|
Albedo (), 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. NOTE: Since it is the ratio of all reflected radiation to incident radiation it will include the diffuse AND the specular radiation reflected. It is, however, common to assume a surface reflects in either a totally specular manner or a totally diffuse manner, as this can simplify calculations. 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.〔Environmental Encyclopedia, 3rd ed., Thompson Gale, 2003, ISBN 0-7876-5486-8〕 The term was introduced into optics by Johann Heinrich Lambert in his 1760 work ''Photometria''. ==Terrestrial albedo== 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.〔(【引用サイトリンク】title=Albedo – from Eric Weisstein's World of Physics )〕 The average albedo of Earth is about 0.3.〔 This is far higher than for the ocean primarily because of the contribution of clouds. 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.〔). 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.〔 If only the continental land masses became covered by glaciers, the mean temperature of the planet would drop to about 0 °C.〔 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.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「albedo」の詳細全文を読む スポンサード リンク
|