GC: n
CT: Albedo is a non-dimensional, unitless quantity that indicates how well a surface reflects solar energy. Albedo (α) varies between 0 and 1. Albedo commonly refers to the “whiteness” of a surface, with 0 meaning black and 1 meaning white. A value of 0 means the surface is a “perfect absorber” that absorbs all incoming energy. Absorbed solar energy can be used to heat the surface or, when sea ice is present, melt the surface. A value of 1 means the surface is a “perfect reflector” that reflects all incoming energy.
Albedo generally applies to visible light, although it may involve some of the infrared region of the electromagnetic spectrum. You understand the concept of low albedo intuitively when you avoid walking barefoot on blacktop on a hot summer day. Blacktop has a much lower albedo than concrete because the black surface absorbs more energy and reflects very little energy.
S: NSIDC – http://nsidc.org/cryosphere/seaice/processes/albedo.html (last access: 31 December 2014)
N: 1. “ratio of light reflected from a surface,” 1859, from Latin albedo, literally “whiteness,” from albus “white” (see alb).
2. Albedo is the fraction of solar energy (shortwave radiation) reflected from the Earth back into space. It is a measure of the reflectivity of the earth’s surface. Ice, especially with snow on top of it, has a high albedo: most sunlight hitting the surface bounces back towards space. Water is much more absorbent and less reflective. So, if there is a lot of water, more solar radiation is absorbed by the ocean than when ice dominates.
3. The term albedo (Latin for white) is commonly used to applied to the overall average reflection coefficient of an object. For example, the albedo of the Earth is 0.39 (Kaufmann) and this affects the equilibrium temperature of the Earth. The greenhouse effect, by trapping infrared radiation, can lower the albedo of the earth and cause global warming.
The albedo of an object will determine its visual brightness when viewed with reflected light. For example, the planets are viewed by reflected sunlight and their brightness depends upon the amount of light received from the sun and their albedo. Mercury receives the maximum amount of sunlight, but its albedo is only 0.1 so it is not as bright as it would be with a higher albedo.
In more technical treatments of albedo, such as that of De Pater and Lissauer, a distinction is made between “bond albedo” and “geometric albedo”, the numbers quoted above being geometric albedos. The geometric albedo is defined as the amount of radiation relative to that from a flat Lambertian surface which is an ideal reflector at all wavelengths. The bond albedo is the total radiation reflected from an object compared to the total incident radiation from the sun. The bond albedo for the Earth is given as 0.29 by De Pater and Lissauer, compared to their value of 0.37 for the geometrical albedo.
S:1. OED – http://www.etymonline.com/index.php?allowed_in_frame=0&search=albedo&searchmode=none (last access: 31 December 2014). 2. ESR – https://www.esr.org/outreach/glossary/albedo.html (last access: 4 November 2014). 3. Hphysics – http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/albedo.html (last access: 4 November 2014).
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