# Difference between revisions of "Albedo"

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Percentage of diffusely reflected sunlight in relation to various surface conditions

Albedo (Template:IPAc-en), or reflection coefficient, derived from Latin albedo "whiteness" (or reflected sunlight) in turn from albus "white", is the diffuse reflectivity or reflecting power of a surface. It is the ratio of reflected radiation from the surface to incident radiation upon it. Its dimensionless nature lets it be expressed as a percentage and is measured on a scale from zero for no reflection of a perfectly black surface to 1 for perfect reflection of a white surface.

Albedo depends on the frequency of the radiation. When quoted unqualified, it usually refers to some appropriate average across the spectrum of visible light. In general, the albedo depends on the directional distribution of incident radiation, except for Lambertian surfaces, which scatter radiation in all directions according to a cosine function and therefore have an albedo that is independent of the incident distribution. In practice, a bidirectional reflectance distribution function (BRDF) may be required to accurately characterize the scattering properties of a surface, but albedo is very useful as a first approximation.

The albedo is an important concept in climatology, astronomy, and calculating reflectivity of surfaces in LEED sustainable-rating systems for buildings. The average overall albedo of Earth, its planetary albedo, is 30 to 35% because of cloud cover, but widely varies locally across the surface because of different geological and environmental features.[1]

The term was introduced into optics by Johann Heinrich Lambert in his 1760 work Photometria.

## Terrestrial albedo

Sample albedos
Surface Typical
albedo
Fresh asphalt 0.04[2]
Worn asphalt 0.12[2]
Conifer forest
(Summer)
0.08,[3] 0.09 to 0.15[4]
Deciduous trees 0.15 to 0.18[4]
Bare soil 0.17[5]
Green grass 0.25[5]
Desert sand 0.40[6]
New concrete 0.55[5]
Ocean ice 0.5–0.7[5]
Fresh snow 0.80–0.90[5]

Albedos of typical materials in visible light range from up to 0.9 for fresh snow to about 0.04 for charcoal, one of the darkest substances. Deeply shadowed cavities can achieve an effective albedo approaching the zero of a black body. When seen from a distance, the ocean surface has a low albedo, as do most forests, whereas desert areas have some of the highest albedos among landforms. Most land areas are in an albedo range of 0.1 to 0.4.[7] The average albedo of the Earth is about 0.3.[8] This is far higher than for the ocean primarily because of the contribution of clouds.

2003–2004 mean annual clear-sky and total-sky albedo

Earth's surface albedo is regularly estimated via Earth observation satellite sensors such as NASA's MODIS instruments on board the Terra and Aqua satellites. As the total amount of reflected radiation cannot be directly measured by satellite, a mathematical model of the BRDF is used to translate a sample set of satellite reflectance measurements into estimates of directional-hemispherical reflectance and bi-hemispherical reflectance (e.g.[9]).

Earth's average surface temperature due to its albedo and the greenhouse effect is currently about 15 °C. If Earth were frozen entirely (and hence be more reflective) the average temperature of the planet would drop below −40 °C.[10] If only the continental land masses became covered by glaciers, the mean temperature of the planet would drop to about 0 °C.[11] In contrast, if the entire Earth is covered by water—a so-called aquaplanet—the average temperature on the planet would rise to just under 27 °C.[12]

### White-sky and black-sky albedo

It has been shown that for many applications involving terrestrial albedo, the albedo at a particular solar zenith angle θi can reasonably be approximated by the proportionate sum of two terms: the directional-hemispherical reflectance at that solar zenith angle, ${\displaystyle {{\bar {\alpha }}(\theta _{i})}}$, and the bi-hemispherical reflectance, ${\displaystyle {\bar {\bar {\alpha }}}}$ the proportion concerned being defined as the proportion of diffuse illumination ${\displaystyle {D}}$.

Albedo ${\displaystyle {\alpha }}$ can then be given as:

${\displaystyle {\alpha }=(1-D){\bar {\alpha }}(\theta _{i})+D{\bar {\bar {\alpha }}}.}$

Directional-hemispherical reflectance is sometimes referred to as black-sky albedo and bi-hemispherical reflectance as white-sky albedo. These terms are important because they allow the albedo to be calculated for any given illumination conditions from a knowledge of the intrinsic properties of the surface.[13]

## Astronomical albedo

The albedos of planets, satellites and asteroids can be used to infer much about their properties. The study of albedos, their dependence on wavelength, lighting angle ("phase angle"), and variation in time comprises a major part of the astronomical field of photometry. For small and far objects that cannot be resolved by telescopes, much of what we know comes from the study of their albedos. For example, the absolute albedo can indicate the surface ice content of outer Solar System objects, the variation of albedo with phase angle gives information about regolith properties, whereas unusually high radar albedo is indicative of high metal content in asteroids.

Enceladus, a moon of Saturn, has one of the highest known albedos of any body in the Solar System, with 99% of EM radiation reflected. Another notable high-albedo body is Eris, with an albedo of 0.96.[14] Many small objects in the outer Solar System[15] and asteroid belt have low albedos down to about 0.05.[16] A typical comet nucleus has an albedo of 0.04.[17] Such a dark surface is thought to be indicative of a primitive and heavily space weathered surface containing some organic compounds.

The overall albedo of the Moon is around 0.12, but it is strongly directional and non-Lambertian, displaying also a strong opposition effect.[18] Although such reflectance properties are different from those of any terrestrial terrains, they are typical of the regolith surfaces of airless Solar System bodies.

Two common albedos that are used in astronomy are the (V-band) geometric albedo (measuring brightness when illumination comes from directly behind the observer) and the Bond albedo (measuring total proportion of electromagnetic energy reflected). Their values can differ significantly, which is a common source of confusion.

In detailed studies, the directional reflectance properties of astronomical bodies are often expressed in terms of the five Hapke parameters which semi-empirically describe the variation of albedo with phase angle, including a characterization of the opposition effect of regolith surfaces.

The correlation between astronomical (geometric) albedo, absolute magnitude and diameter is:[19] ${\displaystyle A=\left({\frac {1329\times 10^{-H/5}}{D}}\right)^{2}}$,

where ${\displaystyle A}$ is the astronomical albedo, ${\displaystyle D}$ is the diameter in kilometers, and ${\displaystyle H}$ is the absolute magnitude.

## Examples of terrestrial albedo effects

### Illumination

Although the albedo–temperature effect is best known in colder, whiter regions on Earth, the maximum albedo is actually found in the tropics where year-round illumination is greater. The maximum is additionally in the northern hemisphere, varying between three and twelve degrees north.[20] The minima are found in the subtropical regions of the northern and southern hemispheres, beyond which albedo increases without respect to illumination.[20]

## Other types of albedo

Single-scattering albedo is used to define scattering of electromagnetic waves on small particles. It depends on properties of the material (refractive index); the size of the particle or particles; and the wavelength of the incoming radiation.

## References

1. Environmental Encyclopedia, 3rd ed., Thompson Gale, 2003, ISBN 0-7876-5486-8
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28. Hall, D.K. and Martinec, J. (1985), Remote sensing of ice and snow. Chapman and Hall, New York, 189 pp.
29. "Changing Greenland – Melt Zone" page 3, of 4, article by Mark Jenkins in National Geographic June 2010, accessed 8 July 2010
30. http://vih.freeshell.org/pp/01-ONW-St.Petersburg/Fresnel.pdf
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32. http://facstaff.uww.edu/travisd/pdf/jetcontrailsrecentresearch.pdf
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37. James Hansen & Larissa Nazarenko, Soot Climate Forcing Via Snow and Ice Albedos, 101 Proc. of the Nat'l. Acad. of Sci. 423 (13 January 2004) ("The efficacy of this forcing is »2 (i.e. for a given forcing it is twice as effective as CO2 in altering global surface air temperature)"); compare Zender Testimony, supra note 7, at 4 (figure 3); See J. Hansen & L. Nazarenko, supra note 18, at 426. ("The efficacy for changes of Arctic sea ice albedo is >3. In additional runs not shown here, we found that the efficacy of albedo changes in Antarctica is also >3."); See also Flanner, M.G., C.S. Zender, J.T. Randerson, and P.J. Rasch, Present-day climate forcing and response from black carbon in snow, 112 J. GEOPHYS. RES. D11202 (2007) ("The forcing is maximum coincidentally with snowmelt onset, triggering strong snow-albedo feedback in local springtime. Consequently, the "efficacy" of black carbon/snow forcing is more than three times greater than forcing by CO2.").