Encyclopedia
Albedo is a ratio of scattered to incident
electromagnetic radiation power, most commonly light. It is a unitless measure of a surface or body's reflectivity. The word is derived from
albus, a
Latin word for "white".
The albedo is an important concept particularly in climatology and
astronomy. In climatology it is sometimes expressed as a percentage from 0% to 100%. 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. An exception are Lambertian surfaces, which scatter radiation equally in all directions, so their albedo does not depend on the incoming distribution. In realistic cases, a bidirectional reflectance distribution function is required to characterise the scattering properties of a surface accurately, although albedos are a very useful first approximation.
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. An exception are deeply shadowed cavities whose effective albedo may approach 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 10 to 40% .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 of various areas around the globe. However, quantification of this effect is difficult on the global scale: it is not clear whether the changes have tended to increase or decrease
global warming.
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.
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 can reasonably be approximated by the proportionate sum of two terms: the directional-hemispherical reflectance at that solar zenith angle, , and the bi-hemispherical reflectance, the proportion concerned being defined as the proportion of diffuse illumination .
Albedo can then be given as:
Directional-hemispherical reflectance is sometimes referred to as black-sky albedo and bi-hemispherical reflectance as white sky albedo, for obvious reasons. These terms are important because they allow the albedo to be calculated for any given illumination conditions from knowledge of intrinsic properties of the surface.
Astronomical albedo
The albedo of planets, satellites and asteroids can be used to infer much about their properties. The study of albedos, their dependence on wavelength, lighting 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 the highest known albedo of any body in the solar system, with 99% of EM radiation reflected, while many objects in the outer solar system and
asteroid belt have low albedos down to about 0.05. Such a dark surface is thought to be indicative of a primitive and heavily
space weathered surface.
The overall albedo of the
Moon is around 12%, but it is strongly directional and non-Lambertian, displaying also a strong opposition effect . Such reflectance properties of moon regolith are different to those of any terrestrial terrains, but common on airless rocky solar system bodies.
Two common definitions of astronomical albedos are the geometric albedo and the Bond albedo, and their values can differ significantly — a common source of confusion.
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 warmer than the airport at Fairbanks, partly because of drainage patterns but also largely because of the lower albedo at the college resulting from a higher concentration of
pine trees and therefore less open snowy ground to reflect the heat back into space. Neunke and Kukla have shown that this difference is especially marked during the late
winter months, when
solar radiation is greater.
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
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 appears to increase by an average of about 3 °C year-round, which is a significant amount.
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 white clothes.
Pine forests
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 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%.
Deciduous trees average about 13%. A
grassy field usually comes in at about 20%. 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. .
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 cool the planet. 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.
Studies by the Hadley Centre have investigated the relative effect of albedo change and 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 were neutral or perhaps warming .
Snow
Snow albedos can be as high as 90%. This is for the ideal example, however: 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 . This is the basis for predictions of enhanced warming in the polar and seasonally snow covered regions as a result of
global warming.
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.
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 September 11 attacks, after which all major airlines in the U.S. shut down for three days, showed a local 1 °C increase in the daily temperature range .
Aerosol effects
Aerosol has two effects, direct and indirect. The direct effect is generally to cool the planet; the indirect effect is less certain .
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 ]]) with a range +0.1 to +0.4 W m
-2". .
References in popular culture
Literature
In "Hand of Chaos" the fifth book in the Deathgate Cycle by Margaret Weis and Tracy Hickman, the Elven wizards known as Weesham are based out of an Albedo Cathedral. In the series "Albedo" has come to denote the light of Elven souls reflecting back to their people.
Music
Greek musician
Vangelis recorded an album called
Albedo 0.39, referring to average albedo of Earth.
References
External links
