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Lapse rate
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== Cause == The temperature profile of the atmosphere is a result of the interaction between radiative heating from [[sunlight]], cooling to space via [[thermal radiation]], and upward heat transport via [[convection|natural convection]] (which carries hot air and [[latent heat]] upward). Above the [[tropopause]], convection does not occur and all cooling is radiative. Within the [[troposphere]], the lapse rate is essentially the consequence of a balance between (a) radiative cooling of the air, which by itself would lead to a high lapse rate; and (b) convection, which is activated when the lapse rate exceeds a critical value; convection stabilizes the environmental lapse rate.<ref name="Manabe64">{{cite journal |last1=Manabe |first1=Syukuro |last2=Strickler |first2=Robert F. |title=Thermal Equilibrium of the Atmosphere with a Convective Adjustment |journal=Journal of the Atmospheric Sciences |date=1964 |volume=21 |issue=4 |pages=361β385 |doi=10.1175/1520-0469(1964)021<0361:TEOTAW>2.0.CO;2 |bibcode=1964JAtS...21..361M |url=https://journals.ametsoc.org/view/journals/atsc/21/4/1520-0469_1964_021_0361_teotaw_2_0_co_2.xml |access-date=1 September 2024}}</ref> Sunlight hits the surface of the earth (land and sea) and heats them. The warm surface heats the air above it. In addition, nearly a third of absorbed sunlight is absorbed within the atmosphere, heating the atmosphere directly.<ref name="nasa-eeb17">{{cite web |title=What is Earth's Energy Budget? Five Questions with a Guy Who Knows |date=10 April 2017 |url=https://www.nasa.gov/centers-and-facilities/langley/what-is-earths-energy-budget-five-questions-with-a-guy-who-knows/ |publisher=NASA |access-date=1 September 2024}}</ref> [[Thermal conduction]] helps transfer heat from the surface to the air; this conduction occurs within the few millimeters of air closest to the surface. However, above that thin interface layer, thermal conduction plays a negligible role in transferring heat within the atmosphere; this is because the thermal conductivity of air is very low.<ref name="ucar-cond">{{cite web |title=Conduction |url=https://scied.ucar.edu/learning-zone/earth-system/conduction |publisher=Center for Science Education |access-date=1 September 2024}}</ref><ref name="WallaceHobbs">{{cite book |last1=Wallace |first1=John M. |last2=Hobbs |first2=Peter V. |title=Atmospheric Science |date=2006 |publisher=Elsevier |isbn=9780080499536 |edition=2}}</ref>{{rp|387}} The air is radiatively cooled by [[greenhouse gases]] (water vapor, carbon dioxide, etc.) and clouds emitting [[Outgoing longwave radiation|longwave thermal radiation]] to space.<ref name="Hartmann22">{{cite journal |last1=Hartmann |first1=Dennis L. |last2=Dygert |first2=Brittany D. |last3=Blossey |first3=Peter N. |last4=Fu |first4=Qiang |last5=Sokol |first5=Adam B. |title=The Vertical Profile of Radiative Cooling and Lapse Rate in a Warming Climate |journal=Journal of Climate |date=2022 |volume=35 |issue=19 |pages=6253β6265 |doi=10.1175/JCLI-D-21-0861.1 |bibcode=2022JCli...35.2653H |url=https://journals.ametsoc.org/view/journals/clim/35/19/JCLI-D-21-0861.1.xml |access-date=1 September 2024}}</ref> If [[radiation]] were the only way to transfer energy within the atmosphere, then the lapse rate near the surface would be roughly 40 Β°C/km and the [[greenhouse effect]] of gases in the atmosphere would keep the ground at roughly {{convert|333.|K|C F}}.<ref name=Goody>{{cite book|last1=Richard M. Goody|last2=James C.G. Walker|title=Atmospheres|chapter=Atmospheric Temperatures|chapter-url=http://lasp.colorado.edu/~bagenal/3720/GoodyWalker/AtmosCh3sm.pdf|publisher=Prentice-Hall|year=1972|archive-url=https://web.archive.org/web/20160603212954/http://lasp.colorado.edu/~bagenal/3720/GoodyWalker/AtmosCh3sm.pdf|archive-date=2016-06-03}}</ref>{{rp|59-60}} However, when air gets hot or humid, its density decreases.<ref>{{cite web |last1=Williams |first1=Jack |title=Understanding Air Density and its Effects |url=https://iflycoast.com/understanding-air-density-and-its-effects/ |publisher=USAToday.com |access-date=1 September 2024}}</ref><ref>{{cite web |title=Is humid air heavier than dry air? |url=https://science.howstuffworks.com/nature/climate-weather/atmospheric/is-humid-air-heavier-than-dry-air.htm |website=howstuffworks |access-date=1 September 2024}}</ref> Thus, air which has been heated by the surface tends to rise and carry internal energy upward, especially if the air has been moistened by evaporation from water surfaces. This is the process of [[convection]]. Vertical convective motion stops when a parcel of air at a given altitude has the same density as the other air at the same elevation. Convection carries hot, moist air upward and cold, dry air downward, with a net effect of transferring heat upward. This makes the air below cooler than it would otherwise be and the air above warmer. Because convection is available to transfer heat within the atmosphere, the lapse rate in the troposphere is reduced to around 6.5 Β°C/km<ref name="Manabe64"/> and the greenhouse effect is reduced to a point where Earth has its observed surface temperature of around {{convert|288.|K|C F}}.
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