Hurricane Dolly intensified to a Category 2 storm in the Gulf of Mexico on July 23, 2008, just before it bore down on South Padre Island off the South Texas coast. At its strongest, the storm’s winds reached sustained speeds of 160 kilometers per hour (100 miles per hour), with gusting winds reaching even higher speeds. Dolly dissipated as it traveled inland, falling back to tropical storm status as of July 24.
This data visualization of Hurricane Dolly shows observations from the QuikSCAT satellite on July 22 at 7:23 p.m. local time (00:23 UTC, July 23), when Dolly was near its peak strength. The image depicts wind speed in color and wind direction with small barbs. White barbs point to areas of heavy rain.
QuikSCAT measurements of the wind strength of Hurricane Dolly and other tropical cyclones can be slower than actual wind speeds. QuikSCAT’s scatterometer 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.
To relate the radar signal 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 exact wind speed difficult. Instead, the scatterometer provides a nice picture of the relative wind speeds within the storm and shows wind direction.