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A Category 5 hurricane that hit the Florida Keys on Labor Day of 1935 derailed an 11-car passenger train and destroyed the devices meant to measure its winds.

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Associated Press

You may have seen it circulating online: a big, bold headline warning that Hurricane Irma is on track to become a “Category 6” storm.

Do not be fooled. There is no such thing.

As Irma churned west with sustained winds of 185 miles per hour on Tuesday, making it among the most powerful Atlantic hurricanes on record, some armchair meteorologists suggested that there should be. On the surface, that makes some sense: The difference between successive categories on the existing scale ranges from 14 to 26 miles per hour, and Irma’s winds were 28 miles per hour past the Category 5 threshold. In the years ahead, hurricanes are quite likely to become stronger, and the strongest ones more frequent. But Category 6 still is not going to happen.

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A water tower in Florida City stood amid the ruins left by Hurricane Andrew in August 1992.

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Associated Press

“The scale was developed 1 to 5,” Joel Myers, the founder and president of AccuWeather, said in an interview Tuesday evening. “When you develop a scale 1 to 5, there can’t be any Category 6.”

The purpose of the categories, known as the Saffir-Simpson scale, is to quantify a hurricane’s destructive power, and the destructive power of a Category 5 hurricane — one with sustained winds of at least 157 miles per hour — is virtually total. “A high percentage of framed homes will be destroyed, with total roof failure and wall collapse,” Dennis Feltgen, a spokesman for the National Hurricane Center, wrote in an email. “Fallen trees and power poles will isolate residential areas. Power outages will last for weeks to possibly months. Most of the area will be uninhabitable for weeks or months.”

The scale classifies this level of damage as “catastrophic,” Mr. Feltgen said, and “what is left after ‘catastrophic’ damage?”

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The main business district of Pass Christian, Miss., after Hurricane Camille in August 1969.

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Jack Thornell/Associated Press

This is not to say there is no difference between a storm that brings 160-mile-per-hour winds and one that reaches 190. Dr. Myers noted that “the force of the winds goes up with the square of the velocity.” In layman’s terms, that means a hurricane with 200-mile-per-hour winds has four times — not just double — the force of one with 100-mile-per-hour winds. And wind is not the only factor; barometric pressure and other characteristics also affect how destructive it will be.

But practically, once a storm has leveled a city, there is little more it can do.

Just three Category 5 hurricanes have made landfall in the United States since 1924. The most recent one, Hurricane Andrew, hit in August 1992, and the destruction it wrought in South Florida was so complete, The Miami Herald reported the next morning, that it “changed the very nature of the region.”

Thirty miles south of Miami, a state official told The Herald at the time, Homestead Air Force Base “no longer exists.”

The only other Category 5 hurricanes to strike the United States in the past nine decades are Camille, which made landfall in Mississippi in August 1969, and an unnamed storm that made landfall in the Florida Keys on Labor Day of 1935. Both decimated the regions they hit.

The “Category 6” reports are not the only hoaxes circulating as Irma moves through the Caribbean. Another one claims that the storm is on track to hit the region just ravaged by Hurricane Harvey, when in reality, current forecasts show it heading for Florida. And that is not an academic question.

“People’s lives are at stake,” Dr. Myers said. “If people get the wrong information and make the wrong decision, it’s a tragedy.”

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