Annotated Color-Coded Map
Despite good rainfall and record-setting snowstorms in the spring of 2005,
most of northeastern Wyoming, the Black Hills, and western South Dakota
remain in the midst of a severe drought. This set of images and maps from
NASA's Multi-angle Imaging SpectroRadiometer (MISR) contrast the
appearance of the Black Hills region of northwestern South Dakota on July
12, 2000 (left column), with views acquired four years later, on July 14,
2004 (right column). The natural-color images along the top are from
MISR's nadir (downward-looking) camera. The browning that appears in 2004
compared with 2000 indicates that the vigor of green vegetation was
significantly diminished in 2004.
The color-coded maps (along the bottom) provide a quantitative measurement
of the sunlight reflected from these surfaces, and the loss of
sunlight-absorbing vegetation between the 2000 and 2004 dates. As the
vegetation faded with the drought, the albedo at the surface increased.
Albedo measures the fraction of incident sunlight that is reflected by a
surface, and can vary between zero (if all the incident sunlight is
absorbed and none is reflected) and one (if all sunlight is reflected and
none is absorbed). Dense forest has a low albedo; bright desert, snow and
clouds, have a high albedo. Here, albedo is provided for the wavelengths
of sunlight that plants use for photosynthesis (400 - 700 nanometers).
This measurement is known as the albedo for Photosynthetically Active
Radiation (PAR). Surfaces with greater absorption of PAR appear here in
blue hues, whereas surfaces with lower absorption appear as green, yellow,
orange or red. Black pixels indicate areas where albedo could not be
derived, usually due to the presence of clouds. In July 2004, low albedo
areas (blue pixels) are notably reduced in extent, and higher albedo
areas (yellow, orange and red pixels) have increased.
Because incoming sunlight is scattered by tiny particles in the
atmosphere, satellite measurements of albedo and other surface properties
must correct for the effects of the intervening atmosphere. These albedo
retrievals make use of MISR's simultaneously derived aerosol properties to
make these corrections. The multiangular nature of MISR data is also used
to account for the fact that most surfaces reflect sunlight into all
upward directions, with intensities that vary with angle of view.
The Multi-angle Imaging SpectroRadiometer observes the daylit Earth
continuously and every 9 days views the entire globe between 82° north
and 82° south latitude. This image area covers about 243 kilometers by
259 kilometers. These data products were generated from a portion of the
imagery acquired during Terra orbits 3020 and 24325 and utilize data from
within blocks 54 to 56 within World Reference System-2 paths 33 and 34.
MISR was built and is managed by NASA's Jet Propulsion Laboratory,
Pasadena, CA, for NASA's Office of Earth Science, Washington, DC. The
Terra satellite is managed by NASA's Goddard Space Flight Center,
Greenbelt, MD. JPL is a division of the California Institute of
Technology.