The calm eye of Typhoon Nabi stands out like a bulls-eye in the center of the concentric circles of color that make up the storm. The colors represent wind speed, with purple and pink showing the highest winds, while tiny barbs show the wind’s direction spinning around the eye of the storm. The white barbs indicate regions of heavy rainfall. The image was created using data collected by the QuikSCAT satellite on September 1, 2005, when Nabi was growing into a powerful super typhoon with winds of 260 kilometers per hour (160 miles per hour, 140 knots) and gusts to 315 km/hr (196 mph, 170 knots). At the time this image was taken, however, Nabi had winds of about 213 km/hr (132 mph, 115 knots) with gusts to 260 km/hr (160 mph, 140 knots), making it the equivalent of a Category 3 hurricane on the Saffir-Simpson Hurricane Scale.
The wind speeds shown in this image don't match the winds reported by the Joint Typhoon Warning Center. This is because QuikSCAT measures near-surface wind speeds over the ocean based on how the winds affect the ocean. The satellite sends out high-frequency radio waves, some of which bounce off the ocean and return to the satellite. Rough, storm-tossed seas return more of the waves, creating a strong signal, while a mirror-smooth surface returns a weaker signal. To learn to match wind speeds with the type of signal that returns to the satellite, scientists compare wind measurements taken by ocean buoys to the strength of the signal received by the satellite. The more measurements scientists have, the more accurately they can correlate wind speed to the returning radar signal.
Typhoons and hurricanes are relatively rare. This means that scientists have few buoy measurements to compare to the data they get from the satellite and can’t match the satellite measurements to exact wind speeds. Instead, the image provides a clear picture of relative wind speeds, showing how large the strong center of the storm is and which direction winds are blowing. To learn more about measuring winds from space, check out NASA’s Winds web site.