Hurricanes have been strengthening – like Beryl. Some scientists propose a Category 6.

The 2024 U.S. hurricane season, now underway, is predicted to be among the worst in decades. Before the National Oceanic and Atmospheric Administration made that forecast, a debate had been swirling about whether new vocabulary is needed to reflect the growing intensity of storms.

Category 5, signifying sustained winds stronger than 156 mph, is the highest marker on the Saffir-Simpson hurricane scale. Currently Hurricane Beryl is churning through the Caribbean and is the earliest Atlantic hurricane on record to reach Category 5 strength. The storm, recently downgraded to Category 4, is expected to pass on a path near Jamaica on Wednesday and then move on toward the Cayman Islands and Mexico on Thursday. 

Many Americans know the categories from weather reports and use them to decide how to prepare for storms. But in the past decade, the sustained wind speeds of eight hurricanes impacting the United States have surpassed 156 mph, killing 133 people and causing roughly $77 billion in damage. In recent months, some scientists have asked whether it’s time to add a Category 6 to the scale.

How did the five-step scale come about?

In 1969, American civil engineer Herbert Saffir was conducting research for the United Nations on how to prevent hurricane damage in low-income, storm-prone areas and saw there was no standard system to quantify the likely damage from hurricanes with winds above 74 mph. Mr. Saffir soon collaborated with National Hurricane Center (NHC) director Robert Simpson to release the Hurricane Disaster-Potential Scale.

In a 1974 article for the Weatherwise journal, Mr. Saffir and Mr. Simpson remarked that their wind-focused scale “gives probable property damage and evacuation recommendations.” Category 1 storms, they wrote, bring “damage primarily to shrubbery, trees, foliage and unanchored mobile homes,” and Category 5 storms are marked by “shrubs and trees blown down” and “complete failure of roofs on many residences and industrial buildings.”

Neil Frank, Mr. Simpson’s successor at the NHC, made it an official NHC metric. It eventually was renamed the Saffir-Simpson Hurricane Wind Scale and is used today to categorize storms in the Atlantic basin and parts of the Pacific Ocean.

Why are some scientists proposing a Category 6?

Michael Wehner, of the Lawrence Berkeley National Laboratory, and James Kossin, of the climate nonprofit First Street Foundation, have published a paper proposing a Category 6 that might better warn people of the risk from severe storms. While not outright advocating for a change yet, Dr. Wehner and Dr. Kossin state that a hypothetical Category 6 would give more room in the Saffir-Simpson scale for hurricanes growing in size and strength. 

After all, there’s a big difference between Hurricane Patricia, which grew in strength to 215 mph in 2015, and Hurricane Lee, which reached peak intensity at 160 mph last September. On the Saffir-Simpson scale, both were classified as Category 5.

In Dr. Wehner and Dr. Kossin’s new classification, Category 5 hurricanes would register sustained wind speeds between 157 and 192 mph. Category 6 hurricanes would be those exceeding 192 mph. 

Suzana Camargo of Columbia University’s Lamont-Doherty Earth Observatory notes that “in terms of communicating risk and having people change their behavior, it might work.”

What are some other limitations of the scale? 

In the past decade or so, many scientists realized the scale’s categories didn’t always explain the severity of storms, says Dr. Camargo. She adds that many hurricane deaths are due to storm surge and flooding from rain – metrics excluded in the scale. The scale also doesn’t take into account tornadoes spawned by the storms.

Hurricane Harvey in 2017 is one example of how the scale might fall short, says James Marshall Shepherd of the University of Georgia’s Department of Meteorology. In its early days, that storm brought severe rain to Texas. Yet on the Saffir-Simpson scale, the risk categorization at the time wasn’t even a 1. Dr. Shepherd notes that the storm nevertheless had devastating effects such as power outages before spinning into a Category 4 hurricane. 

Dr. Camargo says remnants of Hurricane Ida in 2021 had significant impacts on New York due to flooding and storm surge, not sustained winds. “There have been suggestions of different scales that proposed to integrate the risk of rain, the risk of storm surge, the risk together with the wind,” she says. “If you’re going to change the scale, I would think that ... would be a better way to do it.”