Editing Cloud cover

{{short description|Fraction of the sky obscured by clouds when observed from a particular location}}
[[File:Total cloud cover.png|thumb|Total cloud cover fraction averaged over the years 1981-2010 from the CHELSA-BIOCLIM+ data set<ref>Brun, P., Zimmermann, N.E., Hari, C., Pellissier, L., Karger, D.N. (preprint): Global climate-related predictors at kilometre resolution for the past and future. Earth Syst. Sci. Data Discuss. https://doi.org/10.5194/essd-2022-212</ref>]]

[[File:MODIS Map.jpg|thumb|right|[[Satellite image]] based largely on observations from [[NASA]]'s [[Moderate Resolution Imaging Spectroradiometer]] (MODIS) on July 11, 2005, of Earth's cloud cover.]]

'''Cloud cover''' (also known as '''cloudiness''', '''cloudage''', or '''cloud amount''') refers to the fraction of the [[sky]] obscured by [[cloud]]s on average when observed from a particular location.<ref>{{cite book |last=Huschke |first= Ralph E. |orig-year=1959 |url=http://glossary.ametsoc.org/wiki/Cloud_cover |entry=Cloud cover |publisher=[[American Meteorological Society]] |title=Glossary of Meteorology |location=Boston |edition=2nd |year=1970 |access-date=2013-08-24}}</ref> [[Okta]] is the usual unit for measurement of the cloud cover. The cloud cover is correlated to the [[sunshine duration]] as the least cloudy locales are the sunniest ones while the cloudiest areas are the least sunny places, as clouds can block sunlight, especially at [[sunrise]] and [[sunset]] where sunlight is already limited.

[[File: Cloud cover over the North Atlantic Ocean 3.JPG|thumb|right|Partial cloud cover over the North Atlantic Ocean.]]
[[File: Cloud cover over France.jpg|thumb|Complete cloud coverage over [[France]]]]

The global cloud cover averages around 67-68%, though it ranges from 56% to 73% depending on the minimum [[optical depth]] considered (lower when optical depth is large, and higher when it is low, such that subvisible [[cirrus cloud]]s are counted).<ref name="2013stubenrauch">{{cite journal
| last1 = Stubenrauch | first1 = C. J.
| last2 = Rossow | first2 = W. B.
| last3 = Kinne | first3 = S.
| last4 = Ackerman | first4 = S.  
| last5 = Cesana | first5 = G.
| last6 = Chepfer | first6 = H
| last7 = Di Girolamo | first7 = L.
| last8 = Getzewich | first8 = B.
| last9 = Guignard | first9 = A.
| last10 = Heidinger | first10 = A.
| last11 = Maddux | first11 = B. C.
| last12 = Menzel | first12 = W.P
| last13 = Minnis | first13 = P.
| last14 = Pearl | first14 = C.
| last15 = Platnick | first15 = S.
| last16 = Poulsen | first16 = C.
| last17 = Reidi | first17 = J.
| last18 = Sun-Mack | first18 = S
| last19 = Walther | first19 = A.
| last20 = Winker | first20 = D.
| last21 = Zeng | first21 = S.
| last22 = Zhao | first22 = G.
| title = Assessment of global cloud datasets from satellites: Project and Database initiated by GEWEX Radiation Panel
| journal = Bulletin of the American Meteorological Society
| date = 2013
| volume = 94 | issue = 7
| pages = 1031–1049
| doi = 10.1175/BAMS-D-12-00117.1
| bibcode = 2013BAMS...94.1031S| s2cid = 12145499
| url = https://hal.archives-ouvertes.fr/hal-01091218/file/bams-d-12-001172E1.pdf
}}</ref> Average cloud cover is around 72% over the oceans, with low seasonal variation, and about 55% above land, with significant seasonal variation.<ref name="King Platnick Menzel Ackerman 2013 pp. 3826–3852">{{cite journal | last1=King | first1=Michael D. | last2=Platnick | first2=Steven | last3=Menzel | first3=W. Paul | last4=Ackerman | first4=Steven A. | last5=Hubanks | first5=Paul A. | title=Spatial and Temporal Distribution of Clouds Observed by MODIS Onboard the Terra and Aqua Satellites | journal=IEEE Transactions on Geoscience and Remote Sensing | publisher=Institute of Electrical and Electronics Engineers (IEEE) | volume=51 | issue=7 | year=2013 | issn=0196-2892 | doi=10.1109/tgrs.2012.2227333 | pages=3826–3852| bibcode=2013ITGRS..51.3826K | doi-access=free | hdl=2060/20120010368 | hdl-access=free }}</ref>

== Role in the climate system ==

Clouds play multiple critical roles in the [[climate system]] and [[diurnal cycle]]. In particular, being bright objects in the visible part of the [[sun|solar]] [[spectrum]], they efficiently reflect light to space and thus contribute to the cooling of the planet, as well as trapping remaining heat at [[night]]. Cloud cover thus plays an important role in the energetic balance of the atmosphere and a variation of it is a factor and consequence of and to the [[climate change]] expected by recent studies.<ref>{{Cite report | url= http://www.grida.no/publications/other/ipcc_tar/?src=/climate/ipcc_tar/ | title= IPCC Third Assessment Report Chapter 7. Physical Climate Processes and Feedbacks (Atmospheric Processes and Feedbacks 7.2) | publisher= International Panel on Climate Change | access-date= August 24, 2013 | quote= It has extensive coverage of cloud-climate interactions | archive-url= https://web.archive.org/web/20130805091249/http://www.grida.no/publications/other/ipcc_tar/?src=%2Fclimate%2Fipcc_tar%2F | archive-date= August 5, 2013 | url-status= dead }}</ref>

== Variability ==
[[File: MODAL2 M CLD FR.ogv|thumb|The average cloud cover of the Earth, 2005-2013. Colors range from blue (no clouds) to white (totally cloudy).<ref>{{Cite web |date=2018-10-31 |others=Webmaster Paul Przyborski; NASA Official Robert Levy |title=Cloud Fraction |url=http://earthobservatory.nasa.gov/GlobalMaps/view.php?d1=MODAL2_M_CLD_FR |website=NASA Earth Observatory}}</ref> (''click for more detail'')]]

Cloud cover values only vary by 3% from year-to-year averages, whereas the local, day-to-day variability in cloud amounts typically rises to 30% over the globe. Land is generally covered by 10-15% less cloud than the oceans, because the seas are covered with water, allowing for more [[evaporation]].<ref name = "2013stubenrauch" />

Lastly, there is a latitudinal variation in the cloud cover. Areas around 10-15% below the global mean can be found around 20°N and 20°S, due to an absence of equatorial effects and strong winds reducing cloud formation.{{Specify|date=May 2023}} On the other hand, in the storm regions of the Southern Hemisphere midlatitudes were found to have with 15–25% more cloudiness than the global mean at 60°S.<ref name = "2013stubenrauch" /> On average, about 67% of the entire Earth is cloud-covered at any moment.<ref name="NASA Earth Observatory 2020">{{cite web | title=Cloudy Earth | website=NASA Earth Observatory | date=2020-07-07 | url=https://earthobservatory.nasa.gov/images/85843/cloudy-earth | access-date=2022-01-29}}</ref>

On a continental scale, it can be noticed based upon a long-term satellite recording of cloudiness data that on a year-mean basis, [[Europe]], [[North America]], [[South America]] and [[Asia]] are dominated by cloudy skies due to the [[westerlies]], [[Monsoon of South Asia|monsoon]] or other effects. On the other hand, [[Africa]], the [[Middle East]] and [[Australia]] are dominated by clear skies due to their continentality and [[aridity]].<ref name="images.slideplayer.com">https://images.slideplayer.com/17/5328401/slides/slide_4.jpg {{Bare URL image|date=March 2022}}</ref>

On a regional scale, some exceptionally humid areas of Earth experience cloudy conditions virtually all time such as South America's [[Amazon Rainforest]] while some highly arid areas experience clear-sky conditions virtually all the time such as Africa's [[Sahara Desert]].<ref name="images.slideplayer.com"/>

== Altitude of typical cloud cover ==

Although clouds can exist within a wide range of altitudes, typical cloud cover has a base at approximately 4,000m and extends up to an altitude of about 5,000m.<ref>{{Cite web |date=2012 |title=Met Office introduction to clouds |url=https://www.metoffice.gov.uk/binaries/content/assets/metofficegovuk/pdf/research/library-and-archive/library/publications/factsheets/factsheet_1-clouds.pdf |website=Met Office (UK)}}</ref> Clouds height can vary depending on latitude; with cloud cover in polar latitudes being slightly lower and in tropical regions the cloud cover may extend up to 8,000m. The [[List of cloud types|type of cloud]] is also a factor, with low [[cumulus cloud]]s sitting at 300–1,500m while high [[cirrus cloud]]s at 5,500-6,500m.

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== References ==
{{Reflist}}
* McIntosh, D. H. (1972) ''Meteorological Glossary'', Her Majesty's Stationery Office, Met. O. 842, A.P. 897, 319 p.

== External links ==
{{Commons}}
* [https://web.archive.org/web/20050420085336/http://www.nsdl.arm.gov/Library/glossary.shtml#Cloud_cover NSDL.arm.gov], ''Glossary of Atmospheric Terms'', From the National Science Digital Library's [https://web.archive.org/web/20021123031727/http://nsdl.arm.gov/ Atmospheric Visualization Collection].
* [http://earthobservatory.nasa.gov/GlobalMaps/view.php?d1=MODAL2_M_CLD_FR# Earthobersvatory.nasa.gov], Monthly maps of global cloud cover from NASA's [[Earth Observatory]]
* [http://isccp.giss.nasa.gov/# International Satellite Cloud Climatology Project (ISCCP)], '' NASA's data products on their satellite observations ''
* [https://web.archive.org/web/20100212071052/http://earthobservatory.nasa.gov/images/imagerecords/41000/41292/cldfrc_TMO_200910_lrg.png NASA composite satellite image].

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[[Category:Clouds]]
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[[Category:Atmospheric dynamics]]

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