Albedo is the fraction of Sun’s radiation reflected from a surface. The term has its origins from the Latin word albus, meaning “white”. It is quantified as the proportion, or percentage of solar radiation of all wavelengths reflected by a body or surface to the amount incident upon it. An ideal white body has an albedo of 100% and an ideal black body, 0%. Visually we can estimate the albedo of an object’s surface from its tone or color. This method suggests that albedo becomes higher as an object gets lighter in shade. The data in Table 1 verifies this fact. Light toned surfaces like snow do have high albedos. Low albedos are associated with surfaces that appear dark colored to our eyes. Some dark colored surfaces include black top roads, coniferous forest, and dark soil. Table 1 also indicates that the albedo of water varies with Sun angle. When Sun angles are high, water tends to absorb more than 95% of the insolation falling on it. At low Sun angles, the surface of water becomes much more reflective.
On average the Earth and its atmosphere typically reflect about 4% and 26%, respectively, of the Sun’s incoming radiation back to space over the course of one year. As a result, the earth-atmosphere system has a combined albedo of about 30%, a value that is dependent on a number of factors including soil type, vegetation cover, and cloud distribution.
The reflectance of locations on the Earth's surface exhibit large geographic variation. Mean annual albedo values differ considerably between the equator and the poles, largely due to the presence of snow and ice-covered surfaces. As the characteristics of a surface change from one season to another, so do its reflectance properties. This fact is most evident throughout the high latitudes, where snow cover and ice extent reach maximum values during the cold seasons, significantly increasing the surface reflectance values. Melting in the spring exposes bare soils that absorb a significantly greater portion of the incoming solar radiation, decreasing the albedo values.
Global measurements of the Earth’s surface albedo can be determined with the aid of sensors aboard orbiting space satellites. NASA’s Earth Radiation Budget Experiment (ERBE) was one of the first attempts of making such measurements. This experiment used a variety of satellite sensors aboard Nimbus-7, NOAA-9, and the Earth Radiation Budget Satellite (ERBS) to monitor the Earth’s albedo for a period of about four years. Figures 1 and 2 show the monthly average surface albedo of the Earth for January and July, 1987. In these figures, most of the reflective properties of the atmosphere have been removed. The patterns seen here are probably representative for most other years. For both January and July, the lowest surface albedos occur over oceans in a zone that covers more than 100 degrees of latitude. Albedo values of this zone are between 8 and 13%, and the center of this zone shifts seasonally. In July, the low albedo zone is located approximately at the Tropic of Cancer (23.5°N), while in January it migrates to the Tropic of Capricorn (23.5°S). At the higher latitudes, the albedo of the ocean surface increases significantly because of low Sun angles or the presence of sea ice. In the July image, the region occupied by the Arctic Ocean has an albedo between 45 to 60%. On the Earth’s terrestrial surface, vegetated areas have an albedo from 15 to 25%. Non-vegetated regions like the Sahara Desert reflect about 30 to 40% of the Sun’s incoming light. Other land surfaces with high albedos are glaciers and seasonal snowfields. The large glaciers covering Greenland and Antarctica reflect as much as 75% of the insolation falling on their surfaces. Comparing the January and July images, we can see that the albedos of areas with a latitude greater than 45°N vary annually because of seasonal snowfall. In these areas, summer albedos typically are around 20%, while winter values jump to as high as 70%.
Figure 1. Surface reflectivity of the Earth for January 1987. Cells with missing data are colored white. Measured by sensors aboard a variety of satellites for NASA’s Earth Radiation Budget Experiment (ERBE). (Image Source: NASA - Earth Radiation Budget Experiment).
Figure 2. Surface reflectivity of the Earth for January and July 1987. Cells with missing data are colored white. Measured by sensors aboard a variety of satellites for NASA’s Earth Radiation Budget Experiment (ERBE). (Image Source: NASA - Earth Radiation Budget Experiment).
Figures 3 and 4 describe measurements of combined surface and atmosphere albedo for our planet. Comparing these figures to Figures 1 and 2 illustrates the large effect that clouds have on reflecting incoming sunlight back to space. Significant bands of reflective cloud exist over at the equator and in the mid-latitudes. Skies are generally clear of cloud over the major deserts, subtropical oceans, and the large continental glaciers of Greenland and Antarctica.
Figure 3. Combined surface and atmosphere reflectivity (or planetary albedo) of the Earth for January 1987. Cells with missing data are colored white. Measured by sensors aboard a variety of satellites for NASA’s Earth Radiation Budget Experiment (ERBE). (Image Source: NASA - Earth Radiation Budget Experiment).
Figure 4. Combined surface and atmosphere reflectivity (or planetary albedo) of the Earth for July 1987. Cells with missing data are colored white. Measured by sensors aboard a variety of satellites for NASA’s Earth Radiation Budget Experiment (ERBE). (Image Source: NASA - Earth Radiation Budget Experiment).
The proportion of absorbed, emitted, and reflected incoming solar radiation steers the Earth's climate system causing fluctuations in temperature, winds, ocean currents, and precipitation. The climate system remains in equilibrium as long as the amount of absorbed solar radiation is in balance with the amount of terrestrial radiation emitted back to space. Earth's albedo values are very important in shaping local and global climates through the radiation budget, determined as the difference between the amount of absorbed shortwave radiation (input) and the outgoing longwave radiation (output). For instance, clouds control the amount of energy that may reach the Earth’s surface. Since mean cloudiness varies geographically with lowest values observed in the subtropics and highest values in the mid- to high-latitudes, the variation of surface reflectance has a significant impact on the distribution of absorbed solar radiation at the surface. Approximately half of the incident solar energy is absorbed by the Earth's surface. This energy is then used to heat the land and oceans and drive the hydrologic cycle.
Terrestrial factors affecting albedo
A variety of factors affect terrestrial albedo including: (a) soil type; (b) soil moisture or icing; (c) vegetation types; (d) soil and vegetative color; (e) micro-topography and (f) macro-topography.
Soil color certainly affects reflectivity, with lighter colors having greater albedo than dark colors, and hence exhibit higher albedo. Soil texture is also a factor that affects albedo. Some studies have shown that sandy soils have higher albedo, and data clearly demonstrate that albedo is strongly affected by mineral salt content including sodium chloride and magnesium chloride.
A variety of factors influence the ability of plants to reflect sunlight. At the most simplistic level, dark coloration provides the greatest absorbtion and hence the lowest albedo. However, leaf shape is quite important, with leaf shapes that are planar providing a higher reflectivity; this effect explains why conifer forests tend to have lower albedo than angiosperm or broadleaf forests. Furthermore, leaf aspect is also contributory, with leaves that have surfaces parallel to the ground surface having the highest albedo.
Macro-topography implies the recognition of overt slope differences; for example, areas of steep slope can be expected to produce lower effective albedo, simply because the angle of reflection forces incoming radiation to endure a subsequent path that is subject to further absorption by secondary incidence and also due to a longer path length of travel for reflected electromagnetic waves. Micro-topography is the presence of dimpled soils which have small crevices and indentations. In these cases there is a similar reduction in albedo where opportunities for multiple reflections from surface complexities exist.
Measurement of albedo
Surface reflectance has been derived through the use of satellites and remote sensing technology. The International Satellite Cloud Climatology Project (ISCCP) established as part of the World Climate Research Programme (WCRP) has been collecting surface and atmospheric reflectance data since 1983. A traditional technique for estimating the Earth's albedo is observation of the moon's ashgrey light—earthlight reflected from its dark hemisphere.
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