AlbedoAlbedoAlbedo (), or reflection coefficient, derived from LatinLatin albedo "whiteness" (or reflected sunlight) in turn from albus "white," is the Diffuse reflectiondiffuse reflectivity or reflecting power of a surface. It is the ratio of reflected radiation from the surface to incident radiation upon it. Its Dimensionless numberdimensionless 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 Frequencyfrequency of the radiation. When quoted unqualified, it usually refers to some appropriate average across the spectrum of Visible lightvisible light. In general, the albedo depends on the directional distribution of incident radiation, except for Lambertian reflectanceLambertian 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 functionbidirectional 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 Climatologyclimatology, Astronomyastronomy, and calculating Reflectivityreflectivity of surfaces in Leadership in Energy and Environmental DesignLEED 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.The term was introduced into optics by Johann Heinrich LambertJohann Heinrich Lambert in his 1760 work PhotometriaPhotometria.File:Albedo-e hg.svgthumbPercentage of diffusely reflected sunlight in relation to various surface conditions
Terrestrial albedoAlbedos 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 bodyblack 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. The average albedo of the EarthEarth 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 observationEarth observation satellite sensors such as NASANASA's MODISMODIS instruments on board the Terra (satellite)Terra and Aqua (satellite)Aqua satellites. As the total amount of reflected radiation cannot be directly measured by satellite, a Mathematical modelmathematical model of the BRDF is used to translate a sample set of satellite reflectance measurements into estimates of Directional-hemispherical reflectancedirectional-hemispherical reflectance and bi-hemispherical reflectance (e.g.).Earth's average surface temperature due to its albedo and the Greenhouse effectgreenhouse 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.
White-sky and black-sky albedoIt has been shown that for many applications involving terrestrial albedo, the albedo at a particular Solar zenith anglesolar zenith angle θi 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 reflectanceDirectional-hemispherical reflectance is sometimes referred to as black-sky albedo and Bi-hemispherical reflectancebi-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.
File:Ceres 2003 2004 clear sky total sky albedo.pngthumb200pxleft2003–2004 mean annual clear-sky and total-sky albedo
Astronomical albedoThe albedos of Planetplanets, Natural satellitesatellites and Asteroidasteroids 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 (astronomy)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 SystemSolar System objects, the variation of albedo with phase angle gives information about Regolithregolith properties, whereas unusually high radar albedo is indicative of high metal content in Asteroidasteroids.EnceladusEnceladus, 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 (dwarf planet)Eris, with an albedo of 0.96. Many small objects in the outer Solar System and Asteroid beltasteroid belt have low albedos down to about 0.05. A typical Comet nucleuscomet nucleus has an albedo of 0.04. Such a dark surface is thought to be indicative of a primitive and heavily Space weatheringspace weathered surface containing some Organic compoundorganic compounds.The overall albedo of the MoonMoon is around 0.12, but it is strongly directional and non-Lambertian, displaying also a strong Opposition effectopposition effect. Although such reflectance properties are different from those of any terrestrial terrains, they are typical of the Regolithregolith surfaces of airless Solar System bodies.Two common albedos that are used in astronomy are the (V-band) Geometric albedogeometric albedo (measuring brightness when illumination comes from directly behind the observer) and the Bond albedoBond 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 parametersHapke parameters which semi-empirically describe the variation of albedo with Phase angle (astronomy)phase angle, including a characterization of the opposition effect of Regolithregolith surfaces.The correlation between astronomical (geometric) albedo, Absolute magnitudeAbsolute magnitude for planets (H)absolute magnitude and diameter is:,where is the astronomical albedo, is the diameter in kilometers, and is the absolute magnitude.
Examples of terrestrial albedo effects
IlluminationAlthough 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. The minima are found in the subtropical regions of the northern and southern hemispheres, beyond which albedo increases without respect to illumination.
Insolation effectsThe intensity of albedo temperature effects depend on the amount of albedo and the level of local Insolationinsolation; high albedo areas in the Arcticarctic and Antarcticantarctic regions are cold due to low insolation, where areas such as the Sahara DesertSahara Desert, which also have a relatively high albedo, will be hotter due to high insolation. TropicalTropical and Sub-tropicalsub-tropical Rain forestrain forest areas have low albedo, and are much hotter than their Temperate foresttemperate forest counterparts, which have lower insolation. Because insolation plays such a big role in the heating and cooling effects of albedo, high insolation areas like the tropics will tend to show a more pronounced fluctuation in local temperature when local albedo changes.
Climate and weatherAlbedo affects Climateclimate and drives Weatherweather. All weather is a result of the uneven heating of Earth caused by different areas of the planet having different albedos. Essentially, for the driving of weather, there are two types of albedo regions on Earth: Land and ocean. Land and ocean regions produce the four basic different types of Air massesair masses, depending on latitude and therefore Insolationinsolation: Warm and dry, which form over tropical and sub-tropical land masses; warm and wet, which form over tropical and sub-tropical oceans; cold and dry which form over temperate, polar and sub-polar land masses; and cold and wet, which form over temperate, polar and sub-polar oceans. Different temperatures between the air masses result in different air pressures, and the masses develop into Pressure systemspressure systems. High pressure systems flow toward lower pressure, driving weather from north to south in the northern hemisphere, and south to north in the lower; however due to the spinning of Earth, the Coriolis effectCoriolis effect further complicates flow and creates several weather/climate bands and the Jet streamjet streams.
Albedo–temperature feedbackWhen an area's albedo changes due to snowfall, a snow–temperature Feedbackfeedback results. A layer of snowfall increases local albedo, reflecting away sunlight, leading to local cooling. In principle, if no outside temperature change affects this area (e.g. a warm Air massair mass), the lowered albedo and lower temperature would maintain the current snow and invite further snowfall, deepening the snow–temperature feedback. However, because local Weatherweather is dynamic due to the change of Seasonsseasons, eventually warm air masses and a more direct angle of sunlight (higher Insolationinsolation) cause melting. When the melted area reveals surfaces with lower albedo, such as grass or soil, the effect is reversed: the darkening surface lowers albedo, increasing local temperatures, which induces more melting and thus reducing the albedo further, resulting in still more heating.
Small-scale effectsAlbedo 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.
Solar photovoltaic effectsAlbedo can affect the Electrical energyelectrical energy output of solar Photovoltaic systemphotovoltaic devices (PV). For example, the effects of a spectrally responsive albedo are illustrated by the differences between the spectrally weighted albedo of solar PV technology based on hydrogenated amorphous silicon (a-Si:H) and crystalline silicon (c-Si)-based compared to traditional spectral-integrated albedo predictions. Research showed impacts of over 10%. More recently, the analysis was extended to the effects of spectral bias due to the specular reflectivity of 22 commonly occurring surface materials (both human-made and natural) and analyzes the albedo effects on the performance of seven PV materials covering three common PV system topologies: industrial (solar farms), commercial flat rooftops and residential pitched-roof applications.
TreesBecause forests generally have a low albedo, (the majority of the ultraviolet and visible spectrum is absorbed through Photosynthesisphotosynthesis), some scientists have suggested that greater heat absorption by trees could offset some of the carbon benefits of afforestation (or offset the negative climate impacts of deforestation). In the case of evergreen forests with seasonal snow cover albedo reduction may be great enough for deforestation to cause a net cooling effect. Trees also impact climate in extremely complicated ways through Evapotranspirationevapotranspiration. The water vapor causes cooling on the land surface, causes heating where it condenses, acts a strong greenhouse gas, and can increase alebedo when it condenses into clouds Scientists generally treat evapotranspiration as a net cooling impact, and the net climate impact of albedo and evapotranspiration changes from deforestation depends greatly on local climate 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 treesDeciduous trees have an albedo value of about 0.15 to 0.18 whereas Coniferous treesconiferous trees have a value of about 0.09 to 0.15.Studies by the Hadley CentreHadley Centre have investigated the relative (generally warming) effect of albedo change and (cooling) effect of Carbon sequestrationcarbon 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.
SnowSnow albedo is highly variable, ranging from as high as 0.9 for freshly fallen snow, to about 0.4 for melting snow, and as low as 0.2 for dirty snow. Over AntarcticaAntarctica 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 because more radiation is being absorbed by the snowpack (the ice-albedo Positive feedbackpositive feedback). CryoconiteCryoconite, powdery windblown Dustdust containing soot, sometimes reduces albedo on glaciers and ice sheets.Hence, small errors in albedo can lead to large errors in energy estimates, which is why it is important to measure the albedo of snow-covered areas through remote sensing techniques rather than applying a single value over broad regions.
WaterWater reflects light very differently from typical terrestrial materials. The reflectivity of a water surface is calculated using the Fresnel equationsFresnel equations (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 reflectionspecular manner (not Diffuse reflectiondiffusely). The glint of light off water is a commonplace effect of this. At small Angle of incidenceangles of incident light, Wavinesswaviness 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 becomes very high at high angles of incident light such as those that occur on the illuminated side of Earth near the Terminator (solar)terminator (early morning, late afternoon, and near the poles). However, as mentioned above, waviness causes an appreciable reduction. Because 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.File:Water reflectivity.jpgthumbright250pxReflectivity of smooth water at 20 °C (refractive index=1.333)
CloudsCloud albedoCloud albedo 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 Contrailcontrails 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 10 °C colder than temperatures several miles away under clear skies.
Aerosol effectsAerosolsAerosols (very fine particles/droplets in the atmosphere) have both direct and indirect effects on Earth's radiative balance. The direct (albedo) effect is generally to cool the planet; the indirect effect (the particles act as Cloud condensation nucleicloud condensation nuclei and thereby change cloud 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 nucleicloud condensation nuclei. 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 carbonAnother albedo-related effect on the climate is from Black carbonblack carbon particles. The size of this effect is difficult to quantify: the Intergovernmental Panel on Climate ChangeIntergovernmental Panel on Climate Change 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. Black carbon is a bigger cause of the melting of the polar ice cap in the Arctic than carbon dioxide due to its effect on the albedo.
Human activitiesHuman activities (e.g. deforestation, farming, and urbanization) change the albedo of various areas around the globe. However, quantification of this effect on the global scale is difficult.
Other types of albedoSingle-scattering albedoSingle-scattering albedo is used to define scattering of electromagnetic waves on small particles. It depends on properties of the material (Refractive indexrefractive index); the size of the particle or particles; and the wavelength of the incoming radiation.
See also Cool roofCool roof Solar radiation managementSolar radiation management Global dimmingGlobal dimming IrradianceIrradiance Polar see-sawPolar see-saw DaisyworldDaisyworld