Albedo
Albedo is known as surface reflectivity of sun’s radiation. The term has its origins from a 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%. The typical amounts of solar radiation reflected from various objects are shown in Table 1. Albedo values can range between 3% for water at small zenith angles to over 95% for fresh snow. 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 number highly dependent on the local surface makeup, cover, and cloud distribution.
Surface reflectance values exhibit large geographic variation (Figure 1). Mean annual albedo values differ considerably between the equator and the poles, largely due to the presence of snow and ice-covered surfaces along with cloudy skies in high latitudes, which greatly increases albedo values in those areas. Atmospheric reflectance principally varies with dust concentration, the zenith angle of the Sun, and the type and/or amount of cloud cover. Well-developed convective clouds reflect up to 90% of incident solar energy, making thick clouds appear bright from space. 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 (Figures 2 & 3), 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.
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.
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.
Further Reading
- Ahrens, C. D. 2006. Meteorology Today. An Introduction to Weather, Climate, and the Environment. Eighth Edition. Thompson, Brooks/Cole. United States. 537 pp. ISBN: 0495011622
- Gurneym R. J., Foster, J. L., and Parkinson, C. L. 1993. Atlas of Satellite Observations Related to Global Change. Cambridge University Press. Great Britain. 470 pp. ISBN: 052143467X
- International Satellite Cloud Climatology Project (ISCCP)
- Oke, T.R. 1992. Boundary Layer Climates. Second Edition. Routledge. New York. 435 pp. ISBN: 0415043190
- Schiffer, R.A., and Rossow, W.B. 1983. The International Satellite Cloud Climatology Project (ISCCP): The First Project of the World Climate Research Programme. Bulletin of the American Meteorological Society, 64:779-784.