Typhoon Jangmi (also spelled Changmi) was well off the coast of Luzon in the Philippines on September 26, 2008. The storm was Category 2 strength, but forecasts by the Joint Typhoon Warning Center said that it could build to Category 3 or 4 strength, with sustained peak winds of 220 kilometers per hour (140 miles per hour). The predicted storm track indicated the typhoon would pass well offshore of Luzon, and then move northward through the Taiwan Strait.
This data visualization of Typhoon Jangmi shows observations from the QuikSCAT satellite on September 25 at 5:30 p.m. local time (9:30 UTC). Jangmi was a Category 1 typhoon at this time, with peak winds of around 65 knots (75 mph; 120 km/hr). The coast of Luzon peeks into the scene in the lower left corner; the storm system was located about two hundred kilometers offshore from the island. 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 Typhoon Jangmi and other tropical cyclones (the general term for typhoons and hurricanes) 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. This information often helps forecasters pinpoint the location of the eye of the storm.