Cloud cover (also known as
cloudiness,
cloudage or
cloud amount) refers to the fraction of the
skyThe sky is the part of the atmosphere or of outer space visible from the surface of any astronomical object. It is difficult to define precisely for several reasons. During daylight, the sky of Earth has the appearance of a deep blue surface because of the air's scattering of sunlight. The sky is...
obscured by
cloudA cloud is a visible mass of droplets or frozen crystals suspended in the atmosphere above the surface of the Earth or another planetary body. A cloud is also a visible mass attracted by gravity, such as masses of material in space called interstellar clouds and nebulae...
s when observed from a particular location.
Traditionally, cloud cover is estimated by trained observers from a
meteorologicalMeteorology is the interdisciplinary scientific study of the atmosphere that focuses on weather processes and forecasting . Studies in the field stretch back millennia, though significant progress in meteorology did not occur until the eighteenth century...
station on the ground and expressed either in
oktaIn meteorology, an okta is a unit of measurement used to describe cloud cover. Sky conditions are estimated in terms of how many eighths of the sky are obscured by cloud, ranging from completely clear, 0 oktas, through to completely overcast, 8 oktas...
s (or eighths of the sky) or in
tenths. They are called oktas because they are measured with an okta grid.
Cloud cover (also known as
cloudiness,
cloudage or
cloud amount) refers to the fraction of the
skyThe sky is the part of the atmosphere or of outer space visible from the surface of any astronomical object. It is difficult to define precisely for several reasons. During daylight, the sky of Earth has the appearance of a deep blue surface because of the air's scattering of sunlight. The sky is...
obscured by
cloudA cloud is a visible mass of droplets or frozen crystals suspended in the atmosphere above the surface of the Earth or another planetary body. A cloud is also a visible mass attracted by gravity, such as masses of material in space called interstellar clouds and nebulae...
s when observed from a particular location.
Measurement
Traditionally, cloud cover is estimated by trained observers from a
meteorologicalMeteorology is the interdisciplinary scientific study of the atmosphere that focuses on weather processes and forecasting . Studies in the field stretch back millennia, though significant progress in meteorology did not occur until the eighteenth century...
station on the ground and expressed either in
oktaIn meteorology, an okta is a unit of measurement used to describe cloud cover. Sky conditions are estimated in terms of how many eighths of the sky are obscured by cloud, ranging from completely clear, 0 oktas, through to completely overcast, 8 oktas...
s (or eighths of the sky) or in
tenths. They are called oktas because they are measured with an okta grid. 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
visibilityIn meteorology, visibility is a measure of the distance at which an object or light can be clearly discerned. It is reported within surface weather observations and METAR code either in meters or statute miles, depending upon the country. Visibility affects all forms of traffic: roads, sailing...
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 Polar lunar optical prime 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 imagesSatellite imagery consists of photographs of Earth or other planets made by means of artificial satellites.- History : First satellite photographs of Earth were made August 14, 1959 by the US satellite Explorer 6. The first satellite photographs of the Moon might have been made on October 6, 1959...
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
solarThe Sun is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 99.86% of the Solar System's mass....
spectrumA spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...
, 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 gasGreenhouse gases are gases in an atmosphere that absorb and emit radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The main greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone...
es (though this may have other implications as well).
See
climate changeClimate change is a change in the statistical distribution of weather over periods of time that range from decades to millions of years. It can be a change in the average weather or a change in the distribution of weather events around an average...
for a more detailed discussion of these issues...
External links