Cloud cover

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 clouds when observed from a particular location. Okta is the usual unit of 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.

File:Cloud cover over the North Atlantic Ocean 3.JPG

Partial cloud cover over the North Atlantic Ocean.

The global cloud cover averages around 0.68 when analysing clouds with optical depth larger than 0.1. This value is lower when considering clouds with an optical depth larger than 2 (0.56) and higher when counting subvisible cirrus clouds.

Role in the climate system

Global cloud cover, averaged over the month of October, 2009. The image shows that the outlines of the continents can often be traced through observations of clouds alone, with the sharpest outlines where very dry land is surrounded by ocean.

Clouds play multiple critical roles in the climate system. 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. Cloud cover thus plays an important role in the energetic balance of the atmosphere and a variation of it is a consequence of and to the climate change expected by recent studies.

Variability

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). (click for more detail)

Cloud cover values only vary by 0.03 from year to year, 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.

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. 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 52 % of Earth is cloud-covered at any moment but 67 % is typically covered by clouds. Some regions are almost always cloudy such as the Amazon Rainforest and some others are almost always clear such as the Sahara Desert.

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's 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.fr:Nuage#Nébulosité et opacité