Editing Tropopause

{{Short description|The boundary of the atmosphere between the troposphere and stratosphere}}
[[Image:Jetcrosssection.jpg|thumb|right|300px|The tropopause extends to high altitudes in the tropical latitudes and extends to low altitudes in the polar latitudes.]]
The '''tropopause''' is the atmospheric boundary that demarcates the lowest two layers of the [[atmosphere of Earth]] – the [[troposphere]] and [[stratosphere]] – which occurs approximately {{convert|17|km}} above the [[Tropics|equatorial regions]], and approximately {{convert|9|km}} above the [[polar regions]].<ref>{{Cite web |title=Layers of the Atmosphere {{!}} National Oceanic and Atmospheric Administration |url=https://www.noaa.gov/jetstream/atmosphere/layers-of-atmosphere |access-date=2025-04-15 |website=www.noaa.gov |language=en}}</ref>

==Definition==
[[Image:Earth Atmosphere.svg|thumb|'''The atmosphere of planet Earth:''' The ''tropopause'' is between the troposphere and the stratosphere.]]
Rising from the planetary surface of the Earth, the tropopause is the atmospheric level where the air ceases to become cool with increased altitude and becomes dry, devoid of water vapor. The tropopause is the boundary that demarcates the [[troposphere]] below from the [[stratosphere]] above, and is part of the atmosphere where there occurs an abrupt change in the [[lapse rate|environmental lapse rate]] (ELR) of temperature, from a positive rate (of decrease) in the troposphere to a negative rate in the stratosphere. The tropopause is defined as the lowest level at which the lapse rate decreases to 2°C/km or less, provided that the average lapse-rate, between that level and all other higher levels within 2.0 km does not exceed 2°C/km.<ref>{{cite book|title= International Meteorological Vocabulary|edition= 2nd|date= 1992|publisher= Secretariat of the World Meteorological Organization|location= Geneva|isbn= 978-92-63-02182-3|page= 636}}</ref> The tropopause is a [[Continuous function|first-order discontinuity]] surface, in which temperature as a function of height varies continuously through the atmosphere, while the [[temperature gradient]] has a discontinuity.<ref>{{Cite book |last=Panchev |first=S. |title=Dynamic Meteorology |date=1985 |publisher=D. Reidel Publishing Company |isbn=978-90-277-1744-3 |series=Environmental fluid mechanics |location=Dordrecht, Boston, Lancaster |language=en}}</ref>

==Location==
The troposphere is the lowest layer of the Earth's atmosphere; it starts at the [[planetary boundary layer]], and is the layer in which most [[weather]] phenomena occur. The troposphere contains the boundary layer, and ranges in height from an average of {{convert|9|km|mi ft|abbr=on}} at the poles, to {{convert|17|km|mi ft|abbr=on}} at the [[Equator]].<ref>{{cite journal | last1 = Hoinka | first1 = K. P. | date = 1999 | title = Temperature, Humidity, and Wind at the Global Tropopause | journal = [[Monthly Weather Review]] | volume = 127 | issue = 10 | pages = 2248–2265 |bibcode = 1999MWRv..127.2248H |doi = 10.1175/1520-0493(1999)127<2248:THAWAT>2.0.CO;2 | doi-access = free }}</ref><ref>{{Cite journal|title= Distribution and influence of convection in the tropical tropopause region|last1= Gettelman|first1= A.|last2= Salby|first2= M. L.|author-link2=Murry Salby|last3= Sassi|first3= F.|date= 2002|journal= [[Journal of Geophysical Research]]|volume= 107|issue= D10|pages= ACL 6–1–ACL 6–12|doi= 10.1029/2001JD001048|bibcode = 2002JGRD..107.4080G |citeseerx= 10.1.1.469.189}}</ref> In the absence of [[Inversion (meteorology)|inversion]]s and not considering [[moisture]], the [[temperature lapse rate]] for this layer is 6.5&nbsp;°C per kilometer, on average, according to the ''[[U.S. Standard Atmosphere]]''.<ref>{{Cite book |last=Petty |first=Grant W. |title=A first course in atmospheric thermodynamics |date=2008 |publisher=Sundog Publishing |isbn=978-0-9729033-2-5 |location=Madison, Wisconsin}}</ref> A measurement of the tropospheric and the stratospheric lapse rates helps identify the location of the tropopause, since temperature increases with height in the stratosphere, and hence the lapse rate becomes negative. 

Since the tropopause responds to the average temperature of the entire layer that lies underneath it, it is at its maximum levels over the Equator, and reaches minimum heights over the poles. On account of this, the coolest layer in the atmosphere lies at about 17&nbsp;km over the equator. Due to the variation in starting height, the tropopause extremes are referred to as the equatorial tropopause and the polar tropopause. 

Variations of starting height were found to correspond with the thermal structure both above and below where tropopause was recognized. The beginning of tropopause was determined to have a positive correlation to tropospheric temperature anomalies and a negative correlation to stratospheric temperature anomalies.<ref name=":0">{{Cite journal |last=Meng |first=Lingyun |last2=Liu |first2=Jane |last3=Tarasick |first3=David W. |last4=Randel |first4=William J. |last5=Steiner |first5=Andrea K. |last6=Wilhelmsen |first6=Hallgeir |last7=Wang |first7=Lei |last8=Haimberger |first8=Leopold |date=2021-11-05 |title=Continuous rise of the tropopause in the Northern Hemisphere over 1980–2020 |url=https://www.science.org/doi/10.1126/sciadv.abi8065 |journal=Science Advances |language=en |volume=7 |issue=45 |doi=10.1126/sciadv.abi8065 |issn=2375-2548 |pmc=8570593 |pmid=34739322}}</ref> However, since 1980 the tropopause has been warming while the stratosphere cools according to Integrated Global Radiosonde Archive V2 data from [[National Oceanic and Atmospheric Administration|NOAA.]]<ref name=":0" />This impacts tropopause starting height as the value is related to the average temperatures of the layers above and below. Because of a shift in average temperatures – particularly in the troposphere – it is expected the initial height of tropopause will increase given the expansion of warm air in the layer below.<ref name=":0" />

== Alternative definitions ==
Given that the lapse rate is not a conservative quantity when the tropopause is considered for stratosphere-troposphere exchanges studies, there exists an alternative definition named ''dynamic tropopause''.<ref>{{Cite book |last=Andrews |first=David G. |title=Middle atmosphere dynamics |last2=Holton |first2=James R. |last3=Leovy |first3=Conway B. |date=1999 |publisher=Acad. Pr |isbn=978-0-12-058576-2 |edition=3. print |series=International geophysics series |location=Orlando, Fla.}}</ref> It is formed with the aid of [[potential vorticity]], which is defined as the product of the [[isentropic]] [[density]], i.e. the density that is measurable by using [[potential temperature]] as the vertical coordinate, and the [[absolute vorticity]], given that this quantity attains quite different values for the troposphere and the stratosphere.<ref>{{Cite journal|last1= Hoskins|first1= B. J.|last2= McIntyre|first2= M. E.|author2-link=Michael E. McIntyre|last3= Robertson|first3= A. W.|date= 1985|title= On the use and significance of isentropic potential vorticity maps|journal= [[Quarterly Journal of the Royal Meteorological Society]]|volume= 111|issue= 470|pages= 877–946|bibcode = 1985QJRMS.111..877H |doi = 10.1002/qj.49711147002 }}</ref> Instead of using the vertical temperature gradient as the defining variable, the dynamic tropopause surface is expressed in ''[[potential vorticity unit]]s'' (PVU, 1&nbsp;PVU&nbsp;=&nbsp;10{{sup|-6}}&nbsp;K&nbsp;m{{sup|2}}&nbsp;kg{{sup|-1}}&nbsp;s{{sup|-1}}<ref name=":3">{{Cite web |last=World Meteorological Organization (WMO) |last2=National Aeronautics and Space Administration (NASA) |last3=Administration |first3=Federal Aviation |last4=National Oceanic and Atmospheric Administration (NOAA) |last5=United Nations Environment Programme (UNEP) |last6=Commission |first6=European |last7=Bundesministerium für Forschung und Technologie |title=Atmospheric ozone 1985 – volume I |url=https://library.wmo.int/records/item/60466-atmospheric-ozone-1985-volume-i |archive-url=http://web.archive.org/web/20231128090246/https://library.wmo.int/records/item/60466-atmospheric-ozone-1985-volume-i |archive-date=2023-11-28 |access-date=2025-04-15 |website=library.wmo.int |language=en}}</ref>). Given that the absolute vorticity is positive in the Northern Hemisphere and negative in the [[Southern Hemisphere]], the threshold value should be considered as positive north of the Equator and negative south of it.<ref>{{Cite journal |last=Hoinka |first=Klaus P. |date=1998 |title=Statistics of the Global Tropopause Pressure |url=http://journals.ametsoc.org/doi/10.1175/1520-0493(1998)1262.0.CO;2 |journal=Monthly Weather Review |language=en |volume=126 |issue=12 |pages=3303–3325 |doi=10.1175/1520-0493(1998)126<3303:SOTGTP>2.0.CO;2 |issn=0027-0644|doi-access=free }}</ref> Theoretically, to define a global tropopause in this way, the two surfaces arising from the positive and negative thresholds need to be matched near the equator using another type of surface such as a constant [[potential temperature]] surface. Nevertheless, the dynamic tropopause is useless at equatorial latitudes because the isentropes are almost vertical.<ref name=":3" /> For the extratropical tropopause in the [[Northern Hemisphere]] the WMO established a value of 1.6 PVU,<ref name=":3" />{{rp|152}} but greater values ranging between 2 and 3.5 PVU have been traditionally used.<ref>{{Cite journal |last=Zängl |first=Günther |last2=Hoinka |first2=Klaus P. |date=2001 |title=The Tropopause in the Polar Regions |url=http://journals.ametsoc.org/doi/10.1175/1520-0442(2001)0142.0.CO;2 |journal=Journal of Climate |language=en |volume=14 |issue=14 |pages=3117–3139 |doi=10.1175/1520-0442(2001)014<3117:TTITPR>2.0.CO;2 |issn=0894-8755|doi-access=free }}</ref>

It is also possible to define the tropopause in terms of chemical composition.<ref>{{Cite journal |last=Pan |first=L. L. |last2=Randel |first2=W. J. |last3=Gary |first3=B. L. |last4=Mahoney |first4=M. J. |last5=Hintsa |first5=E. J. |date=2004 |title=Definitions and sharpness of the extratropical tropopause: A trace gas perspective |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2004JD004982 |journal=Journal of Geophysical Research: Atmospheres |language=en |volume=109 |issue=D23 |doi=10.1029/2004JD004982 |issn=2156-2202|hdl=1912/3670 |hdl-access=free }}</ref> For example, the lower stratosphere has much higher [[ozone]] concentrations than the upper troposphere, but much lower [[water vapor]] concentrations, so an appropriate boundary can be defined.

Additionally, a stability-based definition can be applied, in which the vertical gradient of potential temperature is used to identify where the chemical composition changes the most. This can be defined as potential temperature gradient tropopause, or PTGT.<ref name=":1">{{Cite journal |last=Tinney |first=Emily N. |last2=Homeyer |first2=Cameron R. |last3=Elizalde |first3=Lexy |last4=Hurst |first4=Dale F. |last5=Thompson |first5=Anne M. |last6=Stauffer |first6=Ryan M. |last7=Vömel |first7=Holger |last8=Selkirk |first8=Henry B. |date=2022 |title=A Modern Approach to a Stability-Based Definition of the Tropopause |url=https://journals.ametsoc.org/view/journals/mwre/150/12/MWR-D-22-0174.1.xml |journal=Monthly Weather Review |volume=150 |issue=12 |pages=3151–3174 |doi=10.1175/MWR-D-22-0174.1 |issn=0027-0644|hdl=11603/26155 |hdl-access=free }}</ref> In order to examine these stability metrics, the relationship between a low-stability troposphere and high-stability troposphere is used to analyze gradients in the transition layer. The composition change of O3 on this vertical gradient is also considered.<ref name=":1" /> This strategy aims to overcome potential failures of more traditional systems with static stability observations.

==Tropical tropopause layer cold trap==
In 1949 [[Alan West Brewer]] proposed that tropospheric air passes through the tropopause into the stratosphere near the equator, then travels through the stratosphere to temperate and polar regions, where it sinks into the troposphere.
<ref>{{cite journal|title=Evidence for a world circulation provided by the measurements of helium and water vapor distribution in the stratosphere
|last=Brewer|first=A. W.|date=Oct 1949
|journal=Quarterly Journal of the Royal Meteorological Society|volume=75|issue=326|pages=351–363|doi=10.1002/qj.49707532603|bibcode=1949QJRMS..75..351B }}</ref>
This is now known as [[Brewer-Dobson circulation]].
Because gases primarily enter the stratosphere by passing through the tropopause in the tropics where the tropopause is coldest, water vapor is condensed out of the air that is entering the stratosphere. This ″tropical tropopause layer [[cold trap (astronomy)|cold trap]]″ theory has become widely accepted.
<ref>{{cite journal|title=Cold trap dehydration in the Tropical Tropopause Layer characterised by SOWER chilled-mirror hygrometer network data in the Tropical Pacific
|last1=Hasebe|first1=F.|last2=Inai|first2=Y.|last3=Shiotani|first3=M.|last4=Fujiwara|first4=M.|last5=Vömel|first5=H.
|last6=Nishi|first6=N.|last7=Ogino|first7=S.-Y.|last8=Shibata|first8=T.|last9=Iwasaki|first9=S.
|last10=Komala|first10=N.|last11=Peter|first11=T.|last12=Oltmans|first12=S. J.|date=Apr 2013
|journal=Atmospheric Chemistry and Physics|volume=13|issue=8|pages=4393–4411 |doi=10.5194/acp-13-4393-2013|bibcode=2013ACP....13.4393H |hdl=20.500.11850/67923|hdl-access=free |doi-access=free }}</ref>
This cold trap limits stratospheric water vapor to 3 to 4 parts per million.
<ref>{{cite book|title=Atmospheric Evolution on Inhabited and Lifeless Worlds
|last1=Catling|first1=David C.|last2=Kasting|first2=James F.|year=2017|bibcode=2017aeil.book.....C }}</ref>
Researchers at [[Harvard]] have suggested that the effects of [[Global Warming]] on air circulation patterns will weaken the tropical tropopause layer cold trap.
<ref>{{cite journal | doi=10.5194/acp-23-7447-2023 | title=Weakening of the tropical tropopause layer cold trap with global warming | date=2023 | last1=Bourguet | first1=Stephen | last2=Linz | first2=Marianna | journal=Atmospheric Chemistry and Physics | volume=23 | issue=13 | pages=7447–7460 | bibcode=2023ACP....23.7447B | s2cid=259520137 | doi-access=free }}</ref>

Water vapor that is able to make it through the cold trap eventually rises to the top of the stratosphere, where it undergoes [[photodissociation]] into [[oxygen]] and [[hydrogen]] or [[hydroxide]] ions and hydrogen.<ref>{{cite journal|title=The aeronomic dissociation of water vapor by solar H Lyman α radiation
|last1=Lewis|first1=B. R.|last2=Vardavas|first2=I. M.|last3=Carver|first3=J. H.|date=June 1983
|journal=Journal of Geophysical Research|volume=88|issue=A6|pages=4935–4940|doi=10.1029/JA088iA06p04935
|bibcode=1983JGR....88.4935L }}</ref>
<ref>{{cite journal|title=On the photodissociation of water vapour in the mesosphere|last=Nicolet|first=Marcel|date=July 1984
|journal=Planetary and Space Science|volume=32|issue=7|pages=871–880|doi=10.1016/0032-0633(84)90011-4
|bibcode=1984P&SS...32..871N }}</ref>
This hydrogen is then able to [[atmospheric escape|escape]] the atmosphere.
Thus, in some sense, the tropical tropopause layer cold trap is what prevents Earth from losing its water to space.
[[James Kasting]] has predicted that [[Future of Earth|in 1 to 2 billion years]], as the [[Sun]] increases in luminosity, the temperature of the Earth will rise enough that the cold trap will no longer be effective, and so the Earth will dry out.<ref>{{cite journal|title=The life span of the biosphere revisited|last1=Caldeira|first1=K|last2=Kasting|first2=J F
|journal=Nature | volume=360 | issue=6406 | pages=721–23 |date=December 1992 | doi=10.1038/360721a0|pmid=11536510 |bibcode=1992Natur.360..721C |s2cid=4360963 }}</ref>

==Phenomena==
The tropopause is not a fixed boundary. Vigorous [[thunderstorm]]s, for example, particularly those of tropical origin, will [[convective overshoot|overshoot]] [[overshooting top|into]] the lower stratosphere and undergo a brief (hour-order or less) low-frequency vertical [[oscillation]].<ref name=":2">{{cite journal|last1= Shenk|first1= W. E.|date= 1974|title= Cloud top height variability of strong convective cells|journal= [[Journal of Applied Meteorology]]|volume = 13|issue= 8|pages= 918{{ndash}}922| doi=10.1175/1520-0450(1974)013<0917:cthvos>2.0.co;2 |bibcode = 1974JApMe..13..917S |doi-access= free}}</ref> Such oscillation results in a low-frequency atmospheric [[gravity wave#Atmosphere dynamics on Earth|gravity wave]] capable of affecting both atmospheric and oceanic currents in the region.<ref name=":2" />

Most commercial aircraft are flown in the lower stratosphere, just above the tropopause, during the [[Cruise (aeronautics)|cruise phase]] of their flights; in this region, the clouds and significant weather perturbations characteristic of the troposphere are usually absent.{{sfn|Petty|2008|p=21}}

==See also==

* [[Jet stream]]
* [[Maximum parcel level]]
* [[Stratosphere]]
* [[Atmosphere of Earth]]
* [[Vorticity equation|Vorticity]]

==References==
{{Reflist|refs=
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==External links==
* [http://www-das.uwyo.edu/~geerts/cwx/notes/chap01/tropo.html The height of the tropopause]

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