Editing Greenhouse effect (section)

== Effect of lapse rate ==
=== Lapse rate ===
{{Further|Lapse rate}}
In the lower portion of the atmosphere, the [[troposphere]], the air temperature decreases (or "lapses") with increasing altitude. The rate at which temperature changes with altitude is called the ''[[lapse rate]]''.<ref name="Nugent5">{{cite web |last1=Nugent |first1=Alison |last2=DeCou |first2=David |title=ATMOSPHERIC PROCESSES AND PHENOMENA: Chapter 5: Atmospheric Stability |url=http://pressbooks-dev.oer.hawaii.edu/atmo/chapter/chapter-5-atmospheric-stability/ |access-date=31 May 2023}}</ref>

On Earth, the air temperature decreases by about 6.5&nbsp;°C/km (3.6&nbsp;°F per 1000&nbsp;ft), on average, although this varies.<ref name="Nugent5" />

The temperature lapse is caused by [[convection]]. Air warmed by the surface rises. As it rises, air [[adiabatic process|expands and cools]].  Simultaneously, other air descends, compresses, and warms. This process creates a vertical temperature gradient within the atmosphere.<ref name="Nugent5" />

This vertical temperature gradient is essential to the greenhouse effect. If the lapse rate was zero (so that the atmospheric temperature did not vary with altitude and was the same as the surface temperature) then there would be no greenhouse effect (i.e., its value would be zero).<ref name="ThomasStamnes1999">{{cite book |last1=Thomas |first1=Gary E. |last2=Stamnes |first2=Knut |title=Radiative Transfer in the Atmosphere and Ocean |date=1999 |publisher=Cambridge University Press |isbn=0-521-40124-0}}</ref>

=== Emission temperature and altitude ===

[[File:Spectral OLR.png|thumb|upright=1.35|The temperature at which thermal radiation was emitted can be determined by comparing the intensity at a particular wavenumber to the intensity of a [[Planck's law|black-body emission curve]]. In the chart, emission temperatures range between T<sub>min</sub> and T<sub>s</sub>. "Wavenumber" is frequency divided by the speed of light).]]
Greenhouse gases make the atmosphere near Earth's surface mostly opaque to longwave radiation. The atmosphere only becomes transparent to longwave radiation at higher altitudes, where the air is less dense, there is less water vapor, and reduced [[Spectral line|pressure broadening]] of absorption lines limits the wavelengths that gas molecules can absorb.<ref name="Plass1950">{{cite journal |last1=Strong |first1=J. |last2=Plass |first2=G. N. |date=1950 |title=The Effect of Pressure Broadening of Spectral Lines on Atmospheric Temperature |url=https://ui.adsabs.harvard.edu/abs/1950ApJ...112..365S/abstract#:~:text=Pressure%20broadening%20causes%20lines%20in,absorbed%20by%20the%20upper%20layers. |journal=Astrophysical Journal |volume=112 |page=365 |bibcode=1950ApJ...112..365S |doi=10.1086/145352}}</ref><ref name="Wallace2006" />

For any given wavelength, the longwave radiation that reaches space is emitted by a particular ''radiating layer'' of the atmosphere. The intensity of the emitted radiation is determined by the weighted average air temperature within that layer. So, for any given wavelength of radiation emitted to space, there is an associated ''effective emission temperature'' (or [[brightness temperature]]).<ref name="Pierrehumbert2011">{{cite web |last1=Pierrehumbert |first1=R. T. |date=January 2011 |title=Infrared radiation and planetary temperature |url=https://geosci.uchicago.edu/~rtp1/papers/PhysTodayRT2011.pdf |website=Physics Today |publisher=American Institute of Physics |pages=33–38}}</ref><ref name="Wallace2006" />

A given wavelength of radiation may also be said to have an ''effective emission altitude'', which is a weighted average of the altitudes within the radiating layer.

The effective emission temperature and altitude vary by wavelength (or frequency). This phenomenon may be seen by examining plots of radiation emitted to space.<ref name="Pierrehumbert2011" />

=== Greenhouse gases and the lapse rate ===
[[File:Greenhouse_Effect_Overview.svg|thumb|upright=1.35|[[Greenhouse gas]]es (GHGs) in dense air near the surface absorb most of the [[Outgoing longwave radiation|longwave radiation emitted]] by the warm surface. GHGs in sparse air at higher altitudes—cooler because of the environmental [[lapse rate]]—emit longwave radiation to space at a lower rate than surface emissions.]]
Earth's surface radiates longwave radiation with wavelengths in the range of 4–100 microns.<ref name="Mitchell-1989">{{cite journal|doi=10.1029/RG027i001p00115|last=Mitchell|first=John F. B.|date=1989|title=The "Greenhouse" effect and Climate Change|journal=Reviews of Geophysics|volume=27|issue=1|pages=115–139|url=http://astrosun2.astro.cornell.edu/academics/courses/astro202/Mitchell_GRL89.pdf|access-date=23 March 2008|bibcode=1989RvGeo..27..115M|citeseerx=10.1.1.459.471|archive-date=15 June 2011|archive-url=https://web.archive.org/web/20110615024949/http://astrosun2.astro.cornell.edu/academics/courses/astro202/Mitchell_GRL89.pdf|url-status=live}}</ref> Greenhouse gases that were largely transparent to incoming solar radiation are more absorbent for some wavelengths in this range.<ref name="Mitchell-1989" />

The atmosphere near the Earth's surface is largely opaque to longwave radiation and most heat loss from the surface is by [[evaporation]] and [[convection]]. However radiative energy losses become increasingly important higher in the atmosphere, largely because of the decreasing concentration of water vapor, an important greenhouse gas.

Rather than thinking of longwave radiation headed to space as coming from the surface itself, it is more realistic to think of this outgoing radiation as being emitted by a layer in the mid-[[troposphere]], which is effectively coupled to the surface by a [[lapse rate]]. The difference in temperature between these two locations explains the difference between surface emissions and emissions to space, i.e., it explains the greenhouse effect.<ref>{{cite web |url=https://www.e-education.psu.edu/meteo469/node/198 |title=METEO 469: From Meteorology to Mitigation - Understanding Global Warming - Lesson 5 - Modelling of the Climate System - One-Layer Energy Balance Model |publisher=[[Pennsylvania State University]] College of Mineral and Earth Sciences - Department of Meteorology and Atmospheric Sciences |last1=Mann |first1=Michael |last2=Gaudet |first2=Brian |access-date=4 November 2022 |archive-date=31 October 2022 |archive-url=https://web.archive.org/web/20221031022823/https://www.e-education.psu.edu/meteo469/node/198 |url-status=live }}</ref><ref name="Tziperman2022">{{cite book |last1=Tziperman |first1=Eli |title=Global Warming Science |date=2022 |publisher=Princeton University Press |isbn=9780691228808}}</ref>