Albedo is the fraction of light that is reflected by a body or surface. It is commonly used in astronomy to describe the reflective properties of planets, satellites, and asteroids.
Albedo is usually differentiated into two general types: normal albedo and bond albedo. Normal albedo, also called normal reflectance, is a measure of a surface's relative brightness when illuminated and observed vertically. The normal albedo of snow, for example, is nearly 1.0, whereas that of charcoal is about 0.04. Investigators frequently rely on observations of normal albedo to determine the surface compositions of satellites and asteroids. The albedo, diameter, and distance of such objects together determine their brightness. If the asteroids Ceres and Vesta, for example, could be observed at the same distance, Vesta would be the brighter of the two by roughly 10 percent. Though Vesta's diameter measures less than half that of Ceres, Vesta appears brighter because its albedo is about 0.35, whereas that of Ceres is only 0.09.
Bond albedo, defined as the
fraction of the total incident solar radiation reflected
by a planet back to space, is a measure of the
planet's energy balance. (It is so named for the American astronomer
George P. Bond, who in 1861
published a comparison of the brightness of the Sun, the Moon, and Jupiter.) The
value of bond albedo is dependent on the spectrum of the
incident radiation because such albedo is defined over the entire range
of wavelengths. Earth-orbiting satellites have been used to measure the
Earth's bond albedo. The most recent values
obtained are approximately 0.33. The
Moon, which has a very tenuous atmosphere and no
clouds, has an albedo of 0.12. By contrast, that of
Venus, which is covered by dense clouds, is
A measure of the reflecting power of a nonluminous object, such as a planet, moon, or asteroid. Albedo (from the Latin "albus" for "white") is expressed as the fraction of light and/or other radiation falling on an object that is reflected or scattered back into space; its value ranges from 0, for a perfectly black surface, to 1, for a totally reflective surface.
Several different types of albedo are defined. The two main categories are normal albedo and Bond albedo. Normal albedo, also known as normal reflectance, is a measure of a surface’s relative brightness when illuminated and observed vertically. Within this category, visual albedo refers to radiation only in the visible part of the spectrum, whereas geometric albedo, also known as physical albedo, is the ratio between the brightness of an object as seen from the direction of the Sun, and the brightness of a hypothetical white, diffusely reflecting sphere of the same size and at the same distance. The normal albedo of a moon or asteroid, which can be calculated if the object’s apparent brightness, size, and distance are known, is an important indicator of surface composition. Bond albedo (named for the American astronomer George Bond (1826-1865)), also known as spherical albedo, is the fraction of the total incident solar radiation – the radiation at all wavelengths – that is reflected or scattered by an object in all directions; this is an important measure of a planetary body’s energy balance.
Among familiar objects, the Earth has a visual albedo of 0.37, Mercuryof 0.11, and Venus of 0.65. Charcoal is among the darkest substances, with an albedo of about 0.04), while pristine snow is among the brightest, with an albedo close to 1. From observations of the albedo of a planet or moon it is possible to make inferences about the nature of its atmosphere (if any) or surface. See also albedo feature and Zöllner, Friedrich.
The albedo is a measure of reflectivity of a surface or body. It is the ratio of electromagnetic radiation (EM radiation) reflected to the amount incident upon it. The fraction, usually expressed as a percentage from 0% to 100%, is an important concept in climatology and astronomy. This ratio depends on the frequency of the radiation considered: unqualified, it refers to an average across the spectrum of visible light. It also depends on the angle of incidence of the radiation: unqualified, normal incidence. Fresh snow albedos are high: up to 90%. The ocean surface has a low albedo. Earth has an average albedo of 39% whereas the albedo of the Moon is about 12%. In astronomy, the albedo of satellites and asteroids can be used to infer surface composition, most notably ice content. Enceladus, a moon of Saturn, has the highest known albedo of any body in the solar system, with 99% of EM radiation reflected.
Human activities have changed the albedo (via forest clearance and farming, for example) of various areas around the globe. However, quantification of this effect is difficult on the global scale: it is not clear whether the changes have tended to increase or decrease global warming.
The "classical" example of albedo effect is the snow-temperature feedback. If a snow covered area warms and the snow melts, the albedo decreases, more sunlight is absorbed, and the temperature tends to increase. The converse is true: if snow forms, a cooling cycle happens. The intensity of the albedo effect depends on the size of the change in albedo and the amount of insolation; for this reason it can be potentially very large in the tropics.