Tropical Cyclone Gamede was spinning in the middle of the Indian Ocean on February 21, 2007, when it was observed by NASA's winds.jpl.nasa.gov/missions/quikscat/index.cfm QuikSCAT satellite at 5:03 p.m. local time (13:03 UTC). The nearest land to the storm system was Diego Garcia, several hundred miles north of the storm.
This data visualization of QuikSCAT's observations shows Cyclone Gamede and its spiral pattern of winds. The image depicts wind speed in color and wind direction with small barbs. White barbs point to areas of heavy rain. Gamede appears as a well-formed spiral of winds centered around a strong eye with a calmer center. This pattern is typical of tropical cyclones. Since the storm is in the Southern Hemisphere, the Coriolos force, which gives all such storms their spin, turns the storm clockwise, the opposite direction of hurricanes and typhoons that form in the Northern Hemisphere. According to the University of Hawaii's www.solar.ifa.hawaii.edu/Tropical/tropical.html Tropical Storm Information Center, Cyclone Gamede had sustained winds around 45 knots (83 kilometers per hour; 52 miles per hour) at the time of the QuikSCAT observations.
Measurements of the actual wind strength of cyclones are often higher than those measured by QuikSCAT. QuikSCAT employs a scatterometer, which sends pulses of microwave energy through the atmosphere to the ocean surface, and measures the energy that bounces back from the wind-roughened surface. The energy of the microwave pulses changes depending on wind speed and direction, giving scientists a way to monitor wind around the world. This technique does not work over land, but allows measurements in storms over oceans.
Tropical cyclones, however, are difficult to measure. To relate the radar energy that returns to the sensor to actual wind speed, scientists compare measurements taken from buoys and other ground stations to data the satellite acquired at the same time and place. Because the high wind speeds generated by cyclones are rare, scientists do not have corresponding ground information to know how to translate data from the satellite for wind speeds above 50 knots (about 93 km/hr or 58 mph). Also, the unusually heavy rain found in a cyclone distorts the microwave pulses in a number of ways, making a conversion to accurate wind speed difficult. Instead, the scatterometer provides a nice picture of the relative wind speeds within the storm and shows wind direction.
NASA image courtesy of David Long, Brigham Young University, on the winds.jpl.nasa.gov/ QuikSCAT Science Team , and the Jet Propulsion Laboratory.