![Albedo-e hg](https://upload.wikimedia.org/wikipedia/commons/1/18/Albedo-e_hg.svg)
Albedo (; ) is the measure of the
diffuse reflection
Diffuse reflection is the reflection (physics), reflection of light or other radiation, waves or particles from a surface such that a ray (optics), ray incident on the surface is scattering, scattered at many angles rather than at just one angle ...
of
solar radiation
Solar irradiance is the power per unit area (surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument.
Solar irradiance is measured in watts per square metre (W/m ...
out of the total
solar radiation
Solar irradiance is the power per unit area (surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument.
Solar irradiance is measured in watts per square metre (W/m ...
and measured on a scale from 0, corresponding to a
black body
A black body or blackbody is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. The name "black body" is given because it absorbs all colors of light. A black body ...
that absorbs all incident radiation, to 1, corresponding to a body that reflects all incident radiation.
Surface albedo is defined as the ratio of
radiosity ''J''
e to the
irradiance In radiometry, irradiance is the radiant flux ''received'' by a ''surface'' per unit area. The SI unit of irradiance is the watt per square metre (W⋅m−2). The CGS unit erg per square centimetre per second (erg⋅cm−2⋅s−1) is often used ...
''E''
e (flux per unit area) received by a surface. The proportion reflected is not only determined by properties of the surface itself, but also by the spectral and angular distribution of solar radiation reaching the Earth's surface. These factors vary with atmospheric composition, geographic location, and time (see
position of the Sun
The position of the Sun in the sky is a function of both the time and the geographic location of observation on Earth's surface. As Earth orbits the Sun over the course of a year, the Sun appears to move with respect to the fixed stars on the cel ...
). While bi-hemispherical
reflectance
The reflectance of the surface of a material is its effectiveness in reflecting radiant energy. It is the fraction of incident electromagnetic power that is reflected at the boundary. Reflectance is a component of the response of the electronic ...
is calculated for a single angle of incidence (i.e., for a given position of the Sun), albedo is the directional integration of reflectance over all solar angles in a given period. The temporal resolution may range from seconds (as obtained from flux measurements) to daily, monthly, or annual averages.
Unless given for a specific wavelength (spectral albedo), albedo refers to the entire spectrum of solar radiation. Due to measurement constraints, it is often given for the spectrum in which most solar energy reaches the surface (between 0.3 and 3 μm). This spectrum includes
visible light
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...
(0.4–0.7 μm), which explains why surfaces with a low albedo appear dark (e.g., trees absorb most radiation), whereas surfaces with a high albedo appear bright (e.g., snow reflects most radiation).
Albedo is an important concept in
climatology
Climatology (from Greek , ''klima'', "place, zone"; and , '' -logia'') or climate science is the scientific study of Earth's climate, typically defined as weather conditions averaged over a period of at least 30 years. This modern field of stud ...
,
astronomy
Astronomy () is a natural science that studies astronomical object, celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and chronology of the Universe, evolution. Objects of interest ...
, and environmental management (e.g., as part of the
Leadership in Energy and Environmental Design
Leadership in Energy and Environmental Design (LEED) is a
green building certification program used worldwide. Developed by the non-profit U.S. Green Building Council (USGBC), it includes a set of rating systems for the design, construction ...
(LEED) program for sustainable rating of buildings). The average albedo of the Earth from the upper atmosphere, its ''planetary albedo'', is 30–35% because of
cloud cover
Cloud cover (also known as cloudiness, cloudage, or cloud amount) refers to the fraction of the sky obscured by clouds on average when observed from a particular location. Okta is the usual unit for measurement of the cloud cover. The cloud co ...
, but widely varies locally across the surface because of different geological and environmental features.
The term albedo was introduced into optics by
Johann Heinrich Lambert
Johann Heinrich Lambert (, ''Jean-Henri Lambert'' in French; 26 or 28 August 1728 – 25 September 1777) was a polymath from the Republic of Mulhouse, generally referred to as either Swiss or French, who made important contributions to the subjec ...
in his 1760 work ''
Photometria
''Photometria'' is a book on the measurement of light by Johann Heinrich Lambert published in 1760.Lambert, Johann Heinrich, Photometria, sive de mensura et gradibus luminis, colorum et umbrae', Augsburg: Eberhard Klett, 1760. It established a com ...
''.
Terrestrial albedo
Any albedo in visible light falls within a range of about 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
A black body or blackbody is an idealized physical body that absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence. The name "black body" is given because it absorbs all colors of light. 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.
The average albedo of
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
is about 0.3.
This is far higher than for the ocean primarily because of the contribution of clouds.
![Ceres 2003 2004 clear sky total sky albedo](https://upload.wikimedia.org/wikipedia/commons/4/40/Ceres_2003_2004_clear_sky_total_sky_albedo.png)
Earth's surface albedo is regularly estimated via
Earth observation
Earth observation (EO) is the gathering of information about the physical, chemical, and biological systems of the planet Earth. It can be performed via remote-sensing technologies (Earth observation satellites) or through direct-contact sensors ...
satellite sensors such as
NASA
The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research.
NASA was established in 1958, succeeding t ...
's
MODIS
The Moderate Resolution Imaging Spectroradiometer (MODIS) is a satellite-based sensor used for earth and climate measurements. There are two MODIS sensors in Earth orbit: one on board the Terra (EOS AM) satellite, launched by NASA in 1999 ...
instruments on board the
Terra
Terra may often refer to:
* Terra (mythology), primeval Roman goddess
* An alternate name for planet Earth, as well as the Latin name for the planet
Terra may also refer to: Geography Astronomy
* Terra (satellite), a multi-national NASA scienti ...
and
Aqua
Aqua is the Latin word for water. It is used in many words which relate to water, such as aquatic life. In English, it may also refer to:
Arts
* Aqua (color), a greenish-blue color
Business
* Aqua (skyscraper), an 82-story residential skysc ...
satellites, and the CERES instrument on the
Suomi NPP
The Suomi National Polar-orbiting Partnership (Suomi NPP), previously known as the National Polar-orbiting Operational Environmental Satellite System Preparatory Project (NPP) and NPP-Bridge, is a weather satellite operated by the United States ...
and
JPSS
The Joint Polar Satellite System (JPSS) is the latest generation of U.S. polar-orbiting, non-geosynchronous, environmental satellites. JPSS will provide the global environmental data used in numerical weather prediction models for forecasts, and ...
. As the amount of reflected radiation is only measured for a single direction by satellite, not all directions, a mathematical model is used to translate a sample set of satellite reflectance measurements into estimates of
directional-hemispherical reflectance Directional-hemispherical reflectance is the reflectance of a surface under direct illumination (with no diffuse component). Directional-hemispherical reflectance is the integral of the bidirectional reflectance distribution function over all viewin ...
and bi-hemispherical reflectance (e.g.,
). These calculations are based on the
bidirectional reflectance distribution function
The bidirectional reflectance distribution function (BRDF; f_(\omega_,\, \omega_) ) is a function of four real variables that defines how light is reflected at an opaque surface. It is employed in the optics of real-world light, in computer ...
(BRDF), which describes how the reflectance of a given surface depends on the view angle of the observer and the solar angle. BDRF can facilitate translations of observations of reflectance into albedo.
Earth's average surface temperature due to its albedo and the
greenhouse effect
The greenhouse effect is a process that occurs when energy from a planet's host star goes through the planet's atmosphere and heats the planet's surface, but greenhouse gases in the atmosphere prevent some of the heat from returning directly ...
is currently about . If Earth were frozen entirely (and hence be more reflective), the average temperature of the planet would drop below .
If only the continental land masses became covered by glaciers, the mean temperature of the planet would drop to about .
In contrast, if the entire Earth was covered by water – a so-called
ocean planet
An ocean world, ocean planet, panthalassic planet, maritime world, water world or aquaplanet, is a type of planet that contains a substantial amount of water in form of oceans, either beneath the surface, as subsurface oceans, or on the surfa ...
– the average temperature on the planet would rise to almost .
In 2021, scientists reported that Earth dimmed by ~0.5% over two decades (1998–2017) as measured by earthshine using modern photometric techniques. This may have both been co-caused by
climate change
In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...
as well as a substantial increase in global warming. However, the link to climate change has not been explored to date and it is unclear whether or not this represents an ongoing trend.
White-sky, black-sky, and blue-sky albedo
For land surfaces, it has been shown that the albedo at a particular
solar zenith angle
The solar zenith angle is the zenith angle of the sun, i.e., the angle between the sun’s rays and the vertical direction. It is the complement to the solar altitude or solar elevation, which is the altitude angle or elevation angle between the s ...
''θ''
''i'' can be approximated by the proportionate sum of two terms:
* the
directional-hemispherical reflectance Directional-hemispherical reflectance is the reflectance of a surface under direct illumination (with no diffuse component). Directional-hemispherical reflectance is the integral of the bidirectional reflectance distribution function over all viewin ...
at that solar zenith angle,
, sometimes referred to as black-sky albedo, and
* the
bi-hemispherical reflectance Bi-hemispherical reflectance is the reflectance of a surface under diffuse illumination (with no direct component). Bi-hemispherical reflectance is an important part of the Bidirectional reflectance distribution function over all viewing and illumi ...
,
, sometimes referred to as white-sky albedo.
with
being the proportion of direct radiation from a given solar angle, and
being the proportion of diffuse illumination, the actual albedo
(also called blue-sky albedo) can then be given as:
:
This formula is important because it allows the albedo to be calculated for any given illumination conditions from a knowledge of the intrinsic properties of the surface.
Examples of terrestrial albedo effects
Illumination
Albedo is not directly dependent on illumination because changing the amount of incoming light proportionally changes the amount of reflected light, except in circumstances where a change in illumination induces a change in the Earth's surface at that location (e.g. through melting of reflective ice). That said, albedo and illumination both vary by latitude. Albedo is highest near the poles and lowest in the subtropics, with a local maximum in the tropics.
Insolation effects
The intensity of albedo temperature effects depends on the amount of albedo and the level of local
insolation
Solar irradiance is the power per unit area (surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument.
Solar irradiance is measured in watts per square metre (W/m ...
(solar irradiance); high albedo areas in the
Arctic
The Arctic ( or ) is a polar regions of Earth, polar region located at the northernmost part of Earth. The Arctic consists of the Arctic Ocean, adjacent seas, and parts of Canada (Yukon, Northwest Territories, Nunavut), Danish Realm (Greenla ...
and
Antarctic
The Antarctic ( or , American English also or ; commonly ) is a polar region around Earth's South Pole, opposite the Arctic region around the North Pole. The Antarctic comprises the continent of Antarctica, the Kerguelen Plateau and other ...
regions are cold due to low insolation, whereas areas such as the
Sahara Desert
, photo = Sahara real color.jpg
, photo_caption = The Sahara taken by Apollo 17 astronauts, 1972
, map =
, map_image =
, location =
, country =
, country1 =
, ...
, which also have a relatively high albedo, will be hotter due to high insolation.
Tropical
The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in
the Southern Hemisphere at S. The tropics are also referred to ...
and
sub-tropical
The subtropical zones or subtropics are geographical and climate zones to the north and south of the tropics. Geographically part of the temperate zones of both hemispheres, they cover the middle latitudes from to approximately 35° north and ...
rainforest
Rainforests are characterized by a closed and continuous tree canopy, moisture-dependent vegetation, the presence of epiphytes and lianas and the absence of wildfire. Rainforest can be classified as tropical rainforest or temperate rainfores ...
areas have low albedo, and are much hotter than their
temperate forest
A temperate forest is a forest found between the tropical and boreal regions, located in the temperate zone. It is the second largest biome on our planet, covering 25% of the world's forest area, only behind the boreal forest, which covers abou ...
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.
Arctic regions notably release more heat back into space than what they absorb, effectively cooling the
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...
. This has been a concern since arctic ice and
snow
Snow comprises individual ice crystals that grow while suspended in the atmosphere—usually within clouds—and then fall, accumulating on the ground where they undergo further changes.
It consists of frozen crystalline water throughout ...
has been melting at higher rates due to higher temperatures, creating regions in the arctic that are notably darker (being water or ground which is darker color) and reflects less heat back into space. This
feedback loop
Feedback occurs when outputs of a system are routed back as inputs as part of a chain of cause-and-effect that forms a circuit or loop. The system can then be said to ''feed back'' into itself. The notion of cause-and-effect has to be handled c ...
results in a reduced albedo effect.
Climate and weather
Albedo affects
climate
Climate is the long-term weather pattern in an area, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteorologic ...
by determining how much
radiation
In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes:
* ''electromagnetic radiation'', such as radio waves, microwaves, infrared, visi ...
a planet absorbs. The uneven heating of Earth from albedo variations between land, ice, or ocean surfaces can drive
weather
Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloudy. On Earth, most weather phenomena occur in the lowest layer of the planet's atmosphere, the ...
.
Albedo–temperature feedback
When an area's albedo changes due to snowfall, a snow–temperature
feedback
Feedback occurs when outputs of a system are routed back as inputs as part of a chain of cause-and-effect that forms a circuit or loop. The system can then be said to ''feed back'' into itself. The notion of cause-and-effect has to be handled ...
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 mass
In meteorology, an air mass is a volume of air defined by its temperature and humidity. Air masses cover many hundreds or thousands of square miles, and adapt to the characteristics of the surface below them. They are classified according to la ...
), the raised albedo and lower temperature would maintain the current snow and invite further snowfall, deepening the snow–temperature feedback. However, because local
weather
Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloudy. On Earth, most weather phenomena occur in the lowest layer of the planet's atmosphere, the ...
is dynamic due to the change of
season
A season is a division of the year based on changes in weather, ecology, and the number of daylight hours in a given region. On Earth, seasons are the result of the axial parallelism of Earth's tilted orbit around the Sun. In temperate and pol ...
s, eventually warm air masses and a more direct angle of sunlight (higher
insolation
Solar irradiance is the power per unit area (surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument.
Solar irradiance is measured in watts per square metre (W/m ...
) cause melting. When the melted area reveals surfaces with lower albedo, such as grass, soil, or ocean, 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.
Snow
Snow 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
Antarctica
Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest contine ...
snow albedo averages 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 feedback
Positive feedback (exacerbating feedback, self-reinforcing feedback) is a process that occurs in a feedback loop which exacerbates the effects of a small disturbance. That is, the effects of a perturbation on a system include an increase in the ...
).
Just as fresh snow has a higher albedo than does dirty snow, the albedo of snow-covered sea ice is far higher than that of sea water. Sea water absorbs more solar radiation than would the same surface covered with reflective snow. When sea ice melts, either due to a rise in sea temperature or in response to increased solar radiation from above, the snow-covered surface is reduced, and more surface of sea water is exposed, so the rate of energy absorption increases. The extra absorbed energy heats the sea water, which in turn increases the rate at which sea ice melts. As with the preceding example of snowmelt, the process of melting of sea ice is thus another example of a positive feedback. Both positive feedback loops have long been recognized as important for
global warming
In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...
.
Cryoconite
Cryoconite is powdery windblown dust made of a combination of small rock particles, soot and microbes which is deposited and builds up on snow, glaciers, or ice caps. The darkening, especially from small amounts of soot, absorbs solar radiation me ...
, powdery windblown
dust
Dust is made of fine particles of solid matter. On Earth, it generally consists of particles in the atmosphere that come from various sources such as soil lifted by wind (an aeolian process), volcanic eruptions, and pollution. Dust in homes ...
containing soot, sometimes reduces albedo on glaciers and ice sheets.
["Changing Greenland – Melt Zone"](_blank)
page 3, of 4, article by Mark Jenkins in ''National Geographic
''National Geographic'' (formerly the ''National Geographic Magazine'', sometimes branded as NAT GEO) is a popular American monthly magazine published by National Geographic Partners. Known for its photojournalism, it is one of the most widely ...
'' June 2010, accessed 8 July 2010
The dynamical nature of albedo in response to positive feedback, together with the effects of small errors in the measurement of albedo, can lead to large errors in energy estimates. Because of this, in order to reduce the error of energy estimates, it is important to measure the albedo of snow-covered areas through remote sensing techniques rather than applying a single value for albedo over broad regions.
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.
Solar photovoltaic effects
Albedo can affect the
electrical energy
Electrical energy is energy related to forces on electrically charged particles and the movement of electrically charged particles (often electrons in wires, but not always). This energy is supplied by the combination of electric current and electr ...
output of solar
photovoltaic devices. For example, the effects of a spectrally responsive albedo are illustrated by the differences between the spectrally weighted albedo of solar photovoltaic 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% for vertically (90°) mounted systems, but such effects were substantially lower for systems with lower surface tilts. Spectral albedo strongly affects the performance of
bifacial solar cells
A bifacial solar cell (BSC) is a photovoltaic solar cell that can produce electrical energy when illuminated on both its surfaces, front or rear. Instead, monofacial solar cells only produce electrical energy when photons impinge on their front sid ...
where rear surface performance gains of over 20% have been observed for c-Si cells installed above healthy vegetation. An analysis on the bias due to the specular reflectivity of 22 commonly occurring surface materials (both human-made and natural) provided effective albedo values for simulating the performance of seven photovoltaic materials mounted on three common photovoltaic system topologies: industrial (solar farms), commercial flat rooftops and residential pitched-roof applications.
Trees
Because forests generally have a low albedo, (the majority of the ultraviolet and
visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visual perception, visible to the human eye. Electromagnetic radiation in this range of wavelengths is called ''visible light'' or simply light. A typical human eye wil ...
is absorbed through
photosynthesis
Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...
), some scientists have suggested that greater heat absorption by trees could offset some of the carbon benefits of
afforestation
Afforestation is the establishment of a forest or stand of trees (forestation) in an area where there was no previous tree cover. Many government and non-governmental organizations directly engage in afforestation programs to create forests a ...
(or offset the negative climate impacts of
deforestation
Deforestation or forest clearance is the removal of a forest or stand of trees from land that is then converted to non-forest use. Deforestation can involve conversion of forest land to farms, ranches, or urban use. The most concentrated d ...
). 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
evapotranspiration
Evapotranspiration (ET) is the combined processes by which water moves from the earth’s surface into the atmosphere. It covers both water evaporation (movement of water to the air directly from soil, canopies, and water bodies) and transpi ...
. The water vapor causes cooling on the land surface, causes heating where it condenses, acts a strong greenhouse gas, and can increase albedo 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 trees
In the fields of horticulture and Botany, the term ''deciduous'' () means "falling off at maturity" and "tending to fall off", in reference to trees and shrubs that seasonally shed leaves, usually in the autumn; to the shedding of petals, aft ...
have an albedo value of about 0.15 to 0.18 whereas
coniferous trees
Conifers are a group of cone-bearing seed plants, a subset of gymnosperms. Scientifically, they make up the division Pinophyta (), also known as Coniferophyta () or Coniferae. The division contains a single extant class, Pinopsida. All extant ...
have a value of about 0.09 to 0.15.
Variation in summer albedo across both forest types is associated with maximum rates of photosynthesis because plants with high growth capacity display a greater fraction of their foliage for direct interception of incoming radiation in the upper canopy.
The result is that wavelengths of light not used in photosynthesis are more likely to be reflected back to space rather than being absorbed by other surfaces lower in the canopy.
Studies by the
Hadley Centre
The Met Office Hadley Centre — named in honour of George Hadley — is one of the United Kingdom's leading centres for the study of scientific issues associated with climate change. It is part of, and based at the headquarters of the Met Off ...
have investigated the relative (generally warming) effect of albedo change and (cooling) effect of
carbon sequestration
Carbon sequestration is the process of storing carbon in a carbon pool. Carbon dioxide () is naturally captured from the atmosphere through biological, chemical, and physical processes. These changes can be accelerated through changes in land ...
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.
Water
Water reflects light very differently from typical terrestrial materials. The reflectivity of a water surface is calculated using the
Fresnel equations
The Fresnel equations (or Fresnel coefficients) describe the reflection and transmission of light (or electromagnetic radiation in general) when incident on an interface between different optical media. They were deduced by Augustin-Jean Fresne ...
.
At the scale of the wavelength of light even wavy water is always smooth so the light is reflected in a locally
specular manner (not
diffusely). The glint of light off water is a commonplace effect of this. At small
angles of incident light,
waviness
Waviness is the measurement 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". It can ...
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
Terminator may refer to:
Science and technology
Genetics
* Terminator (genetics), the end of a gene for transcription
* Terminator technology, proposed methods for restricting the use of genetically modified plants by causing second generation s ...
(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.
Snow on top of this sea ice increases the albedo to 0.9.
Clouds
Cloud albedo
Cloud albedo is a measure of the albedo or reflectivity of a cloud. Clouds regulate the amount of solar radiation absorbed by a planet and its solar surface irradiance. Generally, increased cloud cover correlates to a higher albedo and a lower ab ...
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
Contrails (; short for "condensation trails") or vapor trails are line-shaped clouds produced by aircraft engine exhaust or changes in air pressure, typically at aircraft cruising altitudes several miles above the Earth's surface. Contrails ar ...
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 colder than temperatures several miles away under clear skies.
Aerosol effects
Aerosols
An aerosol is a suspension of fine solid particles or liquid droplets in air or another gas. Aerosols can be natural or anthropogenic. Examples of natural aerosols are fog or mist, dust, forest exudates, and geyser steam. Examples of anthrop ...
(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 nuclei
Cloud condensation nuclei (CCNs), also known as cloud seeds, are small particles typically 0.2 µm, or one hundredth the size of a cloud droplet. CCNs are a unique subset of aerosols in the atmosphere on which water vapour condenses. This c ...
and thereby change cloud properties) is less certain.
As per Spracklen et al.
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 (CCNs), also known as cloud seeds, are small particles typically 0.2 µm, or one hundredth the size of a cloud droplet. CCNs are a unique subset of aerosols in the atmosphere on which water vapour condenses. This c ...
. 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'').
In extremely polluted cities like
Delhi
Delhi, officially the National Capital Territory (NCT) of Delhi, is a city and a union territory of India containing New Delhi, the capital of India. Straddling the Yamuna river, primarily its western or right bank, Delhi shares borders w ...
, aerosol pollutants influence local weather and induce an
urban cool island effect during the day.
Black carbon
Another albedo-related effect on the climate is from
black carbon
Chemically, black carbon (BC) is a component of fine particulate matter (PM ≤ 2.5 µm in aerodynamic diameter). Black carbon consists of pure carbon in several linked forms. It is formed through the incomplete combustion of fossil fue ...
particles. The size of this effect is difficult to quantify: the
Intergovernmental Panel on Climate Change
The Intergovernmental Panel on Climate Change (IPCC) is an intergovernmental body of the United Nations. Its job is to advance scientific knowledge about climate change caused by human activities. The World Meteorological Organization (WMO) a ...
estimates that the global mean
radiative forcing
Radiative forcing (or climate forcing) is the change in energy flux in the atmosphere caused by natural or anthropogenic factors of climate change as measured by watts / metre2. It is a scientific concept used to quantify and compare the external ...
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 activities
![(Almería) Greenhouses of Almería, Spain (cropped)](https://upload.wikimedia.org/wikipedia/commons/9/90/%28Almer%C3%ADa%29_Greenhouses_of_Almer%C3%ADa%2C_Spain_%28cropped%29.jpg)
Human activities (e.g., deforestation, farming, and urbanization) change the albedo of various areas around the globe. As per Campra et al.,
human impacts to "the physical properties of the land surface can perturb the climate by altering the Earth’s radiative energy balance" even on a small scale or when undetected by satellites.
The tens of thousands of
hectares
The hectare (; SI symbol: ha) is a non-SI metric unit of area equal to a square with 100-metre sides (1 hm2), or 10,000 m2, and is primarily used in the measurement of land. There are 100 hectares in one square kilometre. An acre is ab ...
of greenhouses in
Almería, Spain form a large expanse of whitened plastic roofs. A 2008 study found that this anthropogenic change lowered the local surface area temperature of the high-albedo area, although changes were localized.
A follow-up study found that "CO2-eq. emissions associated to changes in surface albedo are a consequence of land transformation" and can reduce surface temperature increases associated with climate change.
It has been found that
urbanization
Urbanization (or urbanisation) refers to the population shift from rural to urban areas, the corresponding decrease in the proportion of people living in rural areas, and the ways in which societies adapt to this change. It is predominantly t ...
generally decreases albedo (commonly being 0.01–0.02 lower than adjacent
croplands
A farm (also called an agricultural holding) is an area of land that is devoted primarily to agricultural processes with the primary objective of producing food and other crops; it is the basic facility in food production. The name is used fo ...
), which contributes to
global warming
In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...
. Deliberately increasing albedo in urban areas can mitigate
urban heat island
An urban heat island (UHI) is an urban or metropolitan area that is significantly warmer than its surrounding rural areas due to human activities. The temperature difference is usually larger at night than during the day, and is most apparen ...
. Ouyang et al. estimated that, on a global scale, "an albedo increase of 0.1 in worldwide urban areas would result in a cooling effect that is equivalent to absorbing ~44
Gt of CO
2 emissions."
Intentionally enhancing the albedo of the Earth's surface, along with its daytime
thermal emittance Thermal emittance or thermal emissivity (\varepsilon) is the ratio of the radiant emittance of heat of a specific object or surface to that of a standard black body. Emissivity and emittivity are both dimensionless quantities given in the range of 0 ...
, has been proposed as a
solar radiation management
Solar geoengineering, or solar radiation modification (SRM), is a type of climate engineering in which sunlight (solar radiation) would be reflected back to outer space to limit or reverse human-caused climate change. It is not a substitute for ...
strategy to mitigate
energy crises
An energy crisis or energy shortage is any significant bottleneck in the supply of energy resources to an economy. In literature, it often refers to one of the energy sources used at a certain time and place, in particular, those that supply n ...
and global warming known as
passive daytime radiative cooling
Passive daytime radiative cooling (PDRC) is a renewable cooling method proposed as a solution to global warming of enhancing terrestrial heat flow to outer space through the installation of thermally-emissive surfaces on Earth that require zero ...
(PDRC).
Efforts toward widespread implementation of PDRCs may focus on maximizing the albedo of surfaces from very low to high values, so long as a thermal emittance of at least 90% can be achieved.
Astronomical albedo
In astronomy, the term albedo can be defined in several different ways, depending upon the application and the wavelength of electromagnetic radiation involved.
Optical or visual albedo
The albedos of
planet
A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a you ...
s,
satellites
A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioisotop ...
and
minor planet
According to the International Astronomical Union (IAU), a minor planet is an astronomical object in direct orbit around the Sun that is exclusively classified as neither a planet nor a comet. Before 2006, the IAU officially used the term ''minor ...
s such as
asteroid
An asteroid is a minor planet of the inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic or icy bodies with no atmosphere.
...
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 composes a major part of the astronomical field of
photometry Photometry can refer to:
* Photometry (optics), the science of measurement of visible light in terms of its perceived brightness to human vision
* Photometry (astronomy), the measurement of the flux or intensity of an astronomical object's electrom ...
. 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
The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...
objects, the variation of albedo with phase angle gives information about
regolith
Regolith () is a blanket of unconsolidated, loose, heterogeneous superficial deposits covering solid rock. It includes dust, broken rocks, and other related materials and is present on Earth, the Moon, Mars, some asteroids, and other terrestria ...
properties, whereas unusually high radar albedo is indicative of high metal content in
asteroid
An asteroid is a minor planet of the inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic or icy bodies with no atmosphere.
...
s.
Enceladus
Enceladus is the sixth-largest moon of Saturn (19th largest in the Solar System). It is about in diameter, about a tenth of that of Saturn's largest moon, Titan. Enceladus is mostly covered by fresh, clean ice, making it one of the most refl ...
, a moon of Saturn, has one of the highest known optical albedos of any body in the Solar System, with an albedo of 0.99. Another notable high-albedo body is
Eris, with an albedo of 0.96.
[
] Many small objects in the outer Solar System
and
asteroid belt
The asteroid belt is a torus-shaped region in the Solar System, located roughly between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies, of many sizes, but much smaller than planets, called ...
have low albedos down to about 0.05.
A typical
comet nucleus
The nucleus is the solid, central part of a comet, once termed a ''dirty snowball'' or an ''icy dirtball''. A cometary nucleus is composed of Rock (geology), rock, dust, and frozen gases. When heated by the Sun, the gases Sublimation (physics), ...
has an albedo of 0.04.
Such a dark surface is thought to be indicative of a primitive and heavily
space weathered surface containing some
organic compound
In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. The ...
s.
The overall albedo of the
Moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
is measured to be around 0.14,
[
] but it is strongly directional and non-
Lambertian, displaying also a strong
opposition effect
The opposition surge (sometimes known as the opposition effect, opposition spike or Seeliger effect) is the brightening of a rough surface, or an object with many particles, when illuminated from directly behind the observer. The term is most wid ...
.
Although such reflectance properties are different from those of any terrestrial terrains, they are typical of the
regolith
Regolith () is a blanket of unconsolidated, loose, heterogeneous superficial deposits covering solid rock. It includes dust, broken rocks, and other related materials and is present on Earth, the Moon, Mars, some asteroids, and other terrestria ...
surfaces of airless Solar System bodies.
Two common optical albedos that are used in astronomy are the (V-band)
geometric albedo
In astronomy, the geometric albedo of a celestial body is the ratio of its actual brightness as seen from the light source (i.e. at zero phase angle) to that of an ''idealized'' flat, fully reflecting, diffusively scattering ( Lambertian) disk w ...
(measuring brightness when illumination comes from directly behind the observer) and the
Bond albedo
The Bond albedo (or ''spheric albedo'' or ''planetary albedo'' or ''bolometric albedo''), named after the American astronomer George Phillips Bond (1825–1865), who originally proposed it, is the fraction of power in the total electromagnetic rad ...
(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 The Hapke parameters are a set of parameters for an empirical model that is commonly used to describe the directional reflectance properties of the airless regolith surfaces of bodies in the Solar System. The model has been developed by astronomer ...
which semi-empirically describe the variation of albedo with
phase angle, including a characterization of the opposition effect of
regolith
Regolith () is a blanket of unconsolidated, loose, heterogeneous superficial deposits covering solid rock. It includes dust, broken rocks, and other related materials and is present on Earth, the Moon, Mars, some asteroids, and other terrestria ...
surfaces. One of these five parameters is yet another type of albedo called the
single-scattering albedo Single-scattering albedo is the ratio of scattering efficiency to total extinction efficiency (which is also termed "attenuance", a sum of scattering and absorption). Most often it is defined for small-particle scattering of electromagnetic waves. ...
. It is used to define scattering of electromagnetic waves on small particles. It depends on properties of the material (
refractive index
In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium.
The refractive index determines how much the path of light is bent, or ...
), the size of the particle, and the wavelength of the incoming radiation.
An important relationship between an object's astronomical (geometric) albedo,
absolute magnitude
Absolute magnitude () is a measure of the luminosity of a celestial object on an inverse Logarithmic scale, logarithmic Magnitude (astronomy), astronomical magnitude scale. An object's absolute magnitude is defined to be equal to the apparent mag ...
and diameter is given by:
where
is the astronomical albedo,
is the diameter in kilometers, and
is the absolute magnitude.
Radar albedo
In planetary
radar astronomy
Radar astronomy is a technique of observing nearby astronomical objects by reflecting radio waves or microwaves off target objects and analyzing their reflections. Radar astronomy differs from ''radio astronomy'' in that the latter is a passive o ...
, a microwave (or radar) pulse is transmitted toward a planetary target (e.g. Moon, asteroid, etc.) and the echo from the target is measured. In most instances, the transmitted pulse is
circularly polarized
In electrodynamics, circular polarization of an electromagnetic wave is a polarization state in which, at each point, the electromagnetic field of the wave has a constant magnitude and is rotating at a constant rate in a plane perpendicular to t ...
and the received pulse is measured in the same sense of polarization as the transmitted pulse (SC) and the opposite sense (OC).
The echo power is measured in terms of
radar cross-section
Radar cross-section (RCS), also called radar signature, is a measure of how detectable an object is by radar. A larger RCS indicates that an object is more easily detected.
An object reflects a limited amount of radar energy back to the source. ...
,
,
, or
(total power, SC + OC) and is equal to the cross-sectional area of a metallic sphere (perfect reflector) at the same distance as the target that would return the same echo power.
Those components of the received echo that return from first-surface reflections (as from a smooth or mirror-like surface) are dominated by the OC component as there is a reversal in polarization upon reflection. If the surface is rough at the wavelength scale or there is significant penetration into the regolith, there will be a significant SC component in the echo caused by multiple scattering.
For most objects in the solar system, the OC echo dominates and the most commonly reported radar albedo parameter is the (normalized) OC radar albedo (often shortened to radar albedo):
where the denominator is the effective cross-sectional area of the target object with mean radius,
. A smooth metallic sphere would have
.
Radar albedos of Solar System objects
The values reported for the Moon, Mercury, Mars, Venus, and Comet P/2005 JQ5 are derived from the total (OC+SC) radar albedo reported in those references.
Relationship to surface
bulk density Bulk density, also called apparent density or volumetric density, is a property of powders, granules, and other "divided" solids, especially used in reference to mineral components (soil, gravel), chemical substances, (pharmaceutical) ingredients ...
In the event that most of the echo is from first surface reflections (
or so), the OC radar albedo is a first-order approximation of the Fresnel reflection coefficient (aka reflectivity)
and can be used to estimate the bulk density of a planetary surface to a depth of a meter or so (a few wavelengths of the radar wavelength which is typically at the decimeter scale) using the following empirical relationships:
:
:
.
See also
*
Cool roof
Reflective surfaces or ground-based albedo modification (GBAM) is a solar radiation management method of enhancing the Earth's albedo (the ability to reflect the visible, infrared and ultraviolet wavelengths of the sun, reducing heat transfer to t ...
*
Daisyworld
Daisyworld, a computer simulation, is a hypothetical world orbiting a star whose radiant energy is slowly increasing or decreasing. It is meant to mimic important elements of the Earth-Sun system, and was introduced by James Lovelock and And ...
*
Emissivity
The emissivity of the surface of a material is its effectiveness in emitting energy as thermal radiation. Thermal radiation is electromagnetic radiation that most commonly includes both visible radiation (light) and infrared radiation, which is n ...
*
Exitance
In radiometry, radiant exitance or radiant emittance is the radiant flux emitted by a surface per unit area, whereas spectral exitance or spectral emittance is the radiant exitance of a surface per unit frequency or wavelength, depending on whether ...
*
Global dimming
Global dimming is the reduction in the amount of global direct irradiance at the Earth's surface that has been observed since systematic measurements began in the 1950s. The effect varies by location, but worldwide it has been estimated to be of ...
*
Irradiance In radiometry, irradiance is the radiant flux ''received'' by a ''surface'' per unit area. The SI unit of irradiance is the watt per square metre (W⋅m−2). The CGS unit erg per square centimetre per second (erg⋅cm−2⋅s−1) is often used ...
*
Kirchhoff's law of thermal radiation
In heat transfer, Kirchhoff's law of thermal radiation refers to wavelength-specific radiative emission and absorption by a material body in thermodynamic equilibrium, including radiative exchange equilibrium. It is a special case of Onsage ...
*
Opposition surge
The opposition surge (sometimes known as the opposition effect, opposition spike or Seeliger effect) is the brightening of a rough surface, or an object with many particles, when illuminated from directly behind the observer. The term is most wid ...
*
Polar see-saw
The polar see-saw (also: bipolar seesaw) is the phenomenon that temperature changes in the northern and southern hemispheres may be out of phase. The hypothesis states that large changes, for example when the glaciers are intensely growing or depl ...
*
Radar astronomy
Radar astronomy is a technique of observing nearby astronomical objects by reflecting radio waves or microwaves off target objects and analyzing their reflections. Radar astronomy differs from ''radio astronomy'' in that the latter is a passive o ...
*
Solar radiation management
Solar geoengineering, or solar radiation modification (SRM), is a type of climate engineering in which sunlight (solar radiation) would be reflected back to outer space to limit or reverse human-caused climate change. It is not a substitute for ...
References
External links
Albedo ProjectAlbedo – Encyclopedia of EarthOcean surface albedo look-up-tableSurface albedo derived from Meteosat observationsreflectivity of metals (chart)
{{Authority control
Land surface effects on climate
Climate change feedbacks
Climate forcing
Climatology
Electromagnetic radiation
Radiometry
Scattering, absorption and radiative transfer (optics)
Radiation
1760s neologisms