Typhoon Mirinae was fading from a typhoon to a tropical storm when the QuikSCAT satellite captured the data used to make this image on November 1, 2009. The image depicts wind speed and direction, revealing the storm’s structure. Although Mirinae was weakening, its winds maintained the classic spiral structure associated with well-organized tropical cyclones. The strongest winds, shown in purple, are on the northwest side of the eye, visible as an area of calmer winds in the center of the storm. A broad field of strong winds, red, spiral around the eye in clear bands. The barbs indicate wind direction, and white barbs point to areas of heavy rain.


QuikSCAT monitors wind speed with a radar that sends out pulses of microwave energy and listens for the echo after the pulse bounces off the wind-roughened ocean surface. Scientists translate radar signals into estimates of wind speed by matching the radar echoes to physical measurements collected from buoys at the same time and place.

Cyclone-strength wind speeds are rare, however, and scientists generally don’t have enough matching buoy observations to convert wind speeds above roughly 50 knots. Intense rain rippling the ocean’s surface can also interfere with the radar signal. Due to these limitations, QuikSCAT images don’t show absolute, maximum wind speeds. Instead, they give forecasters a valuable picture of the wind structure within the storm, for example, revealing whether a storm has a strong or a weak eye and how large an area is experiencing tropical-storm-strength winds. Typhoon Mirinae raked across the Philippines on October 30 as a Category 2 typhoon, killing 20, reported the Associated Press. It was the third typhoon to hit the Philippine in 6 weeks. The storm came ashore over Vietnam on November 2, where it caused severe flooding and left 23 people dead, said the Associated Press. (NASA image courtesy of David Long, Brigham Young University, on the QuikSCAT Science Team, and the Jet Propulsion Laboratory. Caption by Holli Riebeek, NASA Earth Observatory.)