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Albedo and reflectance


Albedo is the fraction of solar energy that is diffusely reflected back from Earth to space. Measurements of albedo are essential for climate research studies and investigations of the Earth’s energy budget. Different parts of the Earth have different albedos. For example, ocean surfaces and rain forests have low albedos, which means that they reflect only a small portion of the Sun’s energy. Deserts, ice and clouds, however, have high albedos; they reflect a large portion of the incoming solar energy. The high albedo of ice helps to insulate the polar oceans from solar radiation.
Detailed MeasurementDescription*InstrumentsTimeline
Black and White Sky AlbedoThe directional albedo or directional-hemispherical reflectance (also called black-sky albedo) is the integration of the bi-directional reflectance over the viewing hemisphere. It assumes all energy is coming from a direct radiation from the sun and is computed for a specific time. The hemispherical albedo or bi-hemispherical reflectance (also called white-sky albedo) is the integration of the directional albedo over the illumination hemisphere. It assumes a complete diffuse illumination.0 instruments
Earth surface albedoHemispherically integrated reflectance of the Earth’s surface in the range 0.4-0.7 µm (or other specific short-wave ranges) - Physical unit: [ % ] - Accuracy unit: [ % ].83 instruments
Fractionally absorbed PAR (FPAR)Fraction of PAR absorbed by vegetation (land or marine) for photosynthesis processes (generally around the “red”) - Physical unit: [ % ] - Accuracy unit: [ % ].16 instruments
Photosynthetically Active Radiation (PAR)Flux of downwelling photons of wavelength 0.4-0.7 µm at surface - Physical unit: [ µ einstein ٠ m-2 s-1 ] (1 einstein = 6 ٠ 1023 photons); most frequently used: [ W/m2 ] - Accuracy unit: [ W/m2 ].9 instruments
Snow albedoHemispherically integrated reflectance of the snow mantle in the range 0.4-0.7 µm (or other specific short-wave ranges) - Physical unit: [ % ] - Accuracy unit: [ % ].0 instruments
* Description adapted from WMO.
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