremes between [[daytime|day]] and [[night]], and keeps it warm through heat retention via the [[greenhouse effect]]. The atmosphere redistributes heat and moisture among different regions via [[air current]]s, and provides the [[atmospheric chemistry|chemical]] and [[climate]] conditions that allow [[life]] to exist and [[evolution|evolve]] on Earth.

By [[mole fraction]] (i.e., by quantity of [[molecule]]s), 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 [[trace gas]]es (see [[#Composition|Composition]] below for more detail).<!-- No ref here, see the Composition section, this intro is a summary--> 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.

Earth's primordial atmosphere consisted of gases [[accretion (astrophysics)|accreted]] from the [[solar nebula]], but the composition changed significantly over time, affected by many factors such as [[volcanism]], [[outgassing]], [[impact event]]s, [[weathering]] and the [[evolution of life]] (particularly the [[photoautotroph]]s). In the present day, [[human]] activity has [[human impact on the environment#Impacts through the atmosphere|contributed to atmospheric changes]], such as [[anthropogenic climate change|climate change]] (mainly through [[deforestation]] and [[fossil fuel|fossil-fuel]]–related [[global warming]]), [[ozone depletion]] and [[acid deposition]].

The atmosphere has a mass of about 5.15{{e|18}}&nbsp;kg,<ref>{{cite book | editor-last=Lide | editor-first=David R. | title=Handbook of Chemistry and Physics | location=Boca Raton, FL | publisher=CRC Press | date=May 28, 1996 | pages=14–17 | edition=77th | isbn=978-0-8493-0477-4 }}</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}} is often used as a conventional definition of the [[edge of space]]. Several [[#Stratification|layers]] can be distinguished in the atmosphere based on characteristics such as temperature and composition, namely the [[troposphere]], [[stratosphere]], [[mesosphere]], [[thermosphere]] (formally the [[ionosphere]]), and [[exosphere]]. Air composition, temperature and [[atmospheric pressure]] vary with [[altitude]]. Air suitable for use in [[photosynthesis]] by [[terrestrial plant]]s and [[respiration (physiology)|respiration]] of [[terrestrial animal]]s is found within the troposphere.<ref>{{Cite web |date=2024-05-13 |title=What Is... Earth's Atmosphere? - NASA |url=https://www.nasa.gov/general/what-is-earths-atmosphere/ |access-date=2024-06-18 |language=en-US}}</ref>

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 | last1=Vázquez | first1=M. | last2=Hanslmeier | first2=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 the 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.<ref name=Allens-2002/><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><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>]]

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. In the lower atmosphere, 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">{{cite book | last1=Wallace | first1=John M. | first2=Peter V. | last2=Hobbs | chapter=Chapter 1. Introduction and Overview | pages=1–21 | publisher=Elsevier | edition=Second | year=2006 | isbn=978-0-12-732951-2 | url=http://cup.aos.wisc.edu/453/2016/readings/Atmospheric_Science-Wallace_Hobbs.pdf | access-date=2018-07-28 | title=Atmospheric Science: An Introductory Survey | archive-url=https://web.archive.org/web/20180728040037/http://cup.aos.wisc.edu/453/2016/readings/Atmospheric_Science-Wallace_Hobbs.pdf | archive-date=2018-07-28 }}</ref>{{rp|8}} The remaining gases are often referred to as trace gases,<ref>{{cite encyclopedia | url=http://www.ace.mmu.ac.uk/eae/Atmosphere/Older/Trace_Gases.html | title=Trace Gases | encyclopedia=Encyclopedia of the Atmospheric Environment | 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 }}</ref> among which are other [[greenhouse gas]]es, principally carbon dioxide, methane, nitrous oxide, and ozone. Besides argon, other [[noble gas]]es, [[neon]], [[helium]], [[krypton]], and [[xenon]] are also present. Filtered air includes trace amounts of many other [[chemical compound]]s.<ref name=Graedel_et_al_2012>{{cite book | title=Atmospheric Chemical Compounds: Sources, Occurrence and Bioassay | display-authors=1 | first1=T. E. | last1=Graedel | first2=Donald T. | last2=Hawkins | first3=Larry D. | last3=Claxton | pages=v–ix | publisher=Elsevier | year=2012 | isbn=978-0-08-091842-6 | url=https://books.google.com/books?id=RphpPlnnfPYC&pg=PA1 }}</ref>

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]].<ref name=Colbeck_Lazaridis_2010>{{cite journal | title=Aerosols and environmental pollution | last1=Colbeck | first1=Ian | last2=Lazaridis | first2=Mihalis | journal=Naturwissenschaften | volume=97 | issue=2 | pages=117–131 | date=February 2010 | doi=10.1007/s00114-009-0594-x | pmid=19727639 | bibcode=2010NW.....97..117C }}</ref> Various industrial [[pollutant]]s also may be present as gases or aerosols, such as [[chlorine]] (elemental or in compounds),<ref name=Hao_Wang_et_al_2017>{{cite journal | title=Mixed Chloride Aerosols and their Atmospheric Implications: A Review | display-authors=1 | first1=Hao | last1=Wang | first2=Xinfeng | last2=Wang | first3=Xue | last3=Yang | first4=Weijun | last4=Li | first5=Likun | last5=Xue | first6=Tao | last6=Wang | first7=Jianmin | last7=Chen | first8=Wenxing | last8=Wang | journal=Aerosol and Air Quality Research | volume=17 | pages=878–887 | year=2017 | issue=4 | publisher=Taiwan Association for Aerosol Research | doi=10.4209/aaqr.2016.09.0383 | bibcode=2017AAQR...17..878W | hdl=10397/103030 | hdl-access=free }}</ref> [[fluorine]] compounds,<ref name=Faust_2023>{{cite journal | title=PFAS on atmospheric aerosol particles: a review | last=Faust | first=J. A. | journal=Environmental Science: Processes & Impacts | date=February 22, 2023 | volume=25 | issue=2 | pages=133–150 | doi=10.1039/d2em00002d | pmid=35416231 }}</ref> and elemental [[Mercury (element)|mercury]] vapor.<ref name=Pacyna_et_al_2016>{{cite journal | title=Current and future levels of mercury atmospheric pollution on a global scale | display-authors=1 | last1=Pacyna | first1=Jozef M. | last2=Travnikov | first2=Oleg | last3=De Simone | first3=Francesco | last4=Hedgecock | first4=Ian M. | last5=Sundseth | first5=Kyrre | last6=Pacyna | first6=Elisabeth G. | last7=Steenhuisen | first7=Frits | last8=Pirrone | first8=Nicola | last9=Munthe | first9=John | last10=Kindbom | first10=Karin | journal=Atmospheric Chemistry and Physics | volume=16 | issue=19 | pages=12495–12511 | date=October 2016 | doi=10.5194/acp-16-12495-2016  | doi-access=free | bibcode=2016ACP....1612495P | hdl=11250/2452800 | hdl-access=free }}</ref> Sulfur compounds such as [[hydrogen sulfide]] and [[sulfur dioxide]] (SO<sub>2</sub>) may be derived from natural sources or from industrial air pollution.<ref name=Colbeck_Lazaridis_2010/><ref name=Kumar_Francisco_2017>{{cite journal | title=Elemental sulfur aerosol-forming mechanism | last1=Kumar | first1=Manoj | last2=Francisco | first2=Joseph S. | journal=Proceedings of the National Academy of Sciences | volume=114 | issue=5 | pages=864–869 | date=January 2017 | doi=10.1073/pnas.1620870114 | pmid=28096368 | pmc=5293086 | doi-access=free | bibcode=2017PNAS..114..864K }}</ref>

[[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 85&nbsp;km.]]

{| class="wikitable" style="text-align: center;"
|+Major constituents of air<ref name=Allens-2002>{{Cite book | url=https://link.springer.com/10.1007/978-1-4612-1186-0 | title=Allen's Astrophysical Quantities | date=2002 | edition=4th | chapter=11. Earth | publisher=Springer New York | isbn=978-1-4612-7037-9 | editor-last=Cox | editor-first=Arthur N. | location=New York, NY | language=en | doi=10.1007/978-1-4612-1186-0}}</ref>{{rp|258}}
! colspan=6 | Dry air
|-
! colspan=2 | Gas
! colspan=2 style="font-weight: normal;" | '''Volume fraction'''<sup>(A)</sup>
! colspan=2 style="font-weight: normal;" | '''Mass fraction'''
|-
! 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|%]]
! style="width: 100px; font-weight: normal;" | '''in [[Parts-per notation|ppm]]'''
! style="width: 100px;" | in [[Percentage|%]]
|-
| align=left | [[Nitrogen]]
| N<sub>2</sub>
| 780,800
| 78.08
| 755,200
| 75.52
|-
| align=left | [[Oxygen]]
| O<sub>2</sub>
| 209,500
| 20.95
| 231,400
| 23.14
|-
| align=left | [[Argon]]
| Ar
| 9,340
| 0.9340
| 12,900
| 1.29
|-
| align=left | [[Carbon&nbsp;dioxide]]<ref name="CO2"/>
| {{CO2}}
| 412
| 0.0412
| 626
| 0.063
|-
| align=left | [[Neon]]
| Ne
| 18.2
| 0.00182
| 12.7
| 0.00127
|-
| align=left | [[Helium]]
| He
| 5.24
| 0.000524
| 0.724
| 0.0000724
|-
| align=left | [[Methane]]<ref name="methane"/>
| CH<sub>4</sub>
| 1.79
| 0.000179
| 0.99
| 0.000099
|-
| align=left | [[Krypton]]
| Kr
| 1.14
| 0.000114
| 3.3
| 0.00033
|-
! colspan=6 | 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=6 style="font-size: 0.85em; padding: 5px 2px 5px 10px; text-align: left; font-weight: normal;" | 
The total ppm above adds up to more than 1 million (currently 83.43 above it) due to [[experimental error]].<br/>
'''Notes'''<br/>
<sup>(A)</sup> In the atmosphere the pressure is low enough for the ideal gas laws to be correct within 1%. Therefore, the mole fraction is very close to the [[volume fraction]].<ref>{{Cite book | last=Jacob | first=Daniel J. | title=Introduction to atmospheric chemistry | date=1999 | publisher=Princeton University Press | isbn=978-1-4008-4154-7 | edition=Online-Ausg | location=Princeton, N.J. }}</ref>{{rp|4}}<br/>
<sup>(B)</sup> ppm: [[parts per million]] by molecular count<br/>
<sup>(C)</sup> The concentration of {{CO2}} has been [[Keeling Curve|increasing in recent decades]], as has that of {{CH4}}.<br/>
<sup>(D)</sup> Water vapor is about 0.25% ''by mass'' over full atmosphere<br/>
<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 (chemistry)|mass fraction]], is about 28.946<ref name="Möller2003">{{cite book | first=Detlev | last=Möller | title=Luft: Chemie, Physik, Biologie, Reinhaltung, Recht | publisher=Walter de Gruyter | year=2003 | isbn=3-11-016431-0 | page=173 | url=https://books.google.com/books?id=7ITJXWDijA0C&pg=PA173 }}</ref> or 28.964<ref>{{cite book | first=Yunus | last=Çengel | title=Termodinamica e trasmissione del calore | publisher=McGraw-Hill Education | edition=4th | year=2013 | isbn=978-88-386-6511-0 }}</ref><ref name=Allens-2002/>{{rp|257}}&nbsp;g/mol. This is decreased when the air is humid.

Up to an altitude of around {{convert|100|km|mi|abbr=on}}, atmospheric [[turbulence]] mixes the component gases so that their relative concentrations remain the same. There exists a transition zone from roughly {{convert|80|to|120|km|mi|abbr=on}} where this turbulent mixing gradually yields to [[molecular diffusion]]. The latter process forms the [[heterosphere]] where the relative concentration of lighter gases increase with altitude.<ref name=Schlatter_2009>{{cite book | chapter=Atmospheric Composition and Vertical Structure | first=Thomas W. | last=Schlatter | title=Environmental Impact and Manufacturing | volume=6 | pages=1–54 | year=2009 | chapter-url=https://citeseerx.ist.psu.edu/document?repid=rep1&type=pdf&doi=03ce42a2a73ca165fac2ccc82e17e13d6cdcdd0f | access-date=2025-07-20 }} See p. 6.</ref>

== Stratification ==
[[File:Earth's atmosphere.svg|thumb|A prism cross-section of the Earth's atmosphere. The strata are drawn to scale, but individual features are not.<ref>{{Cite book |last=Brekke |first=Asgeir |url=https://link.springer.com/10.1007/978-3-642-27401-5 |title=Physics of the Upper Polar Atmosphere |date=2013 |publisher=Springer Berlin Heidelberg |isbn=978-3-642-27400-8 |series=Springer Atmospheric Sciences |location=Berlin, Heidelberg |language=en |doi=10.1007/978-3-642-27401-5}}</ref>{{rp|322}}]]

In general, air pressure and density decrease with altitude in the atmosphere. However, 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).<ref name=Champion_et_al_1985>{{cite book | display-authors=1 | last1=Champion | first1=K. S. W. | last2=Cole | first2=A. E. | last3=Kantor | first3=A. J. | editor-first=Adolf S. | editor-last=Jursa | publisher=Air Force Geophysics Library | year=1985 | chapter=Standard and reference atmospheres | title=Handbook of geophysics and the space environment | url=https://www.cnofs.org/Handbook_of_Geophysics_1985/Chptr14.pdf | access-date=2025-07-20 }}</ref> 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. This ''atmospheric stratification'' divides the Earth's atmosphere into five main layers with these typical altitude ranges:<ref name=Buis_2024>{{cite web | title=Earth's Atmosphere: A Multi-layered Cake | first=Alan | last=Buis | publisher=NASA | date=October 22, 2024 | url=https://science.nasa.gov/earth/earth-atmosphere/earths-atmosphere-a-multi-layered-cake/ | access-date=2025-07-21 }}</ref><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=March 2, 2015 | publisher=NASA | access-date=2017-02-20 | language=en }}</ref>

* Exosphere: {{convert |700 |- |10000 |km |abbr=on |0}}<ref name="UCAR-2011">{{cite web | title=Exosphere - overview | url=https://scied.ucar.edu/shortcontent/exosphere-overview | year=2011 | publisher=University Corporation for Atmospheric Research | 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 }}</ref>
* Thermosphere: {{convert |80 |- |700 |km |abbr=on |0}}<ref name="thermosphere">{{cite web | first=Randy | last=Russell | title=The Thermosphere | year=2008 | publisher=National Earth Science Teachers Association (NESTA) | url=https://www.windows2universe.org/earth/Atmosphere/thermosphere.html | access-date=2013-10-18}}</ref>
* Mesosphere: {{convert |50 |- |80 |km |abbr=on |0}}
* Stratosphere: {{convert |12 |- |50 |km |abbr=on |0}}
* Troposphere: {{convert |0 |- |12 |km |abbr=on |0}}<ref name=tropopauseheight>{{cite web | first1=B. | last1=Geerts | first2=E. | last2=Linacre | date=November 1997 | url=http://www-das.uwyo.edu/~geerts/cwx/notes/chap01/tropo.html | archive-url=https://web.archive.org/web/20010222031650/http://www-das.uwyo.edu/~geerts/cwx/notes/chap01/tropo.html | archive-date=February 22, 2001 | title=The height of the tropopause | publisher=Department of Atmospheric Science, University of Wyoming | 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=https://scied.ucar.edu/shortcontent/exosphere-overview | year=2011 | publisher=University Corporation for Atmospheric Research | 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 | publisher=NASA }}</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, with limited amounts of helium, carbon dioxide, and [[Nascent state (chemistry)|nascent]] oxygen closer to the exobase.<ref name=Singh_2020>{{cite book | title=Environmental Plant Physiology: Botanical Strategies for a Climate Smart Planet | first=Vir | last=Singh | publisher=CRC Press | year=2020 | isbn=978-1-000-02486-9 | url=https://books.google.com/books?id=LTf3DwAAQBAJ&pg=PA87 }}</ref> The atoms and molecules are so far apart that they can travel hundreds of kilometres without colliding with one another.<ref name=Champion_et_al_1985/>{{rp|14–4}} 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">{{cite journal | first1=David C. | last1=Catling | author1-link=David Catling | first2=Kevin J. | last2=Zahnle | author2-link=Kevin J. Zahnle | url=https://faculty.washington.edu/dcatling/Catling2009_SciAm.pdf | title=The Planetary Air Leak | journal=Scientific American | date=May 2009 | volume=300 | issue=5 | pages=36–43 | doi=10.1038/scientificamerican0509-36 | doi-broken-date=21 July 2025 | pmid=19438047 | bibcode=2009SciAm.300e..36C | access-date=25 July 2012 }}</ref>

The exosphere is too far above Earth for [[meteorology|meteorological]] phenomena to be possible.  The exosphere contains many of the [[Satellite|artificial satellite]]s that [[orbit]] Earth.<ref name=Liou_Johnson_2008>{{cite journal | title=Instability of the present LEO satellite populations | first1=J.-C. | last1=Liou | first2=N. L. | last2=Johnson | journal=Advances in Space Research | volume=41 | issue=7 | year=2008 | pages=1046–1053 | doi=10.1016/j.asr.2007.04.081 | bibcode=2008AdSpR..41.1046L | hdl=2060/20060024585 | hdl-access=free }}</ref>

===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|abbr=on|sigfig=2}} up to the [[thermopause]] at an altitude range of {{convert|500|-|1000|km|mi|abbr=on|sigfig=2|comma=5}}. The height of the thermopause varies considerably due to changes in solar activity.<ref name="thermosphere"/> The passage of the dusk and dawn [[Terminator (solar)|solar terminator]] creates background density perturbations up to a factor of two through this layer, forming a dominant feature in this region.<ref name=Fitzpatrick_et_al_2025>{{cite journal | title=Solar Terminator Waves Revealed as Dominant Features of Upper Thermospheric Density | display-authors=1 | first1=D. J. | last1=Fitzpatrick | first2=E. K. | last2=Sutton | first3=M. D. | last3=Pilinski | first4=S. E. | last4=Palo | journal=Geophysical Research Letters | volume=52 | issue=11 | page=1 | date=June 16, 2025 | article-number=e2025GL115612 | doi=10.1029/2025GL115612 | bibcode=2025GeoRL..5215612F | doi-access=free }}</ref> Because the thermopause lies at the lower boundary of the exosphere, it is also referred to as the [[exobase]]. Overlapping the thermosphere, from {{convert|50|to|600|km}} above Earth's surface, is the [[ionosphere]] – a region of enhanced [[Plasma (physics)|plasma]] density.<ref name=Blaunstein_Plohotniuc_2008>{{cite book | title=Ionosphere and Applied Aspects of Radio Communication and Radar | first1=Nathan | last1=Blaunstein | first2=Eugeniu | last2=Plohotniuc | publisher=CRC Press | date=May 13, 2008 | page=1 | isbn=978-1-4200-5517-7 | url=https://books.google.com/books?id=KFmOr2hkeWUC&pg=PA1 }}</ref><ref name=NOAA_layers/>

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. This temperature increase is caused by absorption of [[ion]]izing [[Ultraviolet|UV]] and [[X-ray]] emission from the Sun.<ref name=NOAA_layers/><ref name=NASA_1108>{{cite book | title=Propagation effects on satellite systems at frequencies below 10 GHz – a handbook for satellite system design | first=Warren L. | last=Flock | series=NASA Reference Publication | date=1987 | volume=1108 | pages=1–19 to 1–22 | edition=2nd | publisher=National Aeronautics and Space Administration | url=https://books.google.com/books?id=kZxWAAAAMAAJ&pg=SA1-PA19 }}</ref> 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>{{cite book | last=Ahrens | first=C. Donald | title=Essentials of Meteorology | edition=4th | publisher=Thomson Brooks/Cole | year=2005 | isbn=978-0-534-40679-0 }}</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.<ref name=NOAA_layers>{{cite web | title=Layers of the Atmosphere | date=August 20, 2024 | publisher=National Oceanic and Atmospheric Administration | url=https://www.noaa.gov/jetstream/atmosphere/layers-of-atmosphere | access-date=2025-07-21 }}</ref>

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 at an altitude of around {{cvt|100|km|mi}}.<ref name=Lodders_Fegley_2015>{{cite book | title=Chemistry of the Solar System | first1=Katharina | last1=Lodders | first2=Bruce | last2=Fegley, Jr | publisher=Royal Society of Chemistry | year=2015 | isbn=978-1-78262-601-5 | url=https://books.google.com/books?id=B3UoDwAAQBAJ&pg=PT234 }}</ref> The colors of the aurora are linked to the properties of the atmosphere at the altitude they occur. The most common is the green aurora, which comes from atomic oxygen in the <sup>1</sup>S state, and occurs at altitudes from {{cvt|120|to|400|km|mi|sigfig=2}}.<ref name=NOAA_Aurora>{{cite web | title=Aurora Tutorial | publisher=Space Weather Prediction Center, NOAA | url=https://www.swpc.noaa.gov/content/aurora-tutorial | access-date=2025-07-26 }}</ref> The [[International Space Station]] orbits in the thermosphere, between {{convert|370|and|460|km|mi|abbr=on|sigfig=2}}.<ref name=ISS>{{cite web | title=International Space Station | publisher=NASA | date=May 23, 2023 | url=https://www.nasa.gov/reference/international-space-station/ | access-date=2025-07-23 }}</ref> It is this layer where many of the satellites orbiting the Earth are present.<ref name=Liou_Johnson_2008/>

===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|abbr=on|sigfig=2}} to the mesopause at {{convert|80|-|85|km|mi|abbr=on|sigfig=2}} above sea level.<ref name=NOAA_layers/> 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 encyclopedia | first=Joe | last=Buchdahl | url=http://www.ace.mmu.ac.uk/eae/Atmosphere/Older/Mesosphere.html | title=Atmosphere, Climate & Environment Information Programme | encyclopedia=Encyclopedia of the Atmospheric Environment | publisher=Department for Environment, Food and Rural Affairs | access-date=2012-04-18 | 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> Because the atmosphere absorbs sound waves at a rate that is proportional to the square of the [[frequency]], audible sounds from the ground do not reach the mesosphere. Infrasonic waves can reach this altitude, but they are difficult to emit at a high power level.<ref name=Yang_2016>{{cite book | title=Atmospheric Acoustics | first=Xunren | last=Yang | publisher=Walter de Gruyter GmbH & Co KG | year=2016 | isbn=978-3-11-038302-7 | url=https://books.google.com/books?id=2X8qDAAAQBAJ&pg=PT360 }}</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.<ref name=Gadsden_Parvianinen_2006>{{cite web | title=Observing noctilucent clouds | last1=Gadsden | first1=Michael | last2=Parvianinen | first2=Pekka | year=2006 | publisher=International Association of Geomagnetism & Aeronomy | url=https://e-docs.geo-leo.de/server/api/core/bitstreams/ee627b4f-9c9f-4f53-a0de-f2e2bc3b96fb/content | access-date=2025-07-21 }}</ref>

Lightning-induced discharges known as [[transient luminous event]]s (TLEs) occasionally form in the mesosphere above tropospheric [[thunderclouds]].<ref name=Sato_2015>{{cite journal | title=Overview and early results of the Global Lightning and Sprite Measurements mission | display-authors=1 | last1=Sato | first1=M. | last2=Ushio | first2=T. | last3=Morimoto | first3=T. | last4=Kikuchi | first4=M. | last5=Kikuchi | first5=H. | last6=Adachi | first6=T. | last7=Suzuki | first7=M. | last8=Yamazaki | first8=A. | last9=Takahashi | first9=Y. | last10=Inan | first10=U. | last11=Linscott | first11=I. | last12=Ishida | first12=R. | last13=Sakamoto | first13=Y. | last14=Yoshida | first14=K. | last15=Hobara | first15=Y. | last16=Sano | first16=T. | last17=Abe | first17=T. | last18=Nakamura | first18=M. | last19=Oda | first19=H. | last20=Kawasaki | first20=Z. -I. | journal=Journal of Geophysical Research: Atmospheres | volume=120 | issue=9 | pages=3822–3851 | date=May 2015 | doi=10.1002/2014JD022428 | bibcode=2015JGRD..120.3822S }}</ref> The mesosphere is also the layer where most [[meteor]]s and [[satellite]]s burn up upon [[Atmospheric entry|atmospheric entrance]].<ref name=NOAA_layers/><ref name=Karahan_et_al_2025>{{cite conference | title=Statistical analysis of destructive satellite re-entry uncertainties | display-authors=1 | last1=Karahan | first1=B. | last2=Papa | first2=M. | last3=Galla | first3=D. | last4=Fugmann | first4=M. | last5=Klinkner | first5=S. | conference=Proceedings of the 9th European Conference on Space Debris, Bonn, Germany, 1–4 April 2025 | publisher=ESA Space Debris Office | display-editors=1 | editor1-first=S. | editor1-last=Lemmens | editor2-first=T. | editor2-last=Flohrer | editor3-first=F. | editor3-last=Schmitz | date=April 2025 | url=https://conference.sdo.esoc.esa.int/proceedings/sdc9/paper/342/SDC9-paper342.pdf | access-date=2025-07-22}}</ref> 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]].<ref name=Heatwole_2024>{{cite journal | title=Current usage of sounding rockets to study the upper atmosphere | last=Heatwole | first=Scott E. | journal=Proceedings of the National Academy of Sciences | volume=121 | issue=40 | at=id. e2413285121 | date=September 2024 | article-number=e2413285121 | doi=10.1073/pnas.2413285121 | pmid=39302994 | pmc=11459162 | bibcode=2024PNAS..12113285H }}</ref>

===Stratosphere===
{{main|Stratosphere}}
[[File:Kittinger-jump.jpg|right|thumb|In 1960, [[Joseph Kittinger]] set a record by parachuting from a gondola at {{cvt|31.3|km|mi}}]]
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|abbr=on}} above Earth's surface to the [[stratopause]] at an altitude of about {{convert|50|to|55|km|mi|abbr=on}}.<ref name=Buis_2024/> 99% of the total mass of the atmosphere lies below {{cvt|30|km|mi}},<ref name=Holloway_Wayne_2015/> and the atmospheric pressure at the top of the stratosphere is roughly 1/1000 the [[pressure at sea level]].<ref name=Clouds_2025>{{cite web | title=Introduction to Clouds | date=April 3, 2025 | first=Robert B. | last=Schmunk | publisher=NASA | url=https://www.giss.nasa.gov/edu/icp/education/cloudintro/pressure.html | access-date=2025-07-22 }}</ref> It contains the [[ozone layer]], which is the part of Earth's atmosphere that contains relatively high concentrations of that gas.<ref name=Saha_2012>{{cite book | title=Modern Climatology | first=Pijushkanti | last=Saha | publisher=Allied Publishers | year=2012 | isbn=978-81-8424-756-5 | page=21 | url=https://books.google.com/books?id=X4ymCQAAQBAJ&pg=PA21 }}</ref>

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|−80|C|F K|abbr=on|sigfig=2}} at the tropopause, the top of the stratosphere is much warmer, and may be just below 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 }}</ref><ref name=Saha_2012/> This layer is unique to the Earth; neither Mars nor Venus have a stratosphere because of low abundances of oxygen in their atmospheres.<ref name=de_Pater_Lissauer_2015>{{cite book | title=Planetary Sciences | first1=Imke | last1=de Pater | first2=Jack J. | last2=Lissauer | edition=2 | publisher=Cambridge University Press | year=2015 | isbn=978-1-316-19569-7 | pages=81–82 | url=https://books.google.com/books?id=stFpBgAAQBAJ&pg=PA81 }}</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.<ref name=Saha_2012/> 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.<ref name=Salby_1996>{{cite book | title=Fundamentals of Atmospheric Physics | volume=61 | series=International Geophysics | first=Murry L. | last=Salby | publisher=Elsevier | year=1996 | isbn=978-0-08-053215-8 | pages=283–285 | url=https://books.google.com/books?id=sK1qTDhNBsYC&pg=PA283 }}</ref> The stratosphere is the highest layer that can be accessed by [[jet-powered aircraft]].<ref name=Filippone_2012>{{cite book | title=Advanced Aircraft Flight Performance | volume=34 | series=Cambridge Aerospace Series | first=Antonio | last=Filippone | publisher=Cambridge University Press | year=2012 | isbn=978-1-107-02400-7 | url=https://books.google.com/books?id=6gnO9XoeT5cC&pg=PA210 }}</ref>

===Troposphere===
{{Main|Troposphere}}
[[File:ISS-47_Islands_In_The_Sky,_Indonesia.jpg|thumb|upright=1.25|Earth from orbit, showing sunlight filtered into a reddish hue by the troposphere, while casting shadows from different [[cloud types]]. The [[stratosphere]] forms a thin band of [[Rayleigh scattering|blue scattered]] sunlight along the horizon.]]

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}}, although this [[altitude]] varies from about {{cvt|9|km|mi}} at the [[geographic pole]]s to {{cvt|17|km|mi}} 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=978-0-416-07940-1 }}</ref><ref>{{cite book | last1=Tyson | first1=P. D. | last2=Preston-Whyte | first2=R. A. | year=2000 | title=The Weather and Climate of Southern Africa | edition=2nd | location=Oxford | publisher=[[Oxford University Press]] | page=4 | isbn=0-19-571806-2 }}</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").<ref name=Frederick_2008>{{cite book | title=Principles of Atmospheric Science | first=John E. | last=Frederick | publisher=[[Jones & Bartlett Learning]] | year=2008 | isbn=978-0-7637-4089-4 | pages=15–17 | url=https://books.google.com/books?id=ZDkBKHsgtLMC&pg=PA15 }}</ref> 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 | url=https://archive.org/details/mcgrawhillconcis0000unse_h5v4 | isbn=0-07-045482-5 |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 more severely compressed. Fifty percent of the total mass of the atmosphere is located in the lower {{cvt|5.5|km|mi}} of the troposphere.<ref name=Holloway_Wayne_2015>{{cite book | title=Atmospheric Chemistry | first1=Ann M. | last1=Holloway | first2=Richard P. | last2=Wayne | publisher=Royal Society of Chemistry | year=2015 | isbn=978-1-78262-593-3 | url=https://books.google.com/books?id=qmsoDwAAQBAJ&pg=PT16 }}</ref>

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. The ability of the atmosphere to retain water decreases as the temperature declines, so 90% of the water vapor is held in the lower part of the troposphere.<ref name=Singh_2001>{{cite book | title=Snow and Glacier Hydrology | first1=P. | last1=Singh | first2=Vijay P. | last2=Singh | page=56 | isbn=0-7923-6767-7 | year=2001 | publisher=Springer Netherlands | url=https://books.google.com/books?id=0VW6Tv0LVWkC&pg=PA56 }}</ref> 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.<ref name=Wang_et_al_2009>{{cite journal | title=Further evidences of deep convective vertical transport of water vapor through the tropopause | display-authors=1 | first1=Pao K. | last1=Wang | first2=Martin | last2=Setvák | first3=Walter | last3=Lyons | first4=Willi | last4=Schmid | first5=Hsin-Mu | last5=Lin | journal=Atmospheric Research | volume=94 | issue=3 | date=November 2009 | pages=400–408 | doi=10.1016/j.atmosres.2009.06.018 | bibcode=2009AtmRe..94..400W }}</ref> Most conventional [[aviation]] activity takes place in the troposphere, and it is the only layer accessible by [[Propeller (aeronautics)|propeller-driven aircraft]].<ref name=Filippone_2012/> [[Contrail]]s are formed from jet engine water emission at altitudes where the atmospheric temperature is about {{cvt|−53|C}}; typically around {{cvt|7.7|km|mi}} for modern engines.<ref name=Tiwary_Williams_2018>{{cite book | title=Air Pollution: Measurement, Modelling and Mitigation | edition=Fourth | first1=Abhishek | last1=Tiwary | first2=Ian | last2=Williams | publisher=CRC Press | year=2018 | isbn=978-1-4987-1946-9 | page=218 | url=https://books.google.com/books?id=BzD3DwAAQBAJ&pg=PA218 }}</ref>

===Other layers===
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]] reaches a peak concentration of 15 parts per million at an altitude of {{convert|32|km|mi|abbr=on}}, which is much higher than in the lower atmosphere but still very small compared to the main components of the atmosphere.<ref>{{cite web | title=NASA Ozone Watch | publisher=NASA Goddard Spaceflight Center | url=https://ozonewatch.gsfc.nasa.gov/facts/SH.html | access-date=2025-07-22 }}</ref> It is mainly located in the lower portion of the stratosphere from about {{convert|15|-|35|km|mi|abbr=on}},<ref name=Allens-2002/>{{rp|260}} though the thickness varies seasonally and geographically. About 90% of the ozone in Earth's atmosphere is contained in the stratosphere.<ref name=NOAA_Ozone>{{cite web | title=Science: Ozone Basics | work=Stratospheric Ozone | publisher=National Oceanic and Atmospheric Association | url=https://www.ozonelayer.noaa.gov/science/basics.htm | access-date=2025-07-22 }}</ref>
* The [[ionosphere]] is a region of the atmosphere that is ionized by solar radiation. It plays a significant role in [[Aurora (astronomy)|auroras]], [[airglow]], and [[space weather]] phenomenon.<ref name=Newell_et_al_2001>{{cite journal | title=The role of the ionosphere in aurora and space weather | display-authors=1 | last1=Newell | first1=Patrick T. | last2=Greenwald | first2=Raymond A. | last3=Ruohoniemi | first3=J. Michael | journal=Reviews of Geophysics | volume=39 | issue=2 | pages=137–149 | date=May 2001 | doi=10.1029/1999RG000077 | bibcode=2001RvGeo..39..137N }}</ref><ref name=Basavaiah_2012>{{cite book | title=Geomagnetism: Solid Earth and Upper Atmosphere Perspectives | first=Nathani | last=Basavaiah | publisher=Springer Science & Business Media | year=2012 | isbn=978-94-007-0403-9 | url=https://books.google.com/books?id=uypjt19xtmsC&pg=PA111 }}</ref> During daytime hours, it stretches from {{convert|50|to|1000|km|mi|abbr=on}} and includes the mesosphere, thermosphere, and parts of the exosphere. However, ionization in the mesosphere largely ceases during the night.<ref name=UCAR_ION>{{cite web | title=The Ionosphere | publisher=University Corporation for Atmospheric Research | url=https://scied.ucar.edu/learning-zone/atmosphere/ionosphere | access-date=2025-07-23 }}</ref> The ionosphere forms the inner edge of the [[plasmasphere]] – the inner [[magnetosphere]].<ref name=plasmasphere>{{cite web | title=The Earth's Plasmasphere | first=D. L. | last=Gallagher | publisher=NASA | date=April 26, 2023 | url=https://plasmasphere.nasa.gov/ | access-date=2025-07-23 }}</ref> It has practical importance because it influences, for example, radio propagation on Earth.<ref name=Atiq_2018>{{cite journal | display-authors=1 | last1=Kirby | first1=S. S. | last2=Berkner | first2=L. V. | last3=Stuart | first3=D. M. | year=2006 | title=Studies of the ionosphere and their application to radio transmission | journal=Proceedings of the Institute of Radio Engineers | volume=22 | issue=4 | pages=481–521 | doi=10.1109/JRPROC.1934.225867 | url=http://nvlpubs.nist.gov/nistpubs/jres/12/jresv12n1p15_A2b.pdf | access-date=2025-07-23 }}</ref>
* 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}},<ref name=Schlatter_2009/> 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,<ref name=Schlatter_2009/> 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 | first=Anne Marie | last=Helmenstine | date=June 16, 2018 | access-date=2025-07-21}}</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 |1000|–|1500|m|ft|abbr=on}} or more during the afternoon.<ref name=PBL>{{cite web | title=The Planetary Boundary Layer | first=Jeff | last=Haby | publisher=National Weather Service | url=https://www.weather.gov/source/zhu/ZHU_Training_Page/clouds/planetary_boundary_layer/PBL.html | access-date=2025-07-23 }}</ref>
* The [[barosphere]] is the region of the atmosphere where the [[barometric law]] applies. It ranges from the ground to the thermopause. Above this altitude, the velocity distribution is [[Maxwell–Boltzmann distribution|non-Maxwellian]] due to high velocity atoms and molecules being able to escape the atmosphere.<ref>{{cite book | title=Planetary Aeronomy: Atmosphere Environments in Planetary Systems | series=Physics of Earth and Space Environments | first1=Siegfried | last1=Bauer | first2=Helmut | last2=Lammer | publisher=Springer Science & Business Media | year=2013 | isbn=978-3-662-09362-7 | pages=4–5 | url=https://books.google.com/books?id=PXErBAAAQBAJ&pg=PA4 }}</ref>

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 |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 |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 |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 |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}}.<ref name=Allens-2002/>{{rp|257}} This is sometimes referred to as a unit of [[Atmosphere (unit)|standard atmospheres (atm)]]. Total atmospheric mass is {{cvt|5.1480e18|kg|lb}},<ref>{{Cite journal |title=The Mass of the Atmosphere: A Constraint on Global Analyses |journal=Journal of Climate |volume=18 |issue=6 |page=864 |date=January 1970 |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 mega[[hectare]]s,<ref name=Allens-2002/>{{rp|240}} 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]].

Air pressure [[Exponential decay|decreases exponentially]] with altitude at a rate that depends on the air temperature. The rate of decrease is determined by a temperature-dependent parameter called the [[scale height]]: for each increase in altitude by this height, the pressure decreases by a factor of ''[[E (mathematical constant)|e]]'' (the base of [[natural logarithms]], approximately 2.718). For Earth, this value is typically {{val|5.5|to|6|u=km}} for altitudes up to around {{cvt|80|km|mi}}.<ref name=Daniel_2002>{{cite book | title=Concepts in Space Science | first=R. R. | last=Daniel | publisher=Universities Press | year=2002 | isbn=978-81-7371-410-8 | pages=70–72 | url=https://books.google.com/books?id=WYCltc12Gs8C&pg=PA70 }}</ref> 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, the atmosphere is not well mixed, so each chemical species has its own scale height. At altitudes of {{val|200|to|300|u=km}}, the combined scale height is {{val|20|to|30|u=km}}.<ref name=Daniel_2002/>

The mass of Earth's atmosphere is distributed approximately as follows:<ref>{{cite book | last1=Lutgens | first1=Frederick K. | first2=Edward J. | last2=Tarbuck | year=1995 | title=The Atmosphere | publisher=Prentice Hall | edition=6th | pages=14–17 | isbn=0-13-350612-6 }}</ref>

* 50% is below {{cvt|5.6|km|ft}}
* 90% is below {{cvt|16|km|ft}}
* 99.99997% is below {{cvt|100|km|mi ft}}, 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 {{cvt|8848|m|ft}}; commercial [[airliners]] typically cruise between {{cvt|9|and|11.6|km|ft|adj=ri0|sigfig=2}},<ref name=Sforza_2014>{{cite book | chapter=Fuselage Design | year=2014 | pages=47–79 | title=Commercial Airplane Design Principles | doi=10.1016/B978-0-12-419953-8.00003-6 | first=Pasquale | last=Sforza | isbn=978-0-12-419953-8 }}</ref> where the lower density and temperature of the air improve fuel economy; [[weather balloons]] reach about {{cvt|35|km|ft}};<ref name=Kräuchi_et_al_2016>{{cite journal | title=Controlled weather balloon ascents and descents for atmospheric research and climate monitoring | display-authors=1 | last1=Kräuchi | first1=A. | last2=Philipona | first2=R. | last3=Romanens | first3=G. | last4=Hurst | first4=D. F. | last5=Hall | first5=E. G. | last6=Jordan | first6=A. F. | journal=Atmospheric Measurement Techniques | volume=9 | pages=929–938 | doi=10.5194/amt-9-929-2016 | year=2016 | issue=3 | pmid=29263765 | pmc=5734649 | bibcode=2016AMT.....9..929K | doi-access=free }}</ref> and the highest [[North American X-15|X-15]] flight in 1963 reached {{cvt|108.0|km|ft}}.

Even above the Kármán line, significant atmospheric effects such as [[aurora (astronomy)|auroras]] still occur.<ref name=Lodders_Fegley_2015/> [[Meteors]] begin to glow in this region,<ref name=NOAA_layers/> 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]] typically orbit at 370–460&nbsp;km,<ref name=ISS/> within the [[F-layer]] of the ionosphere,<ref name=Allens-2002/>{{rp|271}} where they encounter enough [[atmospheric drag]] to require reboosts every few months, otherwise [[orbital decay]] will occur, resulting in a return to Earth.<ref name=ISS/> Depending on solar activity, satellites can experience noticeable atmospheric drag at altitudes as high as 600–800&nbsp;km.<ref name=Pisacane_2005>{{cite book | chapter=The Space Environment | first1=Brian J. | last1=Anderson | first2=Donald G. | last2=Mitchell | title=Fundamentals of Space Systems | editor-first=Vincent L. | editor-last=Pisacane | publisher=Oxford University Press | year=2005 | isbn=978-0-19-516205-9 | page=56 | chapter-url=https://books.google.com/books?id=uTwb7d8PTXMC&pg=PA56 }}</ref>

===Temperature===
[[File:Profil temperature atmosphere.png|thumb|Temperature change by altitude]]
{{Main|Atmospheric temperature}}
Starting at sea level, the temperature decreases with altitude until reaching the [[stratosphere]] at around 11&nbsp;km. Above, the temperature stabilizes over a large vertical distance. 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 [[molecular oxygen]] and ozone gas in this region. A second region of increasing temperature with altitude occurs at very high altitudes, in the aptly-named [[thermosphere]] above 90&nbsp;km.<ref name=NOAA_layers/>

During the night, the ground radiates more energy than it gains from the atmosphere. As energy is conducted from the nearby atmosphere to the cooler ground, it creates a [[Inversion (meteorology)|temperature inversion]] where the local temperature increases with altitude up to around 1,000&nbsp;m.<ref>{{cite web | title=Atmospheric controllers of local nighttime temperature | year=2004 | publisher=Pennsylvania State University | url=https://www.weather.gov/source/zhu/ZHU_Training_Page/winds/nighttime_influences/Nighttime_Influences.htm | access-date=2025-07-24 }}</ref>

==== 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.<ref name=SoS>{{cite web | title=Speed of sound | first=Tom | last=Benson | publisher=NASA Glenn Research Center | url=https://www.grc.nasa.gov/WWW/k-12/VirtualAero/BottleRocket/airplane/sound.html | access-date=2025-07-24 }}</ref> For example, at sea level the speed of sound is 340&nbsp;m/s. At the average temperature of the stratosphere, −60&nbsp;°C, the speed of sound decreases to 290&nbsp;m/s.<ref name=Wang_2023>{{cite book | title=A Guide to Fluid Mechanics | first=Hongwei | last=Wang | publisher=Cambridge University Press | year=2023 | isbn=978-1-108-49883-8 | url=https://books.google.com/books?id=ISysEAAAQBAJ&pg=PA194 }}</ref>

===Density and mass===
{{Main|Density of air}}
[[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)]]

The density of air at sea level is about 1.29&nbsp;kg/m<sup>3</sup> (1.29&nbsp;g/L, 0.00129 g/cm<sup>3</sup>).<ref name=Allens-2002/>{{rp|257}} 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]].<ref name=Hall_2021>{{cite web | title=Earth Atmosphere Model | publisher=NASA Glenn Research Center | editor-first=Nancy | editor-last=Hall | date=May 13, 2021 | url=https://www.grc.nasa.gov/WWW/K-12/airplane/atmosmet.html | access-date=2025-07-24 }}</ref> More sophisticated models are used to predict the orbital decay of satellites.<ref name=Kumar_et_al_2022>{{cite journal | title=Simulation of the orbital decay of a spacecraft in low Earth orbit due to aerodynamic drag | display-authors=1 | first1=R. | last1=Kumar | first2=R. | last2=Singh | first3=A. K. | last3=Chinnappan | first4=A. | last4=Appar | journal=The Aeronautical Journal | volume=126 | issue=1297 | date=March 2022 | pages=565–583 | doi=10.1017/aer.2021.83 }}</ref>

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."<ref name=Trenberth_Smith_2005>{{cite journal | title=The Mass of the Atmosphere: A Constraint on Global Analyses | first1=Kevin E. | last1=Trenberth | first2=Lesley | last2=Smith | journal=Journal of Climate | volume=18 | issue=6 | date=March 15, 2005 | pages=864–875 | jstor=26253433 | doi=10.1175/JCLI-3299.1 | bibcode=2005JCli...18..864T }}</ref>

==Optical properties==
{{See also|Sunlight}}
[[File:Earth energy budget.svg|upright=1.2|thumb|The relative absorption, emission and reflection of solar radiation by the atmosphere, clouds and surface]]
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. As energy propagates through the atmosphere, it is impacted by the process of [[radiative transfer]]. That is, some of the incoming and emitted radiation is subject to [[Absorption (electromagnetic radiation)|absorption]], emission, and [[Atmospheric scattering|scattering]] by the atmosphere. Another portion of the incident energy is reflected,<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> with the two most important atmospheric reflectors being [[Fugitive dust|dust]] and clouds. Depending on the properties of the [[aerosol]], clouds can reflect up to 70% of the incident radiation. Globally, clouds reflect 20% of the incoming energy, contributing two thirds of the planet's total [[albedo]].<ref name=Sirvatka>{{cite web | title=Radiation and the Earth-atmosphere system: 'Why is the sky blue?' | first=Paul | last=Sirvatka | work=Introduction to meteorology | publisher=College of DuPage | url=https://weather.cod.edu/sirvatka/scatter.html | access-date=2025-07-25 }}</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 troposphere.<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=May 19, 2017 |work=[[The New York Times]] |access-date=20 May 2017}}</ref><ref name="GRL-201760515">{{cite journal | display-authors=1 |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=May 15, 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.<ref name=Bloomfield_2007>{{cite book | title=How Everything Works: Making Physics Out of the Ordinary | first=Louis A. | last=Bloomfield | publisher=John Wiley & Sons | year=2007 | isbn=978-0-470-17066-3 | page=456 | url=https://books.google.com/books?id=_ZNxDwAAQBAJ&pg=PA456 }}</ref>

===Absorption===
{{Main|Absorption (electromagnetic radiation)}}
[[File:Openstax Astronomy EM spectrum and atmosphere.jpg|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.<ref name=Ondoh_Marubashi_2001>{{cite book | title=Science of Space Environment | series=Wave summit course | editor1-first=Tadanori | editor1-last=Ondoh | editor2-first=Katsuhide | editor2-last=Marubashi | publisher=IOS Press | year=2001 | isbn=978-4-274-90384-7 | page=8 | url=https://books.google.com/books?id=F_UpI_Our3AC&pg=PA8 }}</ref> Water (H<sub>2</sub>O) absorbs at many wavelengths above 700&nbsp;nm.<ref name=Collins_et_al_2006>{{cite journal | title=Effects of increased near-infrared absorption by water vapor on the climate system | display-authors=1 | last1=Collins | first1=William D. | last2=Lee-Taylor | first2=Julia M. | last3=Edwards | first3=David P. | last4=Francis | first4=Gene L. | journal=Journal of Geophysical Research: Atmospheres | volume=111 | issue=D18 | at=ID D18109 | date=September 2006 | doi=10.1029/2005JD006796 | bibcode=2006JGRD..11118109C }}</ref> 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. In [[astronomical spectroscopy]], the absorption of specific frequencies by the atmosphere is referred to as [[telluric contamination]].<ref name=Wang_2022>{{cite journal | title=Characterizing and Mitigating the Impact of Telluric Absorption in Precise Radial Velocities | display-authors=1 | last1=Wang | first1=Sharon Xuesong | last2=Latouf | first2=Natasha | last3=Plavchan | first3=Peter | last4=Cale | first4=Bryson | last5=Blake | first5=Cullen | last6=Artigau | first6=Étienne | last7=Lisse | first7=Carey M. | last8=Gagné | first8=Jonathan | last9=Crass | first9=Jonathan | last10=Tanner | first10=Angelle | journal=The Astronomical Journal | volume=164 | issue=5 | at=id. 211 | date=November 2022 | doi=10.3847/1538-3881/ac947a | doi-access=free | arxiv=2206.07287 | bibcode=2022AJ....164..211W }}</ref>

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.<ref name=McLean_2008>{{cite book | title=Electronic Imaging in Astronomy: Detectors and Instrumentation | series=Astronomy and Planetary Sciences | first=Ian S. | last=McLean | edition=2nd | publisher=Springer Science & Business Media | year=2008 | isbn=978-3-540-76582-0 | pages=38–40 | url=https://books.google.com/books?id=FGHhZf-k8SkC&pg=PA38 }}</ref>

===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 [[wavelength]]s. 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.<ref name=Shelton_2009>{{cite book | title=Hydroclimatology: Perspectives and Applications | first=Marlyn L. | last=Shelton | publisher=Cambridge University Press | year=2009 | isbn=978-0-521-84888-6 | pages=35–37 | url=https://books.google.com/books?id=7a2TspPRWmsC&pg=PA35 }}</ref>

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.<ref name=Bohren_Clothiaux_2006>{{cite book | title=Fundamentals of Atmospheric Radiation | first1=Craig F. | last1=Bohren | first2=Eugene E. | last2=Clothiaux | publisher=John Wiley & Sons | year=2006 | isbn=978-3-527-60837-9 | pages=26–29 | url=https://books.google.com/books?id=VN2RC-xcKioC&pg=PA26 }}</ref> 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 in this manner with sunlight in the visible spectrum. Common examples of these are {{CO2}} and H<sub>2</sub>O.<ref name=Wrigglesworth_1997>{{cite book | title=Energy And Life | series=Lifelines Series | first=John | last=Wrigglesworth | publisher=CRC Press | year=1997 | isbn=978-1-4822-7275-8 | page=155 | url=https://books.google.com/books?id=2H7OBQAAQBAJ&pg=PA155 }}</ref> Without greenhouse gases in the atmosphere, the average temperature of [[Earth's surface]] would be a frozen {{cvt|−18|C|F}}, rather than the present comfortable average of {{cvt|15|C|F}}.<ref name=Ma_1998>{{Cite web | first=Qiancheng | last=Ma | date=March 1998 | title=Greenhouse Gases: Refining the Role of Carbon Dioxide | url=https://www.giss.nasa.gov/research/briefs/archive/1998_ma_01/ | publisher=NASA Goddard Institute for Space Studies | access-date=2025-07-24 }}</ref>

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

The [[refractive index]] of air is close to, but just greater than, 1.<ref>{{cite journal | title=The generalized Sellmeier equation for air | last1=Voronin | first1=A. | last2=Zheltikov | first2=A. | journal=Scientific Reports | volume=7 | id=46111 | year=2017 | article-number=46111 | doi=10.1038/srep46111 | pmid=28836624 | pmc=5569311 | bibcode=2017NatSR...746111V | doi-access=free }} Figure 1 gives a refractive index of  1.000273 at 23&nbsp;C.</ref> 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.<ref name=Basey_2019>{{cite web | title=Atmospheric Refraction | first=David | last=Basey | date=March 2, 2019 | publisher=British Astronomical Association | url=https://britastro.org/2019/atmospheric-refraction | access-date=2025-07-24 }}</ref>

The refractive index of air depends on temperature,<ref name="Edlén">{{cite journal | first=Bengt | last=Edlén |  year=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]].<ref>{{cite web | title=An Introduction to Mirages | first=Andrew T. | last=Young | year=2025 | publisher=San Diego State University | url=https://aty.sdsu.edu/mirages/mirintro.html | access-date=2025-07-25 }}</ref>

==Circulation==
{{Main|Atmospheric circulation}}
[[File:AtmosphCirc2.svg|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. The [[axial tilt]] of the planet means the location of maximum heat is continually changing, resulting in [[Seasonality|seasonal]] variations. The uneven distribution of land and water further breaks up the flow of air.<ref name=GAC/>

The flow of air around the planet is divided into three main [[convection cell]]s by latitude. Around the equator, the [[Hadley cell]] is driven by the rising flow of air along the [[equator]]. In the upper atmosphere, this air flows toward the poles. At mid latitudes, this circulation is reversed, with ground air flowing toward the poles with the [[Ferrel cell]]. Finally, in the high latitudes is the [[Polar cells|Polar cell]], where air again rises and flows toward the poles.<ref name=GAC>{{cite web | title=Global Atmospheric Circulations | date=October 3, 2023 | publisher=National Oceanic and Atmospheric Administration | url=https://www.noaa.gov/jetstream/global/global-atmospheric-circulations | access-date=2025-07-25 }}</ref>

The interface between these cells is responsible for [[jet stream]]s. These are narrow, fast moving bands that flow from west to east and typically form at an elevation of around {{cvt|9100|m|ft|sigfig=2}}. Jet streams can shift around depending on conditions. They are strongest in winter, when the boundaries between hot and cold air are the most pronounced.<ref name=NOAA_Jet_Stream>{{cite web | title=The Jet Stream | publisher=National Oceanic and Atmospheric Administration | date=December 9, 2024 | url=https://www.noaa.gov/jetstream/global/jet-stream | access-date=2025-07-25 }}</ref> In the middle latitudes, it is [[Baroclinic instability|instabilities]] in the jet streams that are responsible for moving [[weather system]]s.<ref name=Moffatt_Shuckburgh_2011>{{cite book | title=Environmental Hazards: The Fluid Dynamics And Geophysics Of Extreme Events | chapter=Weather and Climate | first=Emily | last=Shuckburgh | volume=21 | series=Lecture Notes Series, Institute For Mathematical Sciences, National University Of Singapore | editor1-first=H. Keith | editor1-last=Moffatt | editor2-first=Emily | editor2-last=Shuckburgh | publisher=World Scientific | year=2011 | isbn=978-981-4464-67-3 | page=87 | chapter-url=https://books.google.com/books?id=zK7FCgAAQBAJ&pg=PA87 }}</ref>

As with the [[ocean]]s, the Earth's atmosphere is subject to waves and tidal forces. These are triggered by non-uniform heating by the Sun, and by the daily [[solar cycle]], respectively. Wave-like behavior can occur on a variety of scales, from smaller [[gravity wave]]s that transfer [[momentum]] into the higher atmospheric layers, to much larger [[planetary wave]]s, or [[Rossby wave]]s. [[Atmospheric tide]]s are periodic oscillations of the troposphere and stratosphere that transport energy to the upper atmosphere.<ref name=Volland_2012>{{cite book | title=Atmospheric Tidal and Planetary Waves | volume=12 | series=Atmospheric and Oceanographic Sciences Library | first=Hans | last=Volland | publisher=Springer Science & Business Media | year=2012 | isbn=978-94-009-2861-9 | pages=1–5 | url=https://books.google.com/books?id=jDXwCAAAQBAJ&pg=PA1 }}</ref>

== <span class="anchor" id="History of Earth's atmosphere"></span><span class="anchor" id="Atmosphere of early Earth"></span> Evolution of Earth's atmosphere==
{{See also|History of Earth|Paleoclimatology}}

=== <span class="anchor" id="First atmosphere"></span> Earliest atmosphere===
The first atmosphere, during the [[Early Earth]]'s [[Hadean]] [[geologic eon|eon]], consisted of gases in the [[Solar nebula#Formation of planets|solar nebula]], primarily [[hydrogen]], and probably simple [[hydride]]s such as those now found in the [[gas giant]]s ([[Jupiter]] and [[Saturn]]), notably [[water vapor]], [[methane]] and [[ammonia]]. During this earliest era, the [[giant impact hypothesis|Moon-forming collision]] and numerous [[impact event|impact]]s with large [[meteorite]]s heated the atmosphere, driving off the most volatile gases. The collision with [[Theia (planet)|Theia]], in particular, melted and ejected large portions of [[Earth's mantle]] and [[Earth's crust|crust]] and [[outgass]]ed significant amounts of [[steam]] which eventually cooled and condensed to contribute to ocean water at the end of the Hadean.<ref name=Zahnle>{{Cite journal | display-authors=1 |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 |article-number=a004895 |year=2010 |pmid=20573713 |pmc=2944365}}</ref>{{rp|10}}

===Second atmosphere===
{{See also|Prebiotic atmosphere}}

The increasing [[solidification]] of [[Earth's crust]] at the end of the Hadean closed off most of the [[advection|advective]] [[heat transfer]] to the surface, causing the atmosphere to cool, which [[condense]]d most of the water vapor out of the air [[precipitation|precipitating]] into a [[superocean]]. Further [[outgassing]] from [[volcanism]], supplemented by gases introduced by huge [[asteroid]]s during the [[Late Heavy Bombardment]], created the subsequent [[Archean]] atmosphere, which consisted largely of [[nitrogen]] plus [[carbon dioxide]], [[methane]] and [[inert gas]]es.<ref name=Zahnle/> A major part of carbon dioxide emissions [[dissolution (chemistry)|dissolve]]d in water and reacted with metals such as [[calcium]] and [[magnesium]] during [[weathering]] of [[lithosphere|crustal rocks]] to form [[carbonate]]s that were deposited as [[sediment]]s. Water-related sediments have been found that date from as early as 3.8 billion years ago.<ref>{{cite book | first=B. | last=Windley | title=The Evolving Continents | publisher=Wiley Press | location=New York | year=1984 }}</ref>

About 3.4 billion years ago, nitrogen formed the major component of the then-stable "second atmosphere". The influence of the [[evolution of life]] has to be taken into account rather soon in the history of the atmosphere because hints of [[earliest life forms]] appeared as early as 3.5 billion years ago.<ref>{{cite book | first=J. | last=Schopf | title=Earth's Earliest Biosphere: Its Origin and Evolution | publisher=Princeton University Press | location=Princeton, N.J. | year=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]]".<ref name=Feulner_2012>{{cite journal | last=Feulner | first=Georg | title=The faint young Sun problem | journal=Reviews of Geophysics | date=2012 | volume=50 | issue=2 | page=RG2006 | article-number=2011RG000375 | doi=10.1029/2011RG000375 | arxiv=1204.4449 | bibcode=2012RvGeo..50.2006F | s2cid=119248267 }}</ref>

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 [[Neoarchean]], an oxygen-containing atmosphere began to develop, apparently due to a billion years of [[cyanobacteria]]l [[photosynthesis]] (known as the [[Great Oxygenation Event]]),<ref name=Lyons_et_al_2014>{{cite journal | display-authors=1 | last1=Lyons | first1=Timothy W. | last2=Reinhard | first2=Christopher T. | last3=Planavsky | first3=Noah J. | date=February 2014 | title=The rise of oxygen in Earth's early ocean and atmosphere | journal=Nature | volume=506 | issue=7488 | pages=307–315 | doi=10.1038/nature13068 | pmid=24553238 | bibcode=2014Natur.506..307L | s2cid=4443958 }}</ref>  which have been found as [[stromatolite]] [[fossil]]s 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.<ref name=Hayes_Waldbauer_2006>{{cite journal | title=The Carbon Cycle and Associated Redox Processes through Time | first1=John M. | last1=Hayes | first2=Jacob R. | last2=Waldbauer | journal=Philosophical Transactions of the Royal Society B: Biological Sciences | volume=361 | issue=1470 | date=June 29, 2006 | pages=931–950 | jstor=20209694 | doi=10.1098/rstb.2006.1840 | pmid=16754608 | pmc=1578725 }}</ref>

[[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-Jatuli Carbon Isotope Excursion]].<ref>{{Cite journal | year=2014 | display-authors=1 | first1=Timothy W. | last1=Lyons | first2=Christopher T. | last2=Reinhard  | first3=Noah J. | last3=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===
{{main|Geological history of oxygen}}

The constant re-arrangement of [[continent]]s 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]]<ref name=Cordeiro_Tanaka_2020/> and its appearance is indicated by the end of [[banded iron formation]]s (which signals the depletion of [[substrate (chemistry)|substrate]]s that can react with oxygen to produce [[ferric]] deposits) during the early [[Proterozoic]] eon.<ref>{{cite journal | display-authors=1 | last1=Lantink | first1=Margriet L. | last2=Oonk | first2=Paul B. H. | last3=Floor | first3=Geerke H. | last4=Tsikos | first4=Harilaos | last5=Mason | first5=Paul R. D. | date=February 2018 | title=Fe isotopes of a 2.4&nbsp;Ga hematite-rich IF constrain marine redox conditions around the GOE | journal=[[Precambrian Research]] | volume=305 | pages=218–235 | doi=10.1016/j.precamres.2017.12.025 | bibcode=2018PreR..305..218L | hdl=1874/362652 }}</ref>

Before this time, any oxygen produced by cyanobacterial photosynthesis would be readily removed by the [[oxidation]] of [[reducing agent|reducing substance]]s on the Earth's surface, notably [[ferrous iron]], [[sulfur]] and [[atmospheric methane]]. Free oxygen molecules did not start to accumulate in the atmosphere until the rate of production of oxygen began to exceed the availability of reductant materials that removed oxygen. This point signifies a shift from a [[reducing atmosphere]] to an [[oxidizing]] atmosphere.<ref name=Laakso_Schrag_2017>{{cite journal | title=A theory of atmospheric oxygen | last1=Laakso | first1=T. A. | last2=Schrag | first2=D. P. | journal=Geobiology | volume=15 | issue=3 | pages=366–384 | date=May 2017 | doi=10.1111/gbi.12230 | pmid=28378894 | bibcode=2017Gbio...15..366L | url=http://nrs.harvard.edu/urn-3:HUL.InstRepos:17467352 }}</ref> O<sub>2</sub> showed major variations during the Proterozoic, including [[Boring Billion|a billion-year period]] of [[euxinia]], until reaching a steady state of more than 15% by the end of the [[Precambrian]].<ref>{{cite web | first=Christopher R. | last=Scotese | url=http://www.scotese.com/precamb_chart.htm | title=Back to Earth History: Summary Chart for the Precambrian | publisher=Paleomar Project | year=2010 | access-date=2025-07-21 }}</ref>

The rise of the more robust [[eukaryotic]] [[photoautotroph]]s ([[green algae|green]] and [[red algae]]) injected further oxygenation into the air, especially after the end of the [[Cryogenian]] [[Snowball Earth|global glaciation]], which was followed by an [[evolutionary radiation]] event during the [[Ediacaran]] [[geologic period|period]] known as the [[Avalon explosion]], where complex [[metazoa]]n life forms (including the earliest [[cnidarian]]s, [[placozoa]]ns and [[bilaterian]]s) first proliferated. The following time span from 539 million years ago to the present day is the [[Phanerozoic]] eon, during the earliest [[geologic period|period]] of which, the [[Cambrian]], more [[motility|actively moving]] metazoan life began to appear and rapidly diversify in another radiation event called the [[Cambrian explosion]], whose [[animal locomotion|locomotive]] [[metabolism]] was fuelled by the rising oxygen level.<ref name=Towe_1970>{{cite journal | title=Oxygen-Collagen Priority and the Early Metazoan Fossil Record | last=Towe | first=K. M. | journal=Proceedings of the National Academy of Sciences of the United States of America | volume=65 | issue=4 | pages=781–788 | date=April 1970 | doi=10.1073/pnas.65.4.781 | doi-access=free | pmid=5266150 | pmc=282983 | bibcode=1970PNAS...65..781T }}</ref>

[[File:OxygenLevel-1000ma.svg|thumb|Oxygen content of the atmosphere over the last billion years, showing the [[Neoproterozoic oxygenation event]]<ref>{{cite journal | display-authors=1 |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>{{Cite journal |last=Riding |first=R. |date=October 2009 |title=An atmospheric stimulus for cyanobacterial-bioinduced calcification ca. 350 million years ago? |url=https://pubs.geoscienceworld.org/palaios/article/24/10/685-696/146025 |journal=PALAIOS |language=en |volume=24 |issue=10 |pages=685–696 |doi=10.2110/palo.2009.p09-033r |bibcode=2009Palai..24..685R |issn=0883-1351}}</ref><ref name=Berner_2001>{{Cite journal |last=Berner |first=R. A. |date=March 2001 |title=Modeling atmospheric O2 over Phanerozoic time |url=https://www.sciencedirect.com/science/article/pii/S001670370000572X |journal=Geochimica et Cosmochimica Acta |volume=65 |issue=5 |pages=685–694 |doi=10.1016/S0016-7037(00)00572-X |issn=0016-7037|url-access=subscription }}</ref>]]

The amount of oxygen in the atmosphere has fluctuated over the last 600 million years, reaching a peak of about 35% around 280 million years ago during the [[Carboniferous]] period, significantly higher than today's 21%.<ref name=Berner_2001/> Two main processes govern changes in the atmosphere: the [[evolutionary history of plants|evolution of]] [[plant]]s and their increasing role in [[carbon fixation]], and the consumption of oxygen by rapidly diversifying [[animal]] [[fauna]]e and also by plants for [[photorespiration]] and their own metabolic needs at night. Breakdown of [[pyrite]] and [[volcanic eruption]]s release sulfur into the atmosphere, which reacts and hence reduces oxygen in the atmosphere.<ref name="Calvo-Flores 2025">{{cite book | title=Understanding the Chemistry of the Environment | first=Francisco G. | last=Calvo-Flores | publisher=John Wiley & Sons | year=2025 | isbn=978-1-119-56863-6 | url=https://books.google.com/books?id=wxY4EQAAQBAJ&pg=PA50 }}</ref> However, volcanic eruptions also release carbon dioxide,<ref name=Gerlach_2011>{{cite journal | title=Volcanic versus anthropogenic carbon dioxide | last=Gerlach | first=Terry | journal=Eos, Transactions American Geophysical Union | volume=92 | issue=24 | pages=201–202 | date=June 2011 | doi=10.1029/2011EO240001 | bibcode=2011EOSTr..92..201G }}</ref> which can fuel oxygenic photosynthesis by [[terrestrial plant|terrestrial]] and [[aquatic plant]]s. The cause of the variation of the amount of oxygen in the atmosphere is not precisely understood. Periods with more oxygen in the atmosphere were often associated with more rapid development of animals.<ref name=Cordeiro_Tanaka_2020>{{cite journal | title=Environmental Oxygen is a Key Modulator of Development and Evolution: From Molecules to Ecology | first1=Ingrid Rosenburg | last1=Cordeiro | first2=Mikiko | last2=Tanaka | journal=BioEssays | volume=42 | issue=9 | date=September 2020 | at=2000025 | doi=10.1002/bies.202000025 | pmid=32656788 | doi-access=free }}</ref>

== Air pollution ==
{{Main|Air pollution}}
[[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]].]]

{{em|Air pollution}} is the introduction of airborne [[chemical]]s, [[atmospheric particulate matter|particulate matter]] or [[organic matter|biological materials]] that cause harm or discomfort to organisms.<ref>{{cite web | title=Pollution | url=https://www.merriam-webster.com/dictionary/pollution | access-date=2025-07-25 | publisher=Merriam-Webster }}</ref> The [[population growth]], [[industrialization]] and [[motorization]] of [[human]] [[societies]] have significantly increased the amount of airborne [[pollutant]]s in the Earth's atmosphere, causing noticeable problems such as [[smog]]s, [[acid rain]]s and [[pollution-related diseases]]. The [[ozone depletion|depletion]] of the [[stratospheric]] [[ozone layer]], which shields the surface from harmful [[ionizing radiation|ionizing]] [[ultraviolet]] radiations, is also caused by air pollution, chiefly from [[chlorofluorocarbons]] and other ozone-depleting substances.<ref name=Harrop_2003>{{cite book | title=Air Quality Assessment and Management: A Practical Guide | series=Clay's Library of Health and the Environment | first=Owen | last=Harrop | publisher=CRC Press | year=2003 | isbn=978-0-203-30263-7 | pages=30–49 | url=https://books.google.com/books?id=rYlL5I92MakC&pg=PA30 }}</ref>

Since 1750, human activity, especially after the [[Industrial Revolution]], has increased the concentrations of various [[greenhouse gas]]es, most importantly carbon dioxide, methane and [[nitrous oxide]]. [[Greenhouse gas emissions]], coupled with [[deforestation]] and [[habitat destruction|destruction]] of [[wetland]]s via [[logging]] and [[land development]]s, have caused an observed [[global warming|rise in global temperatures]], with the global average surface temperatures being {{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 }}</ref> It has raised concerns of man-made [[climate change]], which can have significant [[effects of climate change|environmental impacts]] such as [[sea level rise]], [[ocean acidification]], [[glacial retreat]] (which threatens [[water security]]), increasing [[extreme weather]] events and [[wildfire]]s, [[ecological collapse]] and [[Holocene extinction|mass dying of wildlife]].<ref name=OECD>{{cite book | title=Managing Climate Risks, Facing up to Losses and Damages | publisher=OECD Publishing | location=Paris | date=November 2021 | isbn=978-92-64-43966-5 | doi=10.1787/55ea1cc9-en | bibcode=2021mcrf.book.....O | url=https://books.google.com/books?id=2r9LEAAAQBAJ&pg=PA112 }}</ref>

== See also ==
{{Portal|Environment|Weather}}
{{Div col|colwidth=20em}}
* [[Aerial perspective]]
* [[Air (classical element)]]
* [[Airshed]]
* [[Atmospheric dispersion modeling]]
* [[Atmospheric electricity]]
* [[Biosphere]]
* [[Climate system]]
* [[COSPAR International Reference Atmosphere]] (CIRA)
* [[Environmental impact of aviation]]
* [[Global dimming]]
* [[Global surface temperature]]
* [[Hydrosphere]]
* [[Lithosphere]]
* [[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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