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About 50 million square kilometers of the Earth's terrestrial surface undergo a transition each year from freeze to thaw, thus setting off a series of global biospheric processes. Much of this activity can be detected by the temporal changes in the amount of sunlight reflected by the Earth's surface at various wavelengths. A quantitative measure of this reflected sunlight is described by the albedo, which is the fraction of sunlight reflected by a surface area to that incident on the surface area in all directions, typically in a given spectral band. The surface albedo can vary between zero (all incident sunlight is absorbed at the surface and none is reflected) and one (all incident sunlight is reflected from the surface and none is absorbed). Fresh snow is an example of a surface type with an albedo close to one in the visible region of the solar spectrum whereas deep clean ocean water has an albedo that is close to zero. Five years of global surface albedo data are now available as summary maps from NASA's Multi-angle Imaging SpectroRadiometer (MISR). The globes in the image show a particular kind of albedo, formally known as Directional Hemispherical Reflectance (DHR), in which all scattering effects from the atmosphere are removed. Thus the sunlight incident on any part of the global surface is uniquely directional, coming only from the location of the sun in the sky and with no multidirectional sunlight created by scattering in the atmosphere. The first and third line of globes show MISR blue, green, and red band DHR, combined to create a natural color DHR. The second and fourth line of globes show DHR-PAR, that DHR for the sunlight band containing only those wavelengths used in photosynthesis (400 - 700 nanometers), which are absorbed by vegetation. The sunlight in this spectral region is known as photosynthetically active radiation (PAR), thus the label DHR-PAR for the associated albedo. Photo Credit: NASA/GSFC/LaRC/JPL, MISR Team [0003-0612-3114-4943] by 0003
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Keywords
albedo, earth, jpl, nasa, season, seasonal changes in earths surface albedo, seasons
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