Albedo

Albedo

Overview
Albedo or reflection coefficient, is the diffuse reflectivity
Diffuse reflection
Diffuse reflection is the reflection of light from a surface such that an incident ray is reflected at many angles rather than at just one angle as in the case of specular reflection...

 or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it. Being a dimensionless fraction, it may also be expressed as a percentage, and is measured on a scale from zero for no reflecting power of a perfectly black surface, to 1 for perfect reflection of a white surface.

Albedo depends on the frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

 of the radiation.
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Encyclopedia
Albedo or reflection coefficient, is the diffuse reflectivity
Diffuse reflection
Diffuse reflection is the reflection of light from a surface such that an incident ray is reflected at many angles rather than at just one angle as in the case of specular reflection...

 or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it. Being a dimensionless fraction, it may also be expressed as a percentage, and is measured on a scale from zero for no reflecting power of a perfectly black surface, to 1 for perfect reflection of a white surface.

Albedo depends on the frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of 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 incoming radiation. Exceptions are Lambertian surfaces, which scatter radiation in all directions according to a cosine function, so their albedo does not depend on the incident distribution. In practice, a bidirectional reflectance distribution function
Bidirectional reflectance distribution function
The bidirectional reflectance distribution function is a four-dimensional function that defines how light is reflected at an opaque surface...

 (BRDF) may be required to characterize the scattering properties of a surface accurately, although the albedo is a very useful first approximation.

The albedo is an important concept in climatology
Climatology
Climatology is the study of climate, scientifically defined as weather conditions averaged over a period of time, and is a branch of the atmospheric sciences...

 and astronomy
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...

, as well as in calculating reflectivity
Reflectivity
In optics and photometry, reflectivity is the fraction of incident radiation reflected by a surface. In general it must be treated as a directional property that is a function of the reflected direction, the incident direction, and the incident wavelength...

 of surfaces in LEED sustainable rating systems for buildings, computer graphics and computer vision. The average overall albedo of Earth, its planetary albedo, is 30 to 35%, because of the covering by clouds, but varies widely locally across the surface, depending on the geological and environmental features.

The term is derived from Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 albedo "whiteness", in turn from albus "white", and was introduced into optics by Johann Heinrich Lambert
Johann Heinrich Lambert
Johann Heinrich Lambert was a Swiss mathematician, physicist, philosopher and astronomer.Asteroid 187 Lamberta was named in his honour.-Biography:...

 in his 1760 work Photometria.

Terrestrial albedo

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

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
Black body
A black body is an idealized physical body that absorbs all incident electromagnetic radiation. Because of this perfect absorptivity at all wavelengths, a black body is also the best possible emitter of thermal radiation, which it radiates incandescently in a characteristic, continuous spectrum...

. When seen from a distance, the ocean surface has a low albedo, as do most forests, while desert areas have some of the highest albedos among landforms. Most land areas are in an albedo range of 0.1 to 0.4. The average albedo of the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

 is about 0.3. This is far higher than for the ocean primarily because of the contribution of clouds.

Human activities have changed the albedo (via forest clearance and farming, for example) of various areas around the globe. However, quantification of this effect on the global scale is difficult.

The classic example of albedo effect is the snow-temperature feedback
Feedback
Feedback describes the situation when output from an event or phenomenon in the past will influence an occurrence or occurrences of the same Feedback describes the situation when output from (or information about the result of) an event or phenomenon in the past will influence an occurrence or...

. 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
Insolation
Insolation is a measure of solar radiation energy received on a given surface area in a given time. It is commonly expressed as average irradiance in watts per square meter or kilowatt-hours per square meter per day...

; for this reason it can be potentially very large in the tropics.
The Earth's surface albedo is regularly estimated via Earth observation
Earth observation
Earth observation is the gathering of information about planet Earth’s physical, chemical and biological systems. It is used to monitor and assess the status of, and changes in, the natural environment and the built environment. In recent years, Earth observation has become technologically more and...

 satellite sensors such as NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

's MODIS
MODIS
The Moderate-resolution Imaging Spectroradiometer is a payload scientific instrument launched into Earth orbit by NASA in 1999 on board the Terra Satellite, and in 2002 on board the Aqua satellite...

 instruments on board the Terra
Terra (satellite)
Terra is a multi-national NASA scientific research satellite in a sun-synchronous orbit around the Earth. It is the flagship of the Earth Observing System...

 and Aqua
Aqua (satellite)
Aqua is a multi-national NASA scientific research satellite in orbit around the Earth, studying the precipitation, evaporation, and cycling of water. It is the second major component of the Earth Observing System preceded by Terra and followed by Aura .The name "Aqua" comes from the Latin word...

 satellites. As the total amount of reflected radiation cannot be directly measured by satellite, a mathematical model
Mathematical model
A mathematical model is a description of a system using mathematical concepts and language. The process of developing a mathematical model is termed mathematical modeling. Mathematical models are used not only in the natural sciences and engineering disciplines A mathematical model is a...

 of the BRDF is used to translate a sample set of satellite reflectance measurements into estimates of directional-hemispherical reflectance
Directional-hemispherical reflectance
Directional-hemispherical reflectance is the reflectance of a surface under direct illumination . Directional-hemispherical reflectance is the integral of the BRDF over all viewing directions. It is sometimes called "black-sky albedo". See also: Bi-hemispherical reflectance...

 and bi-hemispherical reflectance. (e. g.,
.)

The Earth's average surface temperature due to its albedo and the greenhouse effect
Greenhouse effect
The greenhouse effect is a process by which thermal radiation from a planetary surface is absorbed by atmospheric greenhouse gases, and is re-radiated in all directions. Since part of this re-radiation is back towards the surface, energy is transferred to the surface and the lower atmosphere...

 is currently about 15°C. For the frozen (more reflective) planet the average temperature is below -40°C (If only all continents being completely covered by glaciers - the mean temperature is about 0°C). The simulation for (more absorptive) aquaplanet shows the average temperature close to 27°C.

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
Celestial coordinate system
In astronomy, a celestial coordinate system is a coordinate system for mapping positions on the celestial sphere.There are different celestial coordinate systems each using a system of spherical coordinates projected on the celestial sphere, in analogy to the geographic coordinate system used on...

  can reasonably be approximated by the proportionate sum of two terms: the directional-hemispherical reflectance at that solar zenith angle, , and the bi-hemispherical reflectance, the proportion concerned being defined as the proportion of diffuse illumination .

Albedo can then be given as:


Directional-hemispherical reflectance
Directional-hemispherical reflectance
Directional-hemispherical reflectance is the reflectance of a surface under direct illumination . Directional-hemispherical reflectance is the integral of the BRDF over all viewing directions. It is sometimes called "black-sky albedo". See also: Bi-hemispherical reflectance...

 is sometimes referred to as black-sky albedo and bi-hemispherical reflectance
Bi-hemispherical reflectance
Bi-hemispherical reflectance is the reflectance of a surface under diffuse illumination . Bi-hemispherical reflectance is the integral of the BRDF over all viewing and illumination directions of a hemisphere. It is sometimes called "white-sky albedo"....

 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.

Astronomical albedo


The albedos of planet
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...

s, satellites and asteroid
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...

s 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
Photometry (astronomy)
Photometry is a technique of astronomy concerned with measuring the flux, or intensity of an astronomical object's electromagnetic radiation...

. 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
Regolith
Regolith is a layer of loose, heterogeneous material covering solid rock. It includes dust, soil, broken rock, and other related materials and is present on Earth, the Moon, some asteroids, and other terrestrial planets and moons.-Etymology:...

 properties, while unusually high radar albedo is indicative of high metallic content in asteroid
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...

s.

Enceladus
Enceladus (moon)
Enceladus is the sixth-largest of the moons of Saturn. It was discovered in 1789 by William Herschel. Until the two Voyager spacecraft passed near it in the early 1980s very little was known about this small moon besides the identification of water ice on its surface...

, 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
Eris (dwarf planet)
Eris, formal designation 136199 Eris, is the most massive known dwarf planet in the Solar System and the ninth most massive body known to orbit the Sun directly...

, with an albedo of 0.96
. Many small objects in the outer solar system and asteroid belt
Asteroid belt
The asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets...

 have low albedos down to about 0.05. A typical comet nucleus
Comet nucleus
The nucleus is the solid, central part of a comet, popularly termed a dirty snowball. A cometary nucleus is composed of rock, dust, and frozen gases. When heated by the Sun, the gases sublimate and produce an atmosphere surrounding the nucleus known as the coma...

 has an albedo of 0.04. Such a dark surface is thought to be indicative of a primitive and heavily space weathered
Space weathering
Space weathering is a blanket term used for a number of processes that act on any body exposed to the harsh space environment. Airless bodies incur many weathering processes:* collisions of galactic cosmic rays and solar cosmic rays,* irradiation, implantation, and sputtering from solar wind...

 surface containing some organic compound
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...

s.

The overall albedo of the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

 is around 0.12, but it is strongly directional and non-Lambertian, displaying also a strong opposition effect. While such reflectance properties are different from those of any terrestrial terrains, they are typical of the regolith
Regolith
Regolith is a layer of loose, heterogeneous material covering solid rock. It includes dust, soil, broken rock, and other related materials and is present on Earth, the Moon, some asteroids, and other terrestrial planets and moons.-Etymology:...

 surfaces of airless solar system bodies.

Two common albedos that are used in astronomy are the (V-band) geometric albedo
Geometric albedo
The geometric albedo of an astronomical body is the ratio of its actual brightness at zero phase angle to that of an idealized flat, fully reflecting, diffusively scattering disk with the same cross-section....

 (measuring brightness when illumination comes from directly behind the observer) and the Bond albedo
Bond albedo
The Bond albedo, named after the American astronomer George Phillips Bond , who originally proposed it, is the fraction of power in the total electromagnetic radiation incident on an astronomical body that is scattered back out into space...

 (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
Phase angle (astronomy)
Phase angle in astronomical observations is the angle between the light incident onto an observed object and the light reflected from the object...

, including a characterization of the opposition effect of regolith
Regolith
Regolith is a layer of loose, heterogeneous material covering solid rock. It includes dust, soil, broken rock, and other related materials and is present on Earth, the Moon, some asteroids, and other terrestrial planets and moons.-Etymology:...

 surfaces.

The correlation between astronomical (geometric) albedo, absolute magnitude and diameter is:
,

where is the astronomical albedo, is the diameter in kilometres, and H is the absolute magnitude.

The tropics


Although the albedo-temperature effect is best known in colder regions on Earth, because more snow
Snow
Snow is a form of precipitation within the Earth's atmosphere in the form of crystalline water ice, consisting of a multitude of snowflakes that fall from clouds. Since snow is composed of small ice particles, it is a granular material. It has an open and therefore soft structure, unless packed by...

 falls there, it is actually much stronger in tropical regions which receive consistently more sunlight.

Small-scale effects


Albedo works on a smaller scale, too. In sunlight, dark clothes absorb more heat and light-coloured clothes reflect it better, thus allowing some control over body temperature by exploiting the albedo effect of the colour of external clothing.

Trees


Because trees tend to have a low albedo, removing forests would tend to increase albedo and thereby could produce localized climate cooling (ignoring the lost evaporative cooling effect of trees). Cloud feedback
Cloud feedback
Cloud feedback is the coupling between cloudiness and surface air temperature in which a change in radiative forcing perturbs the surface air temperature, leading to a change in clouds, which could then amplify or diminish the initial temperature perturbation....

s further complicate the issue. In seasonally snow-covered zones, winter albedos of treeless areas are 10% to 50% higher than nearby forested areas because snow does not cover the trees as readily. Deciduous trees have an albedo value of about 0.15 to 0.18 while coniferous trees have a value of about 0.09 to 0.15.

Studies by the Hadley Centre have investigated the relative (generally warming) effect of albedo change and (cooling) effect of carbon sequestration on planting forests. They found that new forests in tropical and midlatitude areas tended to cool; new forests in high latitudes (e.g. Siberia) were neutral or perhaps warming.

Snow


Snow albedos can be as high as 0.9; this, however, is for the ideal example: fresh deep snow over a featureless landscape. Over Antarctica they average a little more than 0.8. If a marginally snow-covered area warms, snow tends to melt, lowering the albedo, and hence leading to more snowmelt (the ice-albedo positive feedback
Positive feedback
Positive feedback is a process in which the effects of a small disturbance on a system include an increase in the magnitude of the perturbation. That is, A produces more of B which in turn produces more of A. In contrast, a system that responds to a perturbation in a way that reduces its effect is...

). Cryoconite
Cryoconite
Cryoconite is powdery windblown dust which is deposited and builds up on snow, glaciers, or icecaps. It contains small amounts of soot which absorbs solar radiation melting the snow or ice beneath the deposit sometimes creating a cryoconite hole. Cryoconite may contain dust from far away...

, powdery windblown dust
Dust
Dust consists of particles in the atmosphere that arise from various sources such as soil dust lifted up by wind , volcanic eruptions, and pollution...

 containing soot, sometimes reduces albedo on glaciers and ice sheets.

Water


Water reflects light very differently from typical terrestrial materials. The reflectivity of a water surface is calculated using the Fresnel equations
Fresnel equations
The Fresnel equations , deduced by Augustin-Jean Fresnel , describe the behaviour of light when moving between media of differing refractive indices...

 (see graph).

At the scale of the wavelength of light even wavy water is always smooth so the light is reflected in a locally specular manner
Specular reflection
Specular reflection is the mirror-like reflection of light from a surface, in which light from a single incoming direction is reflected into a single outgoing direction...

 (not diffusely
Diffuse reflection
Diffuse reflection is the reflection of light from a surface such that an incident ray is reflected at many angles rather than at just one angle as in the case of specular reflection...

). The glint of light off water is a commonplace effect of this. At small angles of incident
Angle of incidence
Angle of incidence is a measure of deviation of something from "straight on", for example:* in the approach of a ray to a surface, or* the angle at which the wing or horizontal tail of an airplane is installed on the fuselage, measured relative to the axis of the fuselage.-Optics:In geometric...

 light, waviness
Waviness
Waviness is the measure of the more widely spaced component of surface texture. It is a broader view of roughness because it is more strictly defined as "the irregularities whose spacing is greater than the roughness sampling length"...

 results in reduced reflectivity because of the steepness of the reflectivity-vs.-incident-angle curve and a locally increased average incident angle.

Although the reflectivity of water is very low at low and medium angles of incident light, it increases tremendously at high angles of incident light such as occur on the illuminated side of the Earth near the terminator
Terminator (solar)
A terminator, twilight zone or "grey line" is a moving line that separates the illuminated day side and the dark night side of a planetary body...

 (early morning, late afternoon and near the poles). However, as mentioned above, waviness causes an appreciable reduction. Since the light specularly reflected from water does not usually reach the viewer, water is usually considered to have a very low albedo in spite of its high reflectivity at high angles of incident light.

Note that white caps on waves look white (and have high albedo) because the water is foamed up, so there are many superimposed bubble surfaces which reflect, adding up their reflectivities. Fresh ‘black’ ice exhibits Fresnel reflection.

Clouds


Cloud albedo
Cloud albedo
Cloud albedo is a measure of the albedo of a cloud - higher values mean that the cloud reflects more solar radiation, or more radiation is transmitted....

 has substantial influence over atmospheric temperatures. Different types of clouds exhibit different reflectivity, theoretically ranging in albedo from a minimum of near 0 to a maximum approaching 0.8. "On any given day, about half of Earth is covered by clouds, which reflect more sunlight than land and water. Clouds keep Earth cool by reflecting sunlight, but they can also serve as blankets to trap warmth."

Albedo and climate in some areas are affected by artificial clouds, such as those created by the contrail
Contrail
Contrails or vapour trails are artificial clouds that are the visible trails of condensed water vapour made by the exhaust of aircraft engines...

s of heavy commercial airliner traffic. A study following the burning of the Kuwaiti oil fields during Iraqi occupation showed that temperatures under the burning oil fires were as much as 10oC colder than temperatures several miles away under clear skies.

Aerosol effects


Aerosols (very fine particles/droplets in the atmosphere) have both direct and indirect effects on the Earth’s radiative balance. The direct (albedo) effect is generally to cool the planet; the indirect effect (the particles act as cloud condensation nuclei
Cloud condensation nuclei
Cloud condensation nuclei or CCNs are small particles typically 0.2 µm, or 1/100 th the size of a cloud droplet ) about which cloud droplets coalesce. Water requires a non-gaseous surface to make the transition from a vapour to a liquid. In the atmosphere, this surface presents itself as tiny...

 and thereby change cloud
Cloud
A cloud is a visible mass of liquid droplets or frozen crystals made of water and/or various chemicals suspended in the atmosphere above the surface of a planetary body. They are also known as aerosols. Clouds in Earth's atmosphere are studied in the cloud physics branch of meteorology...

 properties) is less certain. As per the effects are:

  • Aerosol direct effect. Aerosols directly scatter and absorb radiation. The scattering of radiation causes atmospheric cooling, whereas absorption can cause atmospheric warming.
  • Aerosol indirect effect. Aerosols modify the properties of clouds through a subset of the aerosol population called cloud condensation nuclei
    Cloud condensation nuclei
    Cloud condensation nuclei or CCNs are small particles typically 0.2 µm, or 1/100 th the size of a cloud droplet ) about which cloud droplets coalesce. Water requires a non-gaseous surface to make the transition from a vapour to a liquid. In the atmosphere, this surface presents itself as tiny...

    . Increased nuclei concentrations lead to increased cloud droplet number concentrations, which in turn leads to increased cloud albedo, increased light scattering and radiative cooling (first indirect effect), but also leads to reduced precipitation efficiency and increased lifetime of the cloud (second indirect effect).


Black carbon


Another albedo-related effect on the climate is from black carbon particles. The size of this effect is difficult to quantify: the Intergovernmental Panel on Climate Change
Intergovernmental Panel on Climate Change
The Intergovernmental Panel on Climate Change is a scientific intergovernmental body which provides comprehensive assessments of current scientific, technical and socio-economic information worldwide about the risk of climate change caused by human activity, its potential environmental and...

 estimates that the global mean radiative forcing for black carbon aerosols from fossil fuels is +0.2 W m−2, with a range +0.1 to +0.4 W m−2.

Other types of albedo


Single scattering albedo
Single scattering albedo
Single scattering albedo - the ratio of scattering efficiency to total extinction efficiency . Most often it is defined for small-particle scattering of electromagnetic waves...

 is used to define scattering of electromagnetic waves on small particles. It depends on properties of the material (refractive index
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....

); the size of the particle or particles; and the wavelength of the incoming radiation.

See also


  • Global dimming
    Global dimming
    Global dimming is the gradual reduction in the amount of global direct irradiance at the Earth's surface that was observed for several decades after the start of systematic measurements in the 1950s. The effect varies by location, but worldwide it has been estimated to be of the order of a 4%...

  • Irradiance
    Irradiance
    Irradiance is the power of electromagnetic radiation per unit area incident on a surface. Radiant emittance or radiant exitance is the power per unit area radiated by a surface. The SI units for all of these quantities are watts per square meter , while the cgs units are ergs per square centimeter...

  • Polar see-saw
    Polar see-saw
    The Polar see-saw or Antarctic climate anomaly is the phenomenon that temperature changes in Antarctica are usually of the opposite sign to temperature changes elsewhere in the rest of the world....

  • Solar radiation management
    Solar radiation management
    Solar radiation management projects are a type of geoengineering which seek to reflect sunlight and thus reduce global warming. Examples include the creation of stratospheric sulfur aerosols. They do not reduce greenhouse gas concentrations in the atmosphere, and thus do not address problems...



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