Earth

== Atmosphere ==
{{Main|Atmosphere of Earth}}
[[File:ISS-42 Waning sun.jpg|thumb|A view of Earth with different layers of its atmosphere visible: the [[troposphere]] with its clouds casting shadows, a band of [[stratospheric]] blue sky at the horizon, and a line of green [[airglow]] of the lower [[thermosphere]] around an [[Kármán line|altitude of 100&nbsp;km, at the edge of space]]]]
The [[atmospheric pressure]] at Earth's sea level averages {{convert|101.325|kPa|psi|3|abbr=on}},<ref name="Exline2006">{{cite book|last1=Exline|first1=Joseph D. |url=https://www.nasa.gov/pdf/288978main_Meteorology_Guide.pdf|title=Meteorology: An Educator's Resource for Inquiry-Based Learning for Grades 5–9|last2=Levine|first2=Arlene S. |last3=Levine|first3=Joel S.|date=2006|publisher=NASA/Langley Research Center|page=6|id=NP-2006-08-97-LaRC}}</ref> with a [[scale height]] of about {{convert|8.5|km|mi|abbr=on}}.<ref name="earth_fact_sheet" /> A dry atmosphere is composed of 78.084% [[nitrogen]], 20.946% oxygen, 0.934% [[argon]], and trace amounts of carbon dioxide and other gaseous molecules.<ref name="Exline2006" /> [[Water vapor]] content varies between 0.01% and 4%<ref name="Exline2006" /> but averages about 1%.<ref name="earth_fact_sheet" /> [[Cloud cover|Clouds cover]] around two-thirds of Earth's surface, more so over oceans than land.<ref name="King Platnick Menzel Ackerman 2013 pp. 3826–3852">{{cite journal |last1=King |first1=Michael D. |last2=Platnick |first2=Steven |last3=Menzel |first3=W. Paul |last4=Ackerman |first4=Steven A. |last5=Hubanks |first5=Paul A. |title=Spatial and Temporal Distribution of Clouds Observed by MODIS Onboard the Terra and Aqua Satellites |journal=IEEE Transactions on Geoscience and Remote Sensing |publisher=Institute of Electrical and Electronics Engineers (IEEE) |volume=51 |issue=7 |year=2013 |issn=0196-2892 |doi=10.1109/tgrs.2012.2227333 |pages=3826–3852|bibcode=2013ITGRS..51.3826K |s2cid=206691291 |doi-access=free |hdl=2060/20120010368 |hdl-access=free }}</ref> The height of the [[troposphere]] varies with latitude, ranging between {{convert|8|km|mi|0|abbr=on}} at the poles to {{convert|17|km|mi|0|abbr=on}} at the equator, with some variation resulting from weather and seasonal factors.<ref name="geerts_linacre97" />

Earth's [[biosphere]] has significantly altered its [[Atmosphere of Earth|atmosphere]]. [[Oxygen evolution#Oxygen evolution in nature|Oxygenic photosynthesis]] evolved {{val|2.7|u=Gya}}, [[oxygen catastrophe|forming]] the primarily nitrogen–oxygen atmosphere of today.<ref name="NYT-20131003" /> This change enabled the proliferation of [[aerobic organisms]] and, indirectly, the formation of the ozone layer due to the subsequent [[Ozone–oxygen cycle|conversion of atmospheric {{chem2|O2}} into {{chem2|O3}}]]. The ozone layer blocks [[ultraviolet]] [[solar radiation]], permitting life on land.<ref name="Harrison 2002" /> Other atmospheric functions important to life include transporting water vapor, providing useful gases, causing small meteors to burn up before they strike the surface, and moderating temperature.<ref name="atmosphere" /> This last phenomenon is the [[greenhouse effect]]: trace molecules within the atmosphere serve to capture [[thermal energy]] emitted from the surface, thereby raising the average temperature. Water vapor, carbon dioxide, [[methane]], [[nitrous oxide]], and [[ozone]] are the primary greenhouse gases in the atmosphere. Without this heat-retention effect, the average surface temperature would be {{convert|−18|C|F}}, in contrast to the current {{convert|+15|C|F}},<ref name="Pidwirny2006_7" /> and life on Earth probably would not exist in its current form.<ref name="Narottam2008" />

=== Weather and climate ===
{{Main|Weather|Climate}}
{{Multiple image
| align             = right
| direction         = vertical
| width             = 300
| image1            = IntertropicalConvergenceZone-EO.jpg
| caption1          = The [[ITCZ]]'s band of clouds over the Eastern Pacific and the Americas as seen from space
| image5            = Köppen-Geiger Climate Classification Map (1980–2016) no borders.png
| caption5          = Worldwide [[Köppen climate classification]]s
}}
Earth's atmosphere has no definite boundary, gradually becoming thinner and fading into outer space.<ref>{{cite web|url=https://www.nationalgeographic.com/science/article/where-is-the-edge-of-space-and-what-is-the-karman-line|archive-url=https://web.archive.org/web/20210304132146/https://www.nationalgeographic.com/science/article/where-is-the-edge-of-space-and-what-is-the-karman-line|url-status=dead|archive-date=4 March 2021|title=Where, exactly, is the edge of space? It depends on who you ask|website=[[National Geographic]] |last1=Drake |first1=Nadia |author-link1=Nadia Drake|date=20 December 2018|access-date=4 December 2021}}</ref> Three-quarters of the atmosphere's mass is contained within the first {{convert|11|km|mi|abbr=on}} of the surface; this lowest layer is called the troposphere.<ref>{{cite web|url=https://spaceplace.nasa.gov/troposphere/en/ |title=Troposphere |website=SpacePlace|publisher=[[NASA]]|last1=Erickson|first1=Kristen|last2=Doyle|first2=Heather|date=28 June 2019|access-date=4 December 2021}}</ref> Energy from the Sun heats this layer, and the surface below, causing expansion of the air. This lower-density air then rises and is replaced by cooler, higher-density air. The result is [[atmospheric circulation]] that drives the weather and climate through redistribution of thermal energy.<ref name="moran2005" />

The primary atmospheric circulation bands consist of the [[trade winds]] in the equatorial region below 30° latitude and the [[westerlies]] in the mid-latitudes between 30° and 60°.<ref name="berger2002" /> [[Ocean heat content]] and [[Ocean current|currents]] are also important factors in determining climate, particularly the [[thermohaline circulation]] that distributes thermal energy from the equatorial oceans to the polar regions.<ref name=rahmstorf2003 />

Earth receives 1361&nbsp;W/m<sup>2</sup> of&nbsp;[[solar irradiance]].<ref>{{cite web |title=Earth Fact Sheet |website=NASA Space Science Data Coordinated Archive |date=5 June 2023 |url=https://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html |access-date=17 September 2023}}</ref><ref>{{cite journal | first1=Odele | last1=Coddington | first2=Judith L. | last2=Lean | author2-link=Judith Lean | first3=Peter | last3=Pilewskie | first4=Martin | last4=Snow | first5=Doug | last5=Lindholm |date=2016 |title=A Solar Irradiance Climate Data Record |journal=Bulletin of the American Meteorological Society |volume=97 |issue=7 |pages=1265–1282 |bibcode=2016BAMS...97.1265C |doi=10.1175/bams-d-14-00265.1 |doi-access=free}}</ref> The amount of solar energy that reaches Earth's surface decreases with increasing latitude. At higher latitudes, the sunlight reaches the surface at lower angles, and it must pass through thicker columns of the atmosphere. As a result, the mean annual air temperature at sea level decreases by about {{convert|0.4|C-change|F-change|1}} per degree of latitude from the equator.<ref name="sadava_heller2006" /> Earth's surface can be subdivided into specific latitudinal belts of approximately homogeneous climate. Ranging from the equator to the polar regions, these are the tropical (or equatorial), [[Subtropics|subtropical]], [[temperate]] and [[Polar region|polar]] climates.<ref name="climate_zones" />

Further factors that affect a location's climates are its [[Continentality|proximity to oceans]], the oceanic and atmospheric circulation, and topology.<ref>{{cite book |last1=Rohli |first1=Robert. V.|title=Climatology|last2=Vega|first2=Anthony J.|publisher=Jones & Bartlett Learning|year=2018|isbn=978-1-284-12656-3|edition=fourth|page=49}}</ref> Places close to oceans typically have colder summers and warmer winters, due to the fact that oceans can store large amounts of heat. The wind transports the cold or the heat of the ocean to the land.<ref>{{cite book|last1=Rohli|first1=Robert. V.|title=Climatology|last2=Vega|first2=Anthony J.|publisher=Jones & Bartlett Learning |year=2018 |isbn=978-1-284-12656-3|edition=fourth|page=32}}</ref> Atmospheric circulation also plays an important role: San Francisco and Washington DC are both coastal cities at about the same latitude. San Francisco's climate is significantly more moderate as the prevailing wind direction is from sea to land.<ref>{{cite book |last1=Rohli |first1=Robert. V.|title=Climatology|last2=Vega|first2=Anthony J.|publisher=Jones & Bartlett Learning|year=2018|isbn=978-1-284-12656-3|edition=fourth|page=34}}</ref> Finally, temperatures [[Lapse rate|decrease with height]] causing mountainous areas to be colder than low-lying areas.<ref>{{cite book|last1=Rohli|first1=Robert. V. |title=Climatology |last2=Vega |first2=Anthony J. |publisher=Jones & Bartlett Learning |year=2018 |isbn=978-1-284-12656-3 |edition=fourth |page=46}}</ref>

Water vapor generated through surface evaporation is transported by circulatory patterns in the atmosphere. When atmospheric conditions permit an uplift of warm, humid air, this water condenses and falls to the surface as [[precipitation]].<ref name="moran2005" /> Most of the water is then transported to lower elevations by river systems and usually returned to the oceans or deposited into lakes. This [[water cycle]] is a vital mechanism for supporting life on land and is a primary factor in the erosion of surface features over geological periods. Precipitation patterns vary widely, ranging from several meters of water per year to less than a millimeter. Atmospheric circulation, topographic features, and temperature differences determine the average precipitation that falls in each region.<ref name="hydrologic_cycle" />

The commonly used [[Köppen climate classification]] system has five broad groups ([[tropical climate|humid tropics]], [[arid]], [[humid subtropical climate|humid middle latitudes]], [[Continental climate|continental]] and cold [[polar climate|polar]]), which are further divided into more specific subtypes.<ref name="berger2002" /> The Köppen system rates regions based on observed temperature and precipitation.<ref>{{cite book|last1=Rohli|first1=Robert. V.|title=Climatology|last2=Vega|first2=Anthony J.|publisher=Jones & Bartlett Learning|year=2018|isbn=978-1-284-12656-3|edition=fourth|page=159}}</ref> Surface [[Highest temperature recorded on Earth|air temperature can rise to]] around {{convert|55|C|F}} in [[hot desert]]s, such as [[Death Valley National Park|Death Valley]], and [[Lowest temperature recorded on Earth|can fall as low as]] {{convert|-89|C|F}} in [[Antarctica]].<ref>{{Cite journal | first1=Khalid I. | last1=El Fadli | first2=Randall S. | last2=Cerveny | first3=Christopher C. | last3=Burt | first4=Philip | last4=Eden | first5=David | last5=Parker | first6=Manola | last6=Brunet | first7=Thomas C. | last7=Peterson | first8=Gianpaolo | last8=Mordacchini | first9=Vinicio | last9=Pelino | first10=Pierre | last10=Bessemoulin | first11=José Luis | last11=Stella | first12=Fatima | last12=Driouech | first13=M. M Abdel | last13=Wahab | first14=Matthew B. | last14=Pace |display-authors=1|date=2013|title=World Meteorological Organization Assessment of the Purported World Record 58°C Temperature Extreme at El Azizia, Libya (13 September 1922)|journal=Bulletin of the American Meteorological Society |language=en |volume=94 |issue=2 |pages=199–204 |doi=10.1175/BAMS-D-12-00093.1|bibcode=2013BAMS...94..199E|issn=0003-0007|doi-access=free}}</ref><ref>{{Cite journal|last1=Turner|first1=John|display-authors=et al |date=2009 |title=Record low surface air temperature at Vostok station, Antarctica|journal=Journal of Geophysical Research: Atmospheres |language=en |volume=114 |issue=D24 |page=D24102 |doi=10.1029/2009JD012104|bibcode=2009JGRD..11424102T|issn=2156-2202|doi-access=free}}</ref>

=== Upper atmosphere ===
[[File:Antarctic aurora ESA313457.jpg|thumb|upright=1.3|Earth's atmosphere as it appears from space, as bands of different colours at the horizon. From the bottom, [[afterglow]] illuminates the [[troposphere]] in orange with silhouettes of clouds, and the [[stratosphere]] in white and blue. Next the [[mesosphere]] (pink area) extends to just below the [[Kármán line|edge of space]] at one hundred kilometers and the pink line of [[airglow]] of the lower [[thermosphere]] (invisible), which hosts green and red [[aurora]]e over several hundred kilometers.]]
The upper atmosphere, the atmosphere above the troposphere,<ref>{{cite web |last=Morton |first=Oliver |title=Upper atmosphere Definition und Bedeutung |website=Collins Wörterbuch |date=26 August 2022 |url=https://www.collinsdictionary.com/de/worterbuch/englisch/upper-atmosphere |language=de |access-date=26 August 2022}}</ref> is usually divided into the [[stratosphere]], [[mesosphere]], and [[thermosphere]].<ref name="atmosphere" /> Each layer has a different lapse rate, defining the rate of change in temperature with height. Beyond these, the [[exosphere]] thins out into the magnetosphere, where the geomagnetic fields interact with the solar wind.<ref name=sciweek2004 /> Within the stratosphere is the ozone layer, a component that partially shields the surface from ultraviolet light and thus is important for life on Earth. The [[Kármán line]], defined as {{Convert|100|km|mi|abbr=on}} above Earth's surface, is a working definition for the boundary between the atmosphere and [[outer space]].<ref name=cordoba2004 />

Thermal energy causes some of the molecules at the outer edge of the atmosphere to increase their velocity to the point where they can escape from Earth's gravity. This causes a slow but steady [[Atmospheric escape|loss of the atmosphere into space]]. Because unfixed [[hydrogen]] has a low [[molecular mass]], it can achieve [[escape velocity]] more readily, and it leaks into outer space at a greater rate than other gases.<ref name="jas31_4_1118" /> The leakage of hydrogen into space contributes to the shifting of Earth's atmosphere and surface from an initially [[redox|reducing]] state to its current oxidizing one. Photosynthesis provided a source of free oxygen, but the loss of reducing agents such as hydrogen is thought to have been a necessary precondition for the widespread accumulation of oxygen in the atmosphere.<ref name="sci293_5531_839" /> Hence the ability of hydrogen to escape from the atmosphere may have influenced the nature of life that developed on Earth.<ref name="abedon1997" /> In the current, oxygen-rich atmosphere most hydrogen is converted into water before it has an opportunity to escape. Instead, most of the hydrogen loss comes from the destruction of methane in the upper atmosphere.<ref name="arwps4_265" />