Editing Altitude (section)

===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>