NASA's Terra spacecraft and its JPL-built Multi-angle Imaging SpectroRadiometer (MISR) instrument flew over then Tropical Storm Isaac at 11:30 a.m. CDT on Aug. 28, 2012, a few hours before Isaac was upgraded to a Category 1 hurricane on the Saffir-Simpson scale. At the time of the overpass, MISR recorded low-level wind speeds of up to 75 miles per hour (65 knots) from cloud motion observed outside Isaac's eye. The National Hurricane Center in Miami similarly reports maximum sustained winds of 69 miles per hour (60 knots) with gusts to 86 miles per hour (75 knots) soon after. Isaac made initial landfall in southeastern Louisiana in Plaquemines Parish about 90 miles (145 kilometers) southeast of New Orleans around 6:45 p.m. CDT with maximum sustained winds of 80 miles per hour (70 knots).
The image shows the eastern half of the storm from one of MISR's nine cameras. At the time of the overpass, the storm was located in the Gulf of Mexico, southeast of Louisiana. The eye of the storm is just off the western edge of the image, while the coast of the Florida panhandle is located near the top of the image. By tracking the motion of clouds as they move between views from different cameras and taking into account stereo parallax, it is possible to derive both cloud motion and height from MISR data. Derived wind vectors are superimposed on the image. The arrows show both the direction and speed of the wind, and the color scale indicates the height at which the wind is detected.
Hurricanes can be thought of as heat engines that ingest warm, moist air at low levels in the atmosphere, convert it into energy in the form of wind and rain, and then eject cool, dry air at high levels. Due to Earth's rotation, the low-level air spirals inward in a counterclockwise direction, while the high-level winds move outward from the eye in a clockwise direction. The MISR vector cloud motion retrievals, with a gridded resolution of 11 miles (17.6 kilometers), show this circulation. The motion of the low clouds is counterclockwise, with the strongest winds in the upper part of the image where the motion of the storm couples with the inflow. The motion of the clouds to the southeast of the storm center is clockwise. The imagery shows that these are thin, cirrus clouds characteristic of the storm outflow.
MISR observes the daylit Earth continuously, viewing the entire globe between 82 degrees north and 82 degrees south latitude every nine days. The images shown here span 500 miles (800 kilometers) from north to south. They have been derived from MISR data over a portion of Terra orbit 67,531 from blocks 66-71 within World Reference System-2 Path 19.
MISR was built and is managed by NASA's Jet Propulsion Laboratory, Pasadena, Calif., for NASA's Science Mission Directorate, Washington, D.C. The Terra spacecraft is managed by NASA's Goddard Space Flight Center, Greenbelt, Md. The MISR data were obtained from the NASA Langley Research Center Atmospheric Science Data Center in Hampton, Va. JPL is a division of the California Institute of Technology in Pasadena.