Triggered by seasonal changes in available sunlight, about50 million square kilometers of the Earth’s land surface undergo atransition from freeze to thaw each year. Snow and ice disappear thenreturn, frozen ground softens and hardens, vegetation “greensup” and fades. Satellite sensors can detect many of these processesthrough seasonal changes in the amount of sunlight reflected by theEarth’s surface at various wavelengths. The percentage of light that asurface reflects out of the total light falling on it is the surface’salbedo. Bright, reflective surfaces, such as fresh snow, have a highalbedo; dark, absorptive surfaces, such as dense forest, have a lowalbedo.
These globes display seasonal albedo observations over much of Asiacollected by NASA’s Multi-angle Imaging SpectroRadiometer (MISR) in2004. The left-hand column of globes is natural-color, made by combiningMISR-observed albedos in a way that is similar to what our eyes would see.The right-hand set of globes shows the albedo for only the wavelengths oflight that plants use for photosynthesis or the albedo forPhotosynthetically Active Radiation (PAR). Places where the surface isreflecting most of the photosynthetic wavelengths (high albedo) areoff-white or tan; places where the surface is absorbing most of thephotosynthetic wavelengths (low albedo) are dark green to nearly black. Grayareas indicate lack of data, for example, because of persistent cloudiness.Particles in the atmosphere can interfere with the collection ofsurface-albedo measurements because the particles scatter incoming sunlightin all directions; however, all the scattering effects from the atmospherehave been removed from these albedo globes.
In December through February, snow covers the northern latitudes, makingthe land surface more reflective, and therefore increasing the albedo. Thealbedo in photosynthetically active wavelengths is also high (white, tan,and yellow values) because it is winter, and plants are not very active. Asthe seasons progress, the albedo in photosynthetically active wavelengths oflight decreases because plants are abundant and active; by theJune-through-August period, vegetation across Asia was absorbing almost allof the photosynthetic light coming in from the Sun. Areas with higheralbedos even during the summer growing season include the semi-arid TibetanPlateau (roughly centered in the globes), the Arabian Peninsula (far left),and the high Arctic (top).
For more detail and additional globes, visit the MISR image page. For MISR’sfive-year, global albedo data record, and for other surface and vegetationproducts from the MISR instrument, visit the NASA-Langley AtmosphericSciences Data Center’s MISRLevel 3 Imagery Website.
The Multi-angle Imaging SpectroRadiometer observes the daylit Earthcontinuously from pole to pole, and every 9 days views the entire globebetween 82 degrees north and 82 degrees south latitude. MISR was built andis managed by NASA's Jet Propulsion Laboratory, Pasadena, CA, for NASA'sOffice of Earth Science, Washington, DC. The Terra satellite is managed byNASA's Goddard Space Flight Center, Greenbelt, MD. JPL is a division of theCalifornia Institute of Technology.
References & Resources
Image courtesy NASA/GSFC/LaRC/JPL, MISR Team. Text by John Martonchik (JPL) and Clare Averill (Raytheon ITSS/JPL).
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