cloud cover
{{short description|Fraction of the sky obscured by clouds when observed from a particular location}}
File:MODIS Map.jpg 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 clouds on average when observed from a particular location.{{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}} 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
File: Cloud cover over France.jpg]]
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 clouds are counted).{{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
}} Average cloud cover is around 72% over the oceans, with low seasonal variation, and about 55% above land, with significant seasonal variation.{{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 }}
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 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{{Cite web |last=WMO |title=Luminance |url=https://cloudatlas.wmo.int/appearance-of-clouds-luminance.html |access-date=2025-06-10 |website=International Cloud Atlas |language=EN}}. 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.{{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 }}
Variability
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.
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. On average, about 67% of the entire Earth is cloud-covered at any moment.{{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}}
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 or other effects. On the other hand, Africa, the Middle East and Australia are dominated by clear skies due to their continentality and aridity.https://images.slideplayer.com/17/5328401/slides/slide_4.jpg {{Bare URL image|date=March 2022}}
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.
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.{{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)}} 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 type of cloud is also a factor, with low cumulus clouds sitting at 300–1,500m while high cirrus clouds 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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