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.

Okta In meteorology, an okta is a unit of measurement used to describe the amount of cloud cover at any given location such as a weather station. Sky conditions are estimated in terms of how many eighths of the sky are covered in cloud, ranging from ...

is the usual unit for measurement of the cloud cover. The cloud cover is correlated to the

sunshine duration Sunshine duration or sunshine hours is a climatological indicator, measuring duration of sunshine in given period (usually, a day or a year) for a given location on Earth, typically expressed as an averaged value over several years. It is a gene ...

as the least cloudy locales are the sunniest ones while the cloudiest areas are the least sunny places, as clouds can block sunlight, especially on sunrise and

sunset Sunset, also known as sundown, is the daily disappearance of the Sun below the horizon due to Earth's rotation. As viewed from everywhere on Earth (except the North and South poles), the equinox Sun sets due west at the moment of both the spring ...

where sunlight is already limited. Cloud cover over the North Atlantic Ocean 3

The global cloud cover averages around 0.68 when analyzing clouds with optical depth larger than 0.1. This value is lower (0.56) when considering clouds with an optical depth larger than 2, and higher when counting subvisible cirrus clouds. Particularly over the oceans cloud cover is persistent with an average 72% of cloud cover.

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 Solar may refer to: Astronomy * Of or relating to the Sun ** Solar telescope, a special purpose telescope used to observe the Sun ** A device that utilizes solar energy (e.g. "solar panels") ** Solar calendar, a calendar whose dates indicate t ...

spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of color ...

, they efficiently reflect light to space and thus contribute to the cooling of the planet, as well as trapping remaining heat at

night Night (also described as night time, unconventionally spelled as "nite") is the period of ambient darkness from sunset Sunset, also known as sundown, is the daily disappearance of the Sun below the horizon due to Earth's rotation. As view ...

. 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 In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to ...

expected by recent studies.

Variability

Cloud cover values only vary by 0.03 from year to year averages, whereas the local, day to day variability in cloud amount typically rises to 0.3 over the globe. Most data sets agree on the fact that the land is covered by 0.10–0.15 less cloud than the oceans due to the seas covered with water, meaning much more evaporation possible. Lastly, there is a latitudinal variation in the cloud cover, such that around 20°N there are regions with 0.10 less cloudiness than the global mean. Almost the same variation (0.15 instead of 0.10) is found 20°S. This is because of the absence of equatorial effects and strong winds reducing cloud formation. On the other hand, in the storm regions of the Southern Hemisphere midlatitudes were found to have with 0.15–0.25 more cloudiness than the global mean at 60°S. On average, about 67% of the entire Earth is cloud-covered at any moment. 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 Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a subcontinent of Eurasia and it is located enti ...

North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Car ...

South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the souther ...

and

Asia Asia (, ) is one of the world's most notable geographical regions, which is either considered a continent in its own right or a subcontinent of Eurasia, which shares the continental landmass of Afro-Eurasia with Africa. Asia covers an ...

are dominated by cloudy skies due to the westerlies, monsoon or other effects. On the other hand,

Africa Africa is the world's second-largest and second-most populous continent, after Asia in both cases. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 6% of Earth's total surface area ...

, the Middle East and Australia are dominated by clear skies due to their continentality and

aridity A region is arid when it severely lacks available water, to the extent of hindering or preventing the growth and development of plant and animal life. Regions with arid climates tend to lack vegetation and are called xeric or desertic. Most ar ...

.https://images.slideplayer.com/17/5328401/slides/slide_4.jpg On a regional scale, it can be also worthy of note that 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 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. 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.

References

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

External links

NSDL.arm.gov
''Glossary of Atmospheric Terms'', From the National Science Digital Library'
Atmospheric Visualization Collection

Earthobersvatory.nasa.gov
Monthly maps of global cloud cover from NASA's Earth Observatory
International Satellite Cloud Climatology Project (ISCCP)
'' NASA's data products on their satellite observations ''
NASA composite satellite image
{{Authority control Clouds Articles containing video clips fr:Nuage#Nébulosité et opacité