Definitions

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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.

Measurement

Traditionally, cloud cover is estimated by trained observers from a meteorological station on the ground and expressed either in oktas (or eighths of the sky) or in tenths. These visual estimates are given to the closest value only. A value of 0 refers to clear sky, while 8 oktas or 10 on the decimal scale indicates overcast. Such estimates are representative of conditions within the range of visibility of the observer. The main problems associated with this method include the inability of making observations when visibility is very low (e.g., in case of fog) or the difficulties of estimating the correct fractional cover for clouds that are near the visual horizon.

With the advent of satellite observations, it has become possible to estimate the fractional cloud cover much more accurately (though not without difficulties). Identifying deep bright clouds on satellite images acquired in the solar spectrum is relatively easy. However, thin clouds or clouds with horizontal dimensions smaller than the spatial resolution of the sensor may largely escape detection.

Role in the climate system

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. A small increase in cloud cover could, in principle, balance the heating resulting from greenhouse gases (though this may have other implications as well).

See climate change for a more detailed discussion of these issues.

References

  • Huschke, Ralph E. (1959) Glossary of Meteorology, American Meteorological Society, Boston, Second printing-1970.
  • McIntosh, D. H. (1972) Meteorological Glossary, Her Majesty's Stationery Office, Met. O. 842, A.P. 897, 319 p.
  • IPCC Third Assessment Report, has extensive coverage of cloud-climate interactions: See, in particular, chapter 7.2.

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