The albedo of an object is the extent to which it diffusely reflects light from the sun. It is therefore a more specific form of the term reflectivity. In photometry and Heat transfer, reflectivity is the fraction of incident radiation reflected by a surface Albedo is defined as the ratio of diffusely reflected to incident electromagnetic radiation. Diffuse reflection is the reflection of Light from an uneven or granular surface such that an incident ray is seemingly reflected at a number of angles Electromagnetic radiation takes the form of self-propagating Waves in a Vacuum or in Matter. It is a unitless measure indicative of a surface's or body's diffuse reflectivity. In Dimensional analysis, a dimensionless quantity (or more precisely a quantity with the dimensions of 1) is a Quantity without any Physical units In photometry and Heat transfer, reflectivity is the fraction of incident radiation reflected by a surface The word is derived from Latin albedo "whiteness", in turn from albus "white". Latin ( lingua Latīna, laˈtiːna is an Italic language, historically spoken in Latium and Ancient Rome. The range of possible values is from 0 (dark) to 1 (bright).

Percentage of diffusely reflected sun light in relation to various surface conditions of the earth

The albedo is an important concept in climatology and astronomy. Climatology (from Greek grc κλίμα klima, "region zone" and grc -λογία -logia) is the study of Climate, scientifically Astronomy (from the Greek words astron (ἄστρον "star" and nomos (νόμος "law" is the scientific study In climatology it is sometimes expressed as a percentage. Its value depends on the frequency of radiation considered: unqualified, it usually refers to some appropriate average across the spectrum of visible light. Frequency is a measure of the number of occurrences of a repeating event per unit Time. In general, the albedo depends on the direction and directional distribution of incoming radiation. Exceptions are Lambertian surfaces, which scatter radiation in all directions in a cosine function, so their albedo does not depend on the incoming distribution. See also Lambert's cosine law If a surface exhibits Lambertian reflectance, light falling on it is scattered such that the apparent brightness of the surface to an observer In realistic cases, a bidirectional reflectance distribution function (BRDF) is required to characterize the scattering properties of a surface accurately, although albedos are a very useful first approximation. The bidirectional reflectance distribution function ( BRDF; {f_r(\omega_i, \omega_o\ } is a 4-dimensional function that defines how light is reflected at

Terrestrial albedo

Albedos of typical materials in visible light range from up to 90% for fresh snow, to about 4% for charcoal, one of the darkest substances. Deeply shadowed cavities can achieve an effective albedo approaching the zero of a blackbody. In Physics, a black body is an object that absorbs all light that falls on it 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 . 1 to . 4. ^[5] The average albedo of the Earth is about 30%. EARTH was a short-lived Japanese vocal trio which released 6 singles and 1 album between 2000 and 2001 ^[6] 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 is difficult on the global scale.

The classic 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. Insolation is a measure of Solar radiation energy received on a given surface area in a given time

The Earth's surface albedo is regularly estimated via Earth observation satellite sensors such as NASA's MODIS instruments onboard the Terra and Aqua satellites. Earth observation is the gathering of information about planet Earth’s physical chemical and biological systems The National Aeronautics and Space Administration ( NASA, ˈnæsə is an agency of the United States government, responsible for the nation's public space program MODIS ( Moderate-resolution Imaging Spectroradiometer) is a payload scientific instrument launched into Earth orbit by NASA in 1999 on board Aqua ( EOS PM-1) is a multi-national NASA scientific research Satellite in Orbit around the Earth, studying the precipitation 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. Note The term model has a different meaning in Model theory, a branch of Mathematical logic. Directional-hemispherical reflectance is the Reflectance of a surface under direct illumination (with no diffuse component

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 θ_i 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 D. In Astronomy, a celestial coordinate system is a Coordinate system for mapping positions in the sky

Albedo α can then be given as:

Directional-hemispherical reflectance is sometimes referred to as black-sky albedo and bi-hemispherical reflectance as white sky albedo. Directional-hemispherical reflectance is the Reflectance of a surface under direct illumination (with no diffuse component Bi-hemispherical reflectance is the Reflectance of a surface under diffuse illumination (with no direct component 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 albedo of planets, satellites and asteroids can be used to infer much about their properties. A planet, as defined by the International Astronomical Union (IAU is a celestial body Orbiting a Star or stellar remnant that is A natural satellite or moon is a Celestial body that Orbits a Planet or smaller body which is called the primary. Asteroids, sometimes called Minor planets or planetoids', are bodies—primarily of the inner Solar System —that are smaller than planets but 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 is a technique of Astronomy concerned with measuring the Flux, or intensity of an Astronomical object 's Electromagnetic 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, while unusually high radar albedo is indicative of high metallic content in asteroids. Regolith ( Greek: "blanket rock" is a layer of loose Heterogeneous material covering solid rock. Asteroids, sometimes called Minor planets or planetoids', are bodies—primarily of the inner Solar System —that are smaller than planets but

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. TemplateInfobox Planet.--> This article is about the moon of Another notable high albedo body is Eris, with an albedo of 86%. Many objects in the outer solar system and asteroid belt have low albedos down to about 5%. The asteroid belt is the region of the Solar System located roughly between the orbits of the Planets Mars and Jupiter. Such a dark surface is thought to be indicative of a primitive and heavily space weathered surface containing some organic compounds. Space weathering is a blanket term used for a number of processes that act on any body exposed to the harsh space environment An organic compound is any member of a large class of Chemical compounds whose Molecules contain Carbon.

The overall albedo of the Moon is around 7%, but it is strongly directional and non-Lambertian, displaying also a strong opposition effect. ^[7] While such reflectance properties are different from those of any terrestrial terrains, they are typical of the regolith surfaces of airless solar system bodies. Regolith ( Greek: "blanket rock" is a layer of loose Heterogeneous material covering solid rock.

Two common albedos that are used in astronomy are the geometric albedo (measuring brightness when illumination comes from directly behind the observer) and the Bond albedo (measuring total proportion of electromagnetic energy reflected). The geometric albedo of an astronomical body is the ratio of its actual brightness at zero phase angle (i The Bond albedo is the fraction of power in the total electromagnetic radiation incident on an astronomical body that is scattered back out into space 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 Hapke parameters are a set of parameters for a quasi-experimental model that are commonly used to describe the directional reflectance properties of the airless Regolith Phase angle in astronomical observations is the angle between the light incident onto an observed object and the light reflected from the object The opposition effect (also opposition spike, or opposition surge) is the brightening of a rough surface or an object with many particles when illuminated from directly Regolith ( Greek: "blanket rock" is a layer of loose Heterogeneous material covering solid rock.

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

where A is astronomical albedo, D is diameter in km, and H is the absolute magnitude. In Astronomy, absolute magnitude (also known as absolute visual magnitude) is the Apparent magnitude an object would have if it were at a standard The kilometre ( American spelling: kilometer) symbol km is a unit of Length in the Metric system, equal to one thousand

Other types of albedo

Single scattering albedo - is used to define scattering of electromagnetic waves on small particles. Single scattering albedo - the ratio of scattering efficiency to total light extinction (which is also termed "attenuance" a sum of scattering and absorption It depends on properties of the material (refractive index), the size of the particle(s), and the wavelength of the incoming radiation. The refractive index (or index of Refraction) of a medium is a measure for how much the speed of light (or other waves such as sound waves is reduced inside the medium

Some examples of terrestrial albedo effects

Fairbanks, Alaska

According to the National Climatic Data Center's GHCN 2 data, which is composed of 30-year smoothed climatic means for thousands of weather stations across the world, the college weather station at Fairbanks, Alaska, is about 3 °C (5. The United States National Climatic Data Center ( NCDC) in Asheville, North Carolina is the world's largest active Archive of The Global Historical Climatology Network (GHCN is a database of Temperature, precipitation and Pressure records managed by the National Climatic Fairbanks (ˈfɛrbæŋks is a Home Rule City in Fairbanks North Star Borough, Alaska, United States. 4 °F) warmer than the airport at Fairbanks, partly because of air drainage patterns but also largely because of the lower albedo at the college resulting from a higher concentration of spruce trees and therefore less open snowy ground to reflect the heat back into space. Spruce refers to Trees of the genus Picea, a genus of about 35 species of Coniferous Evergreen trees in the Family Pinaceae A tree is a perennial Woody plant. It is most often defined as a woody plant that has many secondary branches supported clear of the ground on a single main stem or

The tropics

Although the albedo-temperature effect is most famous in colder regions of Earth, because more snow falls there, it is actually much stronger in tropical regions because in the tropics there is consistently more sunlight. "Snowfall" redirects here For other uses see Snow (disambiguation or Snowfall (disambiguation. When Brazilian ranchers cut down dark, tropical rainforest trees to replace them with even darker soil in order to grow crops, the average temperature of the area increases up to 3 °C (5. |utc_offset = -2 to -4 |time_zone_DST = BRST |utc_offset_DST = -2 to -5 |cctld Rainforests are Forests characterized by high Rainfall with definitions setting minimum normal annual rainfall between 1750–2000 mm (68-78 inches 4 °F) year-round,^[8][9] although part of the effect is due to changed evaporation (latent heat flux). In Thermochemistry, latent heat is the amount of Energy in the form of Heat released or absorbed by a substance during a change of phase

Small scale effects

Albedo works on a smaller scale, too. People who wear dark clothes in the summertime put themselves at a greater risk of heatstroke than those who wear lighter color clothes. Hyperthermia, in its advanced state referred to as heat stroke or sunstroke, is an acute condition which occurs when the Body produces or absorbs more ^[10]

Albedo of various terrains

The albedo of a pine forest at 45°N in the winter in which the trees cover the land surface completely is only about 9%, among the lowest of any naturally occurring land environment. This article is about the tree For other uses of the term "pine" see Pine (disambiguation. This is partly due to the color of the pines, and partly due to multiple scattering of sunlight within the trees which lowers the overall reflected light level. Due to light penetration, the ocean's albedo is even lower at about 3. 5%, though this depends strongly on the angle of the incident radiation. Dense swampland averages between 9% and 14%. A swamp is a Wetland featuring temporary or permanent inundation of large areas of land by shallow bodies of water Deciduous trees average about 13%. Botany Autumn leaf color. See --> In Botany and Horticulture, deciduous Plants, including A grassy field usually comes in at about 20%. Grass is the common word that generally describes Monocotyledonous green Plants The family Gramineae ( Poaceae) are the "true grasses" and include A barren field will depend on the color of the soil, and can be as low as 5% or as high as 40%, with 15% being about the average for farmland. A desert or large beach usually averages around 25% but varies depending on the color of the sand. A desert is a Landscape or region that receives very little precipitation.

Urban areas

Urban areas in particular have very unnatural values for albedo because of the many human-built structures which absorb light before the light can reach the surface. In the northern part of the world, cities are relatively dark, and Walker has shown that their average albedo is about 7%, with only a slight increase during the summer. In most tropical countries, cities average around 12%. This is similar to the values found in northern suburban transitional zones. Part of the reason for this is the different natural environment of cities in tropical regions, e. g. , there are more very dark trees around; another reason is that portions of the tropics are very poor, and city buildings must be built with different materials. Warmer regions may also choose lighter colored building materials so the structures will remain cooler.

Trees

Because trees tend to have a low albedo, removing forests would tend to increase albedo and thereby could produce localized climate cooling. Cloud feedbacks further complicate the issue. Cloud feedback is the coupling between Cloudiness and surface air Temperature in which a change in surface air temperature could lead to a change in volume of clouds 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. Botany Autumn leaf color. See --> In Botany and Horticulture, deciduous Plants, including 15 to 0. 18 while coniferous trees have a value of about 0. 09 to 0. 15. ^[11] The difference between deciduous and coniferous is because coniferous trees are darker in general and have cone-shape seeds. The pattern of these seeds trap light energy more than deciduous trees.

Studies by the Hadley Centre have investigated the relative (generally warming) effect of albedo change and (cooling) effect of carbon sequestration on planting forests. The Met Office Hadley Centre for Climate Change — named in honour of George Hadley — is part of and based at the headquarters of the Met Office in Exeter 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. ^[12]

Snow

Snow albedos can be as high as 90%; this, however, is for the ideal example: fresh deep snow over a featureless landscape. Over Antarctica they average a little more than 80%.

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). Feedback is a circular causal Process whereby some proportion of a system's output is returned (fed back to the Input. This is the basis for predictions of enhanced warming in the polar and seasonally snow covered regions as a result of global warming.

Water

Water reflects light very differently from typical terrestrial materials. The reflectivity of a water surface is calculated using the Fresnel equations (see graph).

Reflectivity of smooth water at 20 C (refractive index=1. 333)

At the scale of the wavelength of light even wavy water is always smooth so the light is reflected in a specular manner (not diffusely). Specular reflection is the perfect Mirror -like reflection of light (or sometimes other kinds of Wave) from a surface in which light from a single incoming Diffuse reflection is the reflection of Light from an uneven or granular surface such that an incident ray is seemingly reflected at a number of angles The glint of light off water is a commonplace effect of this. At small angles of incident light, waviness results in reduced reflectivity (from as high as 100%) because of the steepness of the reflectivity-vs. -incident-angle curve and a locally increased average incident angle. ^[13]

Although the reflectivity of water is very low at high and medium angles of incident light, it increases tremendously at small angles of incident light such as occur on the illuminated side of the earth near the terminator (early morning, late afternoon and near the poles). The terminator is a fictive line that delimits the illuminated day side and dark Night side of a planetary body (also known as the "grey line" or 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 low angles of incident light.

Note that white caps on waves look white (and have high albedo) because the water is foamed up (not smooth at the scale of the wavelength of light) so the Fresnel equations do not apply. Fresh ‘black’ ice exhibits Fresnel reflection.

Clouds

Further information: Cloud albedo

Clouds are another source of albedo that play into the global warming equation. Cloud albedo is a measure of the reflectivity of a Cloud - higher values mean that the cloud can reflect more Solar radiation. Different types of clouds have different albedo values, theoretically ranging from a minimum of near 0% to a maximum in the high 70s. "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. "^[14]

Albedo and climate in some areas are already affected by artificial clouds, such as those created by the contrails of heavy commercial airliner traffic. ^[15] A study following the burning of the Kuwaiti oil fields by Saddam Hussein showed that temperatures under the burning oil fires were as much as 10^oC colder than temperatures several miles away under clear skies. Saddam Hussein Abd al-Majid al-Tikriti ( Arabic: ar صدام حسين عبد المجيد التكريتي --> April 28 1937 &ndash December 30 ^[16]

Aerosol effects

Aerosol (very fine particles/droplets in the atmosphere) has two effects, direct and indirect. Particulates, alternatively referred to as particulate matter (PM or fine particles, are tiny particles of solid or liquid suspended in a gas The direct (albedo) effect is generally to cool the planet; the indirect effect (the particles act as CCNs and thereby change cloud properties) is less certain. Cloud condensation nuclei or CCN s (also known as cloud seeds) are small particles (typically 0 ^[17]

Black carbon

Another albedo-related effect on the climate is from black carbon particles. The size of this effect is difficult to quantify: the IPCC say that their "estimate of the global mean radiative forcing for BC aerosols from fossil fuels is . . . +0. 2 W m^-2 (from +0. 1 W m^-2 in the SAR) with a range +0. The Second Assessment Report (SAR of the Intergovernmental Panel on Climate Change was published in 1995 1 to +0. 4 W m. . . ^-2". ^[18]

See also

• Global dimming
• Irradiance
• Insolation

References

External links

• Albedo - Encyclopedia of Earth
• NASA MODIS Terra BRDF/albedo product site
• NASA MODIS BRDF/albedo product site
• Surface albedo derived from Meteosat observations
• A discussion of Lunar albedos