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PolitiFact: No, Hurricane Irma is not a Category 6

Hurricane Irma west of the Leeward Islands on Tuesday as it grew into a Category 5 storm. [National Oceanic and Atmospheric Administration via the Association Press]

Hurricane Irma west of the Leeward Islands on Tuesday as it grew into a Category 5 storm. [National Oceanic and Atmospheric Administration via the Association Press]

A hurricane with winds in excess of 180 mph is scary enough. But some corners of the Internet are stirring additional panic by referring to an extreme hurricane category that doesn't exist.

One web post appeared as Hurricane Irma powered through the Caribbean, days ahead of an expected landfall in Florida. The storm is one of the most powerful to form in the Atlantic Ocean, with sustained winds as high as 185 mph.

A website posing as an arm of CNN posted a page headlined, "Hurricane Irma could be a Category 6 by the time it hits East Coast."

The advertising-filled page contains an audio recording of a folksy woman rambling about Irma. She says, "The wind speeds are way up there into the Category 6 territory."

But there is no such thing as Category 6, despite what this site and others say. The scale used by the National Hurricane Center only goes as high as 5.

The Saffir-Simpson Hurricane Wind Scale was developed by Herbert Saffir, an engineer and expert on wind damage in Coral Gables, and Robert Simpson, who headed the National Hurricane Center from 1967-73. The scale used wind speeds as a guide to predict the extent of structural damage.

One of Simpson's formative experiences came with the approach of Hurricane Camille to the Mississippi coast on Aug. 16, 1969. In the aftermath, "Simpson felt he needed a better way to communicate what a storm is capable of doing," wrote Jack Williams, co-author of Hurricane Watch: Forecasting the Deadliest Storms on Earth. "He correlated Saffir's wind damage rankings with the surge potential to create the scale used today."

Officials at the National Hurricane Center began using Simpson's new scale internally before releasing it to the public in 1975.

Under the scale, Category 1 is for sustained winds of 74 to 95 mph, Category 2 is for 96 to 110 mph, Category 3 is for 111 to 129 mph, Category 4 is for 130 to 156 mph, and Category 5 is for 157 mph or higher. A Category 4 or 5 storm will cause "catastrophic damage."

There has been discussion of whether a new category should be added to the scale. A simple extrapolation from the existing categories would set the threshold for a new Category 6 around 180 mph, making Irma a good candidate for that category if it existed.

But experts said adding Category 6 would add nothing to the understanding of how destructive a storm would be.

"Almost no structures can withstand sustained winds over 155 mph," said Michael M. Bell, an associate professor in atmospheric science at Colorado State University. "At those wind speeds, significant damage or even total destruction of most buildings is expected. By this definition, adding a Category 6 would not add any new information, since the strongest criteria for expected wind damage has already been met."

The scale's co-creator, Simpson, agreed in a 1991 interview, saying "That's the reason why we didn't try to go any higher than that anyway."

The web post rates Pants on Fire!

Contact Louis Jacobson at Follow @LouJacobson.

PolitiFact: No, Hurricane Irma is not a Category 6 09/06/17 [Last modified: Wednesday, September 6, 2017 7:51pm]
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