Hurricane Jimena had weakened significantly from its peak strength on September 1, 2009, when the QuikSCAT satellite made the observations used to create this image. The image depicts wind speed in color and direction with small barbs. White barbs indicate areas of heavy rain. At the time QuikSCAT made these measurements at 6:20 p.m. Pacific Daylight Time (1:20 UTC on September 2), Jimena was a Category 3 hurricane with winds of 120 miles per hour (195 kilometers per hour), according to the National Hurricane Center. Earlier in the day, Jimena had been an extremely powerful Category 4 hurricane, nearing Category 5 strength. In this image, the storm has a core of strong winds, shown in purple and pink, surrounded target-like by gradually weakening winds.
The QuikSCAT scatterometer measures wind speed by bouncing microwave pulses off the ocean’s surface and recording the echo. An ocean tossed by strong winds will scatter the energy differently than a calm ocean, so the measurement provides an indication of both wind speed and direction. However, since hurricanes are relatively rare, scientists have not calibrated the data to accurately estimate wind speeds greater than 50 knots (about 60 miles per hour or 90 kilometers per hour). Heavy rain also distorts the ocean’s surface, making wind speeds more difficult to gauge. As a result, this image shows relative wind speeds and storm structure rather than an accurate reporting of absolute wind speeds.
- National Hurricane Center. (2009, September 4). Hurricane Jimena Advisory Archive. National Oceanic and Atmospheric Administration, National Weather Service. Accessed September 4, 2009.
NASA image courtesy of David Long, Brigham Young University, on the QuikSCAT Science Team, and the Jet Propulsion Laboratory. Caption by Holli Riebeek.