Editing Cloud (section)

== Distribution ==

===Convergence along low-pressure zones===
{{Main|Intertropical Convergence Zone|Extratropical cyclone|Cold front|Warm front}}
[[File:Worldclouds 2009.jpg|thumb|Global cloud cover, averaged over the month of October 2009. [[NASA]] composite satellite image.<ref>For a larger image see [http://earthobservatory.nasa.gov/images/imagerecords/41000/41292/cldfrc_TMO_200910_lrg.png this image] {{Webarchive|url=https://web.archive.org/web/20100529185735/http://earthobservatory.nasa.gov/images/imagerecords/41000/41292/cldfrc_TMO_200910_lrg.png |date=29 May 2010 }} at earthobservatory.nasa.gov</ref>]]

[[File:MODAL2 M CLD FR.ogv|thumb|These maps display the fraction of Earth's area that was cloudy on average during each month from January 2005 to August 2013. The measurements were collected by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Terra satellite. Colors range from blue (no clouds) to white (totally cloudy). Like a digital camera, MODIS collects information in gridded boxes, or pixels. Cloud fraction is the portion of each pixel that is covered by clouds. Colors range from blue (no clouds) to white (totally cloudy).<ref>{{Cite web |title=Cloud Fraction : Global Maps |url=http://earthobservatory.nasa.gov/GlobalMaps/view.php?d1=MODAL2_M_CLD_FR |access-date=26 October 2014 |website=nasa.gov}}</ref> (''click for more detail'')]]

Although the local distribution of clouds can be significantly influenced by topography, the global prevalence of cloud cover in the troposphere tends to vary more by [[latitude]]. It is most prevalent in and along low pressure zones of surface tropospheric convergence which encircle the Earth close to the [[equator]] and near the 50th parallels of latitude in the northern and southern [[Hemispheres of the Earth|hemispheres]].<ref>{{Cite book |last=Kondratʹev, Kirill Iakovlevich |url=https://books.google.com/books?id=tcCTsENwgGYC&pg=PA411 |title=Atmospheric aerosol properties: formation, processes and impacts |publisher=Springer |year=2006 |isbn=978-3-540-26263-3 |page=403}}</ref> The adiabatic cooling processes that lead to the creation of clouds by way of lifting agents are all associated with convergence; a process that involves the horizontal inflow and accumulation of air at a given location, as well as the rate at which this happens.<ref name="Convergence/divergence">{{Cite web |last=Wei-hung, Leung |year=2010 |title=Meteorology Basics: Convergence and Divergence |url=http://www.hko.gov.hk/education/edu01met/wxphe/ele-condiv-e.htm |url-status=dead |archive-url=https://web.archive.org/web/20191026053144/http://www.hko.gov.hk/education/edu01met/wxphe/ele-condiv-e.htm |archive-date=26 October 2019 |access-date=8 December 2014 |publisher=Hong Kong Observatory}}</ref> Near the equator, increased cloudiness is due to the presence of the low-pressure [[Intertropical Convergence Zone]] (ITCZ) where very warm and unstable air promotes mostly cumuliform and cumulonimbiform clouds.<ref name="noaa">{{Cite web |date=24 October 2007 |title=Inter-Tropical Convergence Zone |url=http://www.srh.noaa.gov/jetstream//tropics/itcz.htm |access-date=4 June 2009 |website=JetStream – Online School for Weather |publisher=[[NOAA]]}}</ref> Clouds of virtually any type can form along the mid-latitude convergence zones depending on the stability and moisture content of the air. These extratropical convergence zones are occupied by the [[polar front]]s where [[air mass]]es of polar origin meet and clash with those of tropical or subtropical origin.<ref name="The Climate System: General Circulation and Climate Zones">{{Cite web |last=Kushnir, Yochanan |year=2000 |title=The Climate System: General Circulation and Climate Zones |url=http://eesc.columbia.edu/courses/ees/climate/lectures/gen_circ/index.html |url-status=dead |archive-url=https://web.archive.org/web/20040822022845/http://eesc.columbia.edu/courses/ees/climate/lectures/gen_circ/index.html |archive-date=22 August 2004 |access-date=13 March 2012}}</ref> This leads to the formation of weather-making [[extratropical cyclone]]s composed of cloud systems that may be stable or unstable to varying degrees according to the stability characteristics of the various airmasses that are in conflict.<ref>{{Cite news |last=Williams, Jack |date=27 June 1997 |title=Extratropical storms are major weather makers |work=USA Today |url=https://www.usatoday.com/weather/wstorm.htm |access-date=13 March 2012}}</ref>

===Divergence along high pressure zones===
{{Main|Subtropical ridge|Polar high}}

Divergence is the opposite of convergence. In the Earth's troposphere, it involves the horizontal outflow of air from the upper part of a rising column of air, or from the lower part of a subsiding column often associated with an area or ridge of high pressure.<ref name="Convergence/divergence"/> Cloudiness tends to be least prevalent near the poles and in the subtropics close to the 30th parallels, north and south. The latter are sometimes referred to as the [[horse latitudes]]. The presence of a large-scale high-pressure [[subtropical ridge]] on each side of the equator reduces cloudiness at these low latitudes.<ref name="subtropical-ridge">{{Cite journal |last1=Cai |first1=Wenju |last2=Van Rensch |first2=Peter |last3=Cowan |first3=Tim |year=2011 |title=Subtropical Ridge |journal=Journal of Climate |volume=24 |issue=23 |pages=6035 |bibcode=2011JCli...24.6035C |doi=10.1175/2011JCLI4149.1 |s2cid=59145525|doi-access=free }}</ref> Similar patterns also occur at higher latitudes in both hemispheres.<ref name="pressure-belts">{{Cite web |year=2015 |editor-last=PMF IAS |title=Atmospheric Pressure Belts and Wind Systems PMF IAS Pressure Belts |url=https://www.pmfias.com/pressure-belts-pressure-systems-equatorial-low-sub-tropical-high-sub-polar-low-polar-high/ |access-date=5 April 2018}}</ref>