Tropical Storm Hilda was about 540 miles southeast of Hawaii on the morning of August 25, 2009, heading west at about 9 miles per hour. According to the 5:00 a.m. (HST) advisory from the Central Pacific Hurricane Center, wind shear in the vicinity of Hilda was predicted to decline, increasing the probability that Hilda would reach hurricane strength after 48 hours.
This view of the wind speed and direction of Tropical Storm Hilda was captured by the QuikSCAT satellite on August 24. Colors indicate wind speed (highest speeds are purple), barbed lines indicate direction. White lines indicate areas where winds were accompanied by heavy rain. Like the air flow in all Northern Hemisphere tropical cyclones, the winds spiral inward in a counterclockwise direction toward the eye—where the air pressure is lowest.
Quikscat measures wind speeds over the ocean by sending pulses of microwave energy through the atmosphere to the ocean and measuring the echo—the energy that bounces back from the wind-roughened surface. The intensity of the microwave echo changes depending on wind speed and direction, and scientists relate the radar echo to actual wind speed by correlating it with observations from ground stations and buoys.
Because the high wind speeds generated by cyclones are rare, scientists do not have corresponding ground information to know how to translate QuikSCAT data 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, making a conversion to absolute wind speed difficult. Instead, QuikSCAT images provide a picture of the wind structure within the storm, which, among other things, can reveal whether a storm has developed a strong eye.