"Category Five": How a Hurricane Yardstick Came to Be
for National Geographic News
|December 20, 2005|
The stormy 2005 hurricane season has pushed new phrases into Americans'
everyday speech. News reporters now refer to "Category Five political
scandals." A vacuum cleaner commercial boasts that the machine "has the
suction power of a Category Two hurricane."
The bluster refers to the Saffir-Simpson scale, which ranks hurricanes from one to five based on their wind speeds and destructive potential.
The scale was devised about 35 years ago by engineer Herbert Saffir and meteorologist Robert Simpson. At the time, Simpson directed the National Hurricane Center in Miami and Saffir ran his own engineering firm in neighboring Coral Gables.
Simpson and Saffir both chuckled when asked about the recent fame of their rating scale. But they also were gratified that their brainchild had found its way into popular speech.
"People are looking for yardsticks," Simpson said. "Our language has demonstrated the use of many expressions that started out to be used for one phenomenon, then were used to refer to many other things."
Saffir said he's "very pleased that the public is aware of the scale and can see the difference between a Category One and a Category Five storm. That's extremely important."
Saffir and Simpson created the scale to help local relief agencies and emergency management officials better prepare for approaching hurricanes. Until recently, the scale was mostly known to meteorologists and officials at emergency management and relief agencies.
But the unprecedented 2005 hurricane seasonthe most active and expensive on recordmade the Saffir-Simpson scale's ranking system a household phrase.
Four powerful hurricanes ranking Category Three or higher made landfall on the U.S. Gulf Coast between July and October. And the world's attention was focused on the epic Hurricane Katrina as it climbed rapidly up the scale before virtually destroying New Orleans and parts of the Mississippi Gulf Coast on August 29.
When politicians started talking about a "Category Five budget crisis" soon after Katrina, they were discovering what weather professionals have known for a long time.
"It's a great tool for emergency management people as well as for us to pass on information to the public," said Randy McKee, meteorologist in charge of the National Weather Service office in Mobile, Alabama.
"It catches their attention and inspires them to take more action to prepare for a storm than if there was no scale at all," he added. "It's been really good in the respect of having the public be aware of strong storms and take more drastic measures to protect themselves."
Major or Minor
Simpson and Saffir each witnessed powerful hurricanes long before they devised the scale. Simpson watched a hurricane deliver a devastating blow to his hometown of Corpus Christi, Texas, in 1919. Although he was only six years old at the time, he still has vivid memories of that catastrophe.
In the summer of 1947, Saffir, a New York City native, took a job as a county engineer in Miami. Soon after he arrived, a pair of hurricanes put much of South Florida under water.
About 12 years later, Saffir had set up his own engineering firm when the United Nations commissioned him to write a report on providing low-cost housing in areas of the world that are subject to hurricanes.
As he worked on the report, Saffir realized that there were only two ways of categorizing hurricanesmajor and minor.
"I worked up five scales [of hurricanes] based on a subjective description of the damage that could occur for each category," Saffir said.
Soon after Saffir's report to the UN, Hurricane Camillethe second-most powerful hurricane in U.S. historystruck in August 1969. The storm blasted coastal Mississippi with a 25-foot (7.6-meter) storm surge and winds exceeding 190 miles an hour (300 kilometers an hour).
The hurricane's torrential rains also unleashed deadly flooding far inland as the storm moved across the southeastern U.S.
"Hurricane Camille was sort of a red light to the federal government to learn what can happen with a hurricane," Saffir said. "Before Camille, I think most people were thinking of a hurricane as a sort of local event. Camille was a warning."
In Camille's wake, federal officials decided a system was needed to alert local officials and relief agencies of what to expect when a hurricane is predicted to make landfall.
"We had so many agenciesthe Red Cross, the Salvation Armythat didn't understand the characteristics of storms enough to make decisions about how they needed to plan their responses," Simpson said. "We could tell them the location where the storm would strike, but not what they needed to send."
Saffir conferred with Simpson, who had become director of the National Hurricane Center in 1967. Simpson came up with the idea to refine Saffir's five categories by including a hurricane's lowest barometric pressure readings and likely storm surge levels in the ranking system.
The barometric pressure reading in the eye of a hurricane drops as the storm intensifies. Extremely powerful hurricanes have very low barometric pressure readings in their centers.
For example, in October 2005 Hurricane Wilma's barometric pressure dropped to 26.04 inches, or 882 millibarsthe lowest ever recorded for a hurricane in the Atlantic Ocean.
At sea level during calm weather, the normal barometric pressure is roughly 30.00 inches, or about a thousand millibars.
"The bottom line is that central pressure is the most conservative thing that you have in a hurricane," Simpson said. "It's the best yardstick as to what the potential damage is going to be. The central maximum wind may be going up or down, all within the same category of hurricane."
The National Hurricane Center started using the Saffir-Simpson scale experimentally in 1970. At first, the scale was made available only to meteorologists and relief agencies. Then the federal government decided that it should be released to the general public.
Irene Toner, emergency management director of Monroe County, Florida, is one of hundreds of public officials who now routinely use the Saffir-Simpson scale to prepare for hurricanes. But Toner, whose district includes the Florida Keys, adds an extra twist to hurricane preparation.
"If they tell me a Category One is coming, I'll prepare for a Category Two," she said.
Willie Drye is the author of Storm of the Century: The Labor Day hurricane of 1935, published by National Geographic Books.
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