The MODIS instrument, flying aboard NASA's Terra and Aqua satellites, measures albedo. Albedo measures the proportion of incoming solar radiation reaching a surface that is reflected back to the atmosphere and to space. For an unchanging surface, albedo can vary somewhat, depending on the sky and atmospheric conditions. This image maps the white-sky albedo, which is the albedo under conditions of a uniform, dense cloud cover, in which downwelling light energy comes uniformly from all directions. The color bar indicates the albedo value ranging from 0.0 to 0.4 over the Earth's land surfaces. Areas colored red show the brightest, most reflective regions; yellows and greens are intermediate values; and blues and violets show relatively dark surfaces. White indicates no data is available. Typically, vegetated surfaces and water have low albedos, while soil and urban surfaces have somewhat higher values. Note that solar energy that is not reflected away from a surface is absorbed by that surface. Thus, albedo also provides information about the amount of energy absorbed by a surface. Since this energy serves to heat the soil and the air just above the surface, albedo is an important factor in weather and climate studies, and especially is important for modeling of weather and climate on scales of days to years. This image was produced using data composited over a 16-day period, from April 7-22, 2002.