Cloud cover (also known as cloudiness, cloudage, or cloud amount) refers to the fraction of the

sky The sky is an unobstructed view upward from the planetary surface, surface of the Earth. It includes the atmosphere of Earth, atmosphere and outer space. It may also be considered a place between the ground and outer space, thus distinct from ...

obscured by clouds on average when observed from a particular location. 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 Sunset (or sundown) is the disappearance of the Sun at the end of the Sun path, below the horizon of the Earth (or any other astronomical object in the Solar System) due to its Earth's rotation, rotation. As viewed from everywhere on Earth, it ...

where sunlight is already limited. 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). Average cloud cover is around 72% over the oceans, with low seasonal variation, and about 55% above land, with significant seasonal variation.

Role in the climate system

Clouds play multiple critical roles in the

climate system Earth's climate system is a complex system with five interacting components: the Atmosphere of Earth, atmosphere (air), the hydrosphere (water), the cryosphere (ice and permafrost), the lithosphere (earth's upper rocky layer) and the biosphere ( ...

and

diurnal cycle A diurnal cycle (or diel cycle) is any pattern that recurs every 24 hours as a result of one full Earth's rotation, rotation of the planet Earth around its axis. Earth's rotation causes surface diurnal temperature variation, temperature fluctuati ...

. In particular, being bright objects in the visible part of the solar

spectrum A spectrum (: spectra or spectrums) is a set of related ideas, objects, or properties whose features overlap such that they blend to form a continuum. The word ''spectrum'' was first used scientifically in optics to describe the rainbow of co ...

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

night Night, or nighttime, is the period of darkness when the Sun is below the horizon. Sunlight illuminates one side of the Earth, leaving the other in darkness. The opposite of nighttime is daytime. Earth's rotation causes the appearance of ...

. 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 Present-day climate change includes both global warming—the ongoing increase in Global surface temperature, global average temperature—and its wider effects on Earth's climate system. Climate variability and change, Climate change in ...

expected by recent studies.

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 Evaporation is a type of vaporization that occurs on the Interface (chemistry), surface of a liquid as it changes into the gas phase. A high concentration of the evaporating substance in the surrounding gas significantly slows down evapora ...

. 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. 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. 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 continent located entirely in the Northern Hemisphere and mostly in the Eastern Hemisphere. It is bordered by the Arctic Ocean to the north, the Atlantic Ocean to the west, the Mediterranean Sea to the south, and Asia to the east ...

North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...

South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a considerably smaller portion in the Northern Hemisphere. It can also be described as the southern Subregion#Americas, subregion o ...

and

Asia Asia ( , ) is the largest continent in the world by both land area and population. It covers an area of more than 44 million square kilometres, about 30% of Earth's total land area and 8% of Earth's total surface area. The continent, which ...

are dominated by cloudy skies due to the westerlies,

monsoon A monsoon () is traditionally a seasonal reversing wind accompanied by corresponding changes in precipitation but is now used to describe seasonal changes in Atmosphere of Earth, atmospheric circulation and precipitation associated with annu ...

or other effects. On the other hand,

Africa Africa is the world's second-largest and second-most populous continent after Asia. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 20% of Earth's land area and 6% of its total surfac ...

, the

Middle East The Middle East (term originally coined in English language) is a geopolitical region encompassing the Arabian Peninsula, the Levant, Turkey, Egypt, Iran, and Iraq. The term came into widespread usage by the United Kingdom and western Eur ...

and

Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...

are dominated by clear skies due to their continentality and aridity.https://images.slideplayer.com/17/5328401/slides/slide_4.jpg On a regional scale, some exceptionally humid areas of Earth experience cloudy conditions virtually all time such as South America's

Amazon Rainforest The Amazon rainforest, also called the Amazon jungle or Amazonia, is a Tropical and subtropical moist broadleaf forests, moist broadleaf tropical rainforest in the Amazon biome that covers most of the Amazon basin of South America. This basin ...

while some highly arid areas experience clear-sky conditions virtually all the time such as Africa's

Sahara Desert The Sahara (, ) is a desert spanning across North Africa. With an area of , it is the largest hot desert in the world and the list of deserts by area, third-largest desert overall, smaller only than the deserts of Antarctica and the northern Ar ...

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 cloud Cumulus clouds are clouds that have flat cloud base, bases and are often described as puffy, cotton-like, or fluffy in appearance. Their name derives from the Latin , meaning "heap" or "pile". Cumulus clouds are low-level clouds, generally less ...

s 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 Atmospheric dynamics fr:Nuage#Nébulosité et opacité