Typhoon Haiyan, which ravaged the Philippines earlier this month, might have been one of the strongest tropical cyclones ever to hit land, but its exact strength was a matter of dispute. Forecasting agencies said it would pack winds of up to 195 miles (about 315 kilometers) per hour, but meteorologists in the Philippines said that top measured speeds were closer to 150 mph, or 240 kph.
The difference means little to the millions of people who were affected, but precise data on the intensity of hurricanes and typhoons — monsters like Haiyan, as well as far more frequent weaker storms — is important to scientists, especially those who are trying to determine the possible effects of climate change on weather.
But the tools for determining the strength of tropical cyclones have limitations, and not all of them are available everywhere or for every storm. Other than the United States, for example, few countries make direct measurements by flying planes into these kinds of storms.
Worldwide, hurricanes and typhoons are analyzed primarily with imagery from satellites that measure the temperature of the sea surface and clouds by sensing the infrared radiation emitted from them. Called the Dvorak technique, after Vernon Dvorak, the scientist who developed it four decades ago, the method is used by trained analysts and, increasingly, computers.
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“It’s basically an inkblot test for hurricanes,” said James Franklin, chief of the hurricane specialist unit at the National Hurricane Center.
Analysts or automated programs pore over heat maps of clouds in search of patterns that reveal the degree to which they are organized into a storm. “You can relate the appearance of the storm to intensity,” Franklin said.
Among other things, the analysis includes how cold the cloud tops are, which is an indication of their depth, and the width and temperature of the eye, he said. A storm system is then given a rating that corresponds to certain wind speeds and air pressures as determined through direct measurements of past storms.
John A. Knaff, a researcher with the National Oceanic and Atmospheric Administration’s Center for Satellite Applications and Research, who has evaluated the Dvorak technique, said that it has been tweaked over the years and worked well, especially for stronger storms with a high degree of organization.
Haiyan, for example, was rated a T-8, the highest number, through Dvorak analysis. It was clear from the satellite imagery that this was a superstorm. “From the technique’s standpoint, this was about as good as you can get,” Knaff said.
But the technique is not without its problems. Two analysts looking at the same image can come up with different ratings, especially for weaker storms. And some meteorological agencies relate the same ratings to different wind speeds, which can lead to confusion.
Ultimately the wind speeds are only estimates, and can be off by 25 mph or more. That may account for some of the differences between the forecast and the actual measurements during Haiyan. It is also not clear that the instruments in the Philippines captured peak wind speeds, which occur in the wall of clouds surrounding the eye.
“You’d have to be pretty lucky, or unlucky, to encounter them,” Franklin said.
For direct measurements before a storm hits land, the only real tools are airplanes. The U.S. flies aircraft through storms in the Atlantic and the eastern Pacific, but has not conducted flights in the western Pacific since 1987. Hong Kong and Taiwan fly into typhoons, but less extensively than the U.S.
“Nobody out there does routine inner-core reconnaissance in the way we do it here,” Franklin said.
But even the U.S. flies into only about one-third of Atlantic storms — those that threaten land, he said.
A flight passes through the eyewall at 10,000 feet, and instruments gather wind speed and other data. If there is no other way to get data near the surface, “we will take 90 percent of whatever we measure at 10,000 feet and use that,” Franklin said.
But usually the flights also eject parachute-equipped instruments, called dropsondes, that are caught in the winds and transmit data as they descend.
Even with them, there is no guarantee of measuring maximum winds, as they have to be precisely ejected to fall into the eyewall. “It’s kind of like a bombing exercise,” Franklin said.