The hurricane category is not complete. Scientists suggest it’s time for a new scale

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Nothing drives hurricane experts crazier than the feeling “Oh, it’s just a Category 1.”

It’s not just a boring attitude, it’s deadly. Hardly a hurricane season passes without another example of a low-category storm causing more problems than expected.

Last year, Hurricane Ida swept from Louisiana to New York, killing 87 people. But more people died in the northeast — where remnants of the storm, not even a tropical depression at the time, caused catastrophic flash floods — than in Louisiana, where Ida reached Category 4.

Sixteen years earlier, Hurricane Katrina made landfall in almost the same location, cementing its place as one of the most destructive hurricanes in modern history. More than 1,800 people died.

It was “only” a Category 3.

Scientists have long argued that the familiar Category 1-5 system that hurricane watchers are used to doesn’t explain storm risk well enough. And they came up with all kinds of new ways to categorize them.

“If Katrina is a 3 on a scale of 1 to 5, then that scale is broken. We need to do something else,” said Phil Klotzbach, meteorologist at Colorado State University.

The main problem is that the scale, developed by Coral Gables civil engineer Herbert Saffir and National Hurricane Center director Robert Simpson in 1973, was only designed to classify storms by wind speed. But that’s not how the public sees it.

“It’s effectively morphed to not just be the threat of the wind, but all of the threats of a hurricane,” Klotzbach said.

And there are many. Hurricanes and tropical storms can not only bring devastating high winds, but also strong storm surges, torrential rains and even tornadoes.

With another six-month “above average” hurricane season beginning June 1, scientists like Klotzbach are wondering if a new categorization system could better explain the enormous danger these storms pose and help save lives. lives.

Suggested new indices and scales

The problem some scientists have with the Saffir-Simpson scale is that wind speeds don’t necessarily translate to land. They are usually measured thousands of feet in the air with hurricane fighter planes while storms are over the ocean.

“You’ll never find a sustained 150 mph wind on land,” Klotzbach said. “The minute this storm hits land, that wind no longer exists.”

And one measure doesn’t speak for the whole storm. Hurricanes are not a single point. They cover a wide swath, with the strongest winds generally on the east side of the direction the storm is moving.

This is where an alternative measure comes in, the integrated kinetic energy scale. It ranks storms from 0.00 to 4.99 based on the energy of a storm’s entire wind field. Scientists say it offers a more realistic picture of a storm’s total strength.

There’s also the Cyclone Damage Potential Index, which rates storms from 1 to 10 based on wind speed, storm size, and how fast it is moving. This can be useful in telling the difference between a storm with a high wind speed that zooms in and a lower one that lingers longer in one place.

Private industry is also on board with alternative scales. In 2019, Accuweather introduced the RealImpact scale, a six-point scale that rates wind speed, storm size, flood potential, and population of the area a storm is expected to hit.

Klotzbach favors a scale of 1 to 5 based on barometric pressure, which decreases as a storm strengthens. Hurricane chasers measure pressure by placing a sensor in the heart of the storm that sends data back to researchers as it falls.

In a 2020 paper, he and other researchers found that categorizing storms by pressure correlated better with damage and fatalities than wind speed alone. They also found that this scale closely matched the actual impacts of older storms; it recorded no powerful storms as weak, or vice versa.

“That’s my argument. Simplicity,” he said.

The role of precipitation

For Marshall Shepherd, director of the University of Georgia’s atmospheric science program, the biggest problem with the current hurricane ranking system is rain.

He pointed to Hurricane Florence, which rumbled toward the Atlantic coast as a Category 4 but crashed to Category 1 before making landfall in North Carolina. When categorization downgrades hit the headlines in 2018, some residents decided to ignore earlier evacuation orders and stay put. But the rain and storm surge from the weaker storm still caused record flooding that killed dozens of people and left hundreds more in need of emergency evacuations.

Over a career of research, Shepherd discovered that small storms can have a huge impact in terms of precipitation, rain that storm watchers paying attention only to categories may not be prepared for.

“We are all preparing as we should for categories 3, 4, 5, but often the most significant in terms of precipitation can be a tropical storm,” he said. “I see debates on Twitter all the time, is it a strong Cat 2 or Cat 1? And I say it doesn’t matter, the impacts are the same.”

Shepherd and other researchers have proposed a metric to solve this problem, which goes hand in hand with hurricane categories. They call it the “extreme precipitation multiplier”.

The researchers found that historic storms ranged between 1 and 7 on the scale. Hurricane Florence ranked 5.7 on the day it made landfall, the second highest of any historic storm after Hurricane Harvey.

Shepherd said the scale could be used for all rainstorms, not just hurricanes. People die every year after driving on flooded roads and bridges because they don’t understand the danger.

“Until we fundamentally amplify the risk of flooding and water in general for rainstorms, it’s still a background challenge to convey hurricane risk as well,” he said. .

“A fairly institutionalized thing”

The fact that the Saffir-Simpson scale does not accurately reflect the danger of a storm is no mystery to experts at the National Hurricane Center. But it’s hard to shake off one of the world’s most recognizable scales.

“It’s a pretty institutionalized thing; even if we choose not to use it, people are going to talk about it,” said Michael Brennan, branch chief of the specialized hurricane unit at the hurricane center.

In defense of Saffir-Simpson, Brennan said it works well for ships and some of the more mountainous islands in the Caribbean, where the main threat from a storm is wind. And besides, there is no lead candidate for a replacement.

“There really is no other single scale that would encompass all of the risks in a way that would be easy to communicate to the public,” he said.

Instead, the hurricane center’s approach has been to downplay the magnitude and focus on individual hazards. The center has launched a map that shows the estimated arrival time of hurricane-force winds and another that predicts the amount of storm surge a community could see.

The hurricane center also launched a new set of storm surge watches and warnings alongside its hurricane watches and warnings.

“If you tell someone it’s a letter X storm, what does that person do about it? It doesn’t tell them anything about it,” Brennan said. “How do you get actionable insights for people they take the right action on.”


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