Editing Altitude (section)

==In atmospheric studies==
{{See also|Atmospheric pressure#Altitude variation}}

===Atmospheric layers===
{{main|Atmospheric layers}}
The [[Earth's atmosphere]] is divided into several altitude regions. These regions start and finish at varying heights depending on season and distance from the poles.  The altitudes stated below are averages:<ref>{{cite web | title=Layers of the Atmosphere |work=JetStream, the National Weather Service Online Weather School | publisher=National Weather Service | url=http://www.srh.noaa.gov/srh/jetstream/atmos/layers.htm | access-date=22 December 2005| archive-url= https://web.archive.org/web/20051219190158/http://www.srh.noaa.gov/srh/jetstream/atmos/layers.htm| archive-date= 19 December 2005 | url-status= live}}</ref>
* [[Troposphere]]: surface to {{convert|8000|m|mi}} at the poles, {{convert|18000|m|miles}} at the [[Equator]], ending at the Tropopause
* [[Stratosphere]]: Troposphere to {{convert|50|km|mi}}
* [[Mesosphere]]: Stratosphere to {{convert|85|km|mi}}
* [[Thermosphere]]: Mesosphere to {{convert|675|km|mi}}
* [[Exosphere]]: Thermosphere to {{convert|10000|km|mi}}

The [[Kármán line]], at an altitude of {{convert|100|km|mi}} above [[sea level]], by convention defines represents the demarcation between the atmosphere and [[outer space|space]].<ref>{{cite web|url=http://www.fai.org/icare-records/100km-altitude-boundary-for-astronautics|title=The 100&nbsp;km Boundary for Astronautics|author=Dr. S. Sanz Fernández de Córdoba|publisher=[[Fédération Aéronautique Internationale]]|date=24 June 2004|archive-url=https://web.archive.org/web/20110809093537/http://www.fai.org/astronautics/100km.asp|archive-date=9 August 2011}}</ref> The thermosphere and exosphere (along with the higher parts of the mesosphere) are regions of the atmosphere that are conventionally defined as space.

===High altitude and low pressure===
{{anchor|High altitude and low air pressure}}
Regions on the [[Earth]]'s surface (or in its atmosphere) that are high above mean sea level are referred to as '''high altitude'''. High altitude is sometimes defined to begin at {{convert|8000|ft|meters|order=flip}} above sea level.<ref name=websterMed>{{cite book|title=Webster's New World Medical Dictionary|url=http://www.medterms.com/script/main/art.asp?articlekey=8578|publisher=Wiley|isbn=978-0-470-18928-3|year=2008|access-date=27 April 2010|archive-date=8 December 2011|archive-url=https://web.archive.org/web/20111208154830/http://www.medterms.com/script/main/art.asp?articlekey=8578}}</ref><ref>{{cite web|url=http://www.ismmed.org/np_altitude_tutorial.htm |title=An Altitude Tutorial |publisher=International Society for Mountain Medicine |access-date=22 June 2011 |archive-url=https://web.archive.org/web/20110719194849/http://www.ismmed.org/np_altitude_tutorial.htm |archive-date=19 July 2011  }}</ref><ref name=MedicalProblems/>

At high altitude, [[atmospheric pressure]] is lower than that at sea level. This is due to two competing physical effects: gravity, which causes the air to be as close as possible to the ground; and the heat content of the air, which causes the molecules to bounce off each other and expand.<ref name=nova>{{cite web|url=https://www.pbs.org/wgbh/nova/everest/exposure/pressure.html|title=Atmospheric pressure|work=NOVA Online Everest|publisher=Public Broadcasting Service|access-date=23 January 2009| archive-url= https://web.archive.org/web/20090125053918/http://www.pbs.org/wgbh/nova/everest/exposure/pressure.html| archive-date= 25 January 2009 | url-status= live}}</ref>

===Temperature profile===
{{main|Lapse rate}}
{{Further|Atmospheric temperature}}

The temperature profile of the atmosphere is a result of an interaction between [[radiation]] and [[convection]]. Sunlight in the [[visible spectrum]] hits the ground and heats it. The ground then heats the air at the surface. If [[radiation]] were the only way to transfer heat from the ground to space, the [[greenhouse effect]] of gases in the atmosphere would keep the ground at roughly {{convert|333|K|C F}}, and the temperature would decay exponentially with height.<ref name=goodywilson>{{cite book|first1=Richard M.|last1=Goody|first2=James C.G.|last2=Walker|title=Atmospheres|chapter=Atmospheric Temperatures|chapter-url=http://lasp.colorado.edu/~bagenal/3720/GoodyWalker/AtmosCh3sm.pdf|publisher=Prentice-Hall|year=1972|access-date=2 May 2016|archive-date=29 July 2016|archive-url=https://web.archive.org/web/20160729075851/http://lasp.colorado.edu/~bagenal/3720/GoodyWalker/AtmosCh3sm.pdf}}</ref>

However, when air is hot, it tends to expand, which lowers its density. Thus, hot air tends to rise and transfer heat upward. This is the process of [[convection]]. Convection comes to equilibrium when a parcel of air at a given altitude has the same density as its surroundings. Air is a poor conductor of heat, so a parcel of air will rise and fall without exchanging heat. This is known as an [[adiabatic process]], which has a characteristic pressure-temperature curve. As the pressure gets lower, the temperature decreases. The rate of decrease of temperature with elevation is known as the [[adiabatic lapse rate]], which is approximately 9.8&nbsp;°C per kilometer (or {{convert|5.4|F-change|C-change|abbr=on|disp=sqbr}} per 1000&nbsp;feet) of altitude.<ref name=goodywilson/>

The presence of water in the atmosphere complicates the process of convection. Water vapor contains latent [[heat of vaporization]]. As air rises and cools, it eventually becomes [[Dew point|saturated]] and cannot hold its quantity of water vapor. The water vapor condenses (forming [[cloud]]s), and releases heat, which changes the lapse rate from the [[dry adiabatic lapse rate]] to the [[moist adiabatic lapse rate]] (5.5&nbsp;°C per kilometer or {{convert|3|F-change|C-change|abbr=on|disp=sqbr}} per 1000&nbsp;feet).<ref>{{cite web|url=http://meteorologytraining.tpub.com/14312/css/14312_47.htm |title=Dry Adiabatic Lapse Rate |publisher=tpub.com |access-date=2 May 2016  |archive-url=https://web.archive.org/web/20160603041448/http://meteorologytraining.tpub.com/14312/css/14312_47.htm |archive-date=3 June 2016 }}</ref>
As an average, the International Civil Aviation Organization (ICAO) defines an [[international standard atmosphere]] (ISA) with a temperature [[lapse rate]] of 6.49&nbsp;°C per kilometer (3.56&nbsp;°F per 1,000&nbsp;feet).<ref name="ICAO 1993">{{cite book|publisher=[[International Civil Aviation Organization]]|title=Manual of the ICAO Standard Atmosphere (extended to 80 kilometres (262 500 feet))|id=Doc 7488-CD|edition=Third|year=1993|isbn=978-92-9194-004-2}}</ref> The actual lapse rate can vary by altitude and by location.

Finally, only the [[troposphere]] (up to approximately {{convert|11|km|ft}} of altitude) in the Earth's atmosphere undergoes notable convection; in the [[stratosphere]], there is little vertical convection.<ref>{{cite web|url=http://scied.ucar.edu/shortcontent/stratosphere-overview|title=The stratosphere: overview|publisher=UCAR|access-date=2 May 2016}}</ref>