Albedo

albedo originates from a Latin word albus, which means white. Albedo is the amount of sunlight (of all wavelengths) that is reflected back from an object or a substance. The more the amount of light reflected back, the brighter the color of the object. A lesser amount of light is reflected back from darker objects. The albedo of an object varies from 0-1. Black objects have zero albedo, while white objects have an albedo of one. Sometimes it is also expressed in terms of percentage, 1-100. An ideal white body thus has an albedo of 100 percent, while an ideal black body has an albedo of zero percent. Some standard amounts of sunlight reflected from certain objects are shown in Table 1.

Usually, albedo is used in the field of astronomy to describe reflective properties of planets, satellites, and asteroids. There are two types of astronomical albedo: normal and bond albedo. Normal albedo is a measure of a surface's brightness when illuminated and observed vertically, while bond albedo is defined as the fraction of total solar light reflected back to space and is a measure of a planet's energy balance. The value of bond is defined over the entire range of wavelengths.

Surface reflectance values vary across the globe, mainly because of variation in presence or absence of snow, ice, or clouds, which increases albedo values in those areas. The presence of ice and snow, for example, on poles and the absence of snow and ice on the equator reflects the difference in albedo values at the poles and equator. It is interesting to note that the reflectance value (and hence the albedo value) changes with the change in dust concentration, thickness of the clouds (or amount of cloud cover) and zenith of sunlight falling in that zone, which is also reflected in seasonal variation in albedo value for the same region. This can be observed best at higher altitudes, where in winter the surface is covered significantly by snow (or ice), thus increasing the surface reflectance values, while in spring, when most of this snow (or ice) melts, the surface (bare soil) absorbs a lot more sunlight, thus decreasing the albedo values for the same place.

Table 1: Reflectivity values of various surfaces

Surface

Details

Albedo

Soil

Dark and wet

0.05

Light and dry

0.40

Sand

0.15-0.45

Grass

Long Short

0.16 0.26

Crops

0.18-0.25

Tundra

0.18-0.25

Forests

Deciduous

0.15-0.20

Coniferous

0.05-0.15

Water

Small zenith angle Large zenith angle

0.03-0.10 0.10-1.0

Snow

Fresh

0.40 0.95

Ice

Sea

0.30-0.45

Glacier

0.20-0.40

Clouds

Thick Thin

0.60-0.90 0.30-0.50

Sources: Oke; Ahrens.

Sources: Oke; Ahrens.

Albedo is an important concept in climatology. When albedo is expressed in percentages, snow has an albedo of 90 percent and charcoal has an albedo of 4 percent. When seen from a distance, the ocean surface has a low albedo as do most forests, while desert has one of the higher albedo values.

The role of the concept of albedo in climate change can be seen in the following example: ice reflects back more sun radiation than water; with the snow cover getting smaller and the water in lakes (and seas and oceans) increasing, the amount of sunlight absorbed (and, hence, heat retained) is increasing, leading to further increases in the temperature of lake, sea, and ocean water. On the other hand, if more snow is formed, a cooling cycle starts. The amount of sunlight (radiation) absorbed or reflected back causes fluctuations in temperature, wind, ocean currents, and precipitation. In a way, the hydrological cycle changes with the fluctuations in temperature (which is related to how much evapo-transpiration takes place). Also, the climate system equilibrium is dependent on the balance between the amount of solar radiation absorbed and the amount of terrestrial radiation emitted back to space.

Thus, Earth's albedo values are important in shaping both local and global climate through their radiation budget (difference between the amount of absorbed short-wave radiation and outgoing longwave radiation). For example, clouds have an impact on the amount of energy (sunlight/radiation) that reaches the Earth's surface. Because cloudiness varies geographically, with the lowest values of cloudiness observed in the subtropics and the highest values observed in mid- to high-latitudes, this has an impact on surface reflectance globally. The variation in surface reflectance determines how much of the sunlight is absorbed or reflected back. Approximately half of the solar energy is absorbed by the Earth's surface, which causes evapo-transpiration and precipitation, and thus, impacts the hydrologic cycle as well.

Table 1 shows albedo values for different land-forms and covers. Land areas have a higher albedo value than oceans, mainly due to cloud contributions over land. Human activities such as clearing of forests for farming, human settlements, urbanization, and industrialization have changed the landscape and, in turn, have changed the albedo value of different places. Since forests/trees have a low albedo, removal of forests/trees tends to increase the albedo, thereby cooling the planet. In winter, in snow-covered areas, the albedo of treeless areas is about 10 to 50 percent higher than forested areas (where snow does not cover the trees that readily). Studies have also shown that new forests in tropical or mid-latitude areas tend to be cooler, while new forests in higher latitudes are more neutral or may be warming. Scientists have also focused on planting forests and carbon sequestration and its use in dealing with climate change.

Urbanization, especially, has led to changes in natural albedo values, because many human-built structures absorb light before the light actually reaches the Earth's surface. Studies have shown that cities in the northern part of the world are relatively dark, with an average albedo of 7 percent, which increases a bit during summer, while cities in tropical countries have an average albedo of 12 percent. Some of the reasons for this difference may include different natural environments of cities in tropical regions, such as the presence of darker trees. Also, city buildings are built with different materials, and in warmer regions might be made of light-colored material to keep structures cooler; asphalt (used in the building of roads in urban areas) also changes the albedo value of the surface. Remote-sensing technology is generally used to measure surface reflectance and albedo. Values observed through satellites are fed into mathematical models to get the albedo values.

SEE ALSO: Atmospheric Absorption of Solar Radiation; Sunlight; Weather.

BIBLIOGRAPHY. C.D. Ahrens, Meteorology Today: An Introduction to Weather, Climate, and the Environment (Thompson, Brooks/Cole, 2006); T.R. Oke, Boundary Layer Climates (Routledge, 1992).

Velma I. Grover Natural Resource Consultant

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