As climate change leads to more destructive hurricanes, some experts have proposed adding a “Category 6” to the Saffir-Simpson Hurricane Wind Scale, but not everyone thinks it’s a good idea.
For more than 50 years, forecasters at the National Hurricane Center used the Saffir-Simpson Hurricane Wind Scale (SSHWS) to classify hurricane strength. This scale, which ranks hurricanes from Category 1 to Category 5, is based on only one metric: maximum sustained wind speed.
That wasn’t always the case. Until 2012, the SSHWS also took central pressure and storm surge into account, but the NHC eliminated these factors to reduce public confusion. The trouble is, rising global temperatures are exacerbating multiple hurricane hazards, not just wind speed. In recent years, exceptionally intense storms, such as Milton, Patricia, and Typhoon Haiyan, have sparked a debate over whether it’s time to create a Category 6.
For this Giz asks, we asked multiple experts which side of that debate they’re on. While some are more open to the idea than others, all agreed that simply adding a Category 6 to the SSHWS isn’t the answer—even though several past hurricanes have exceeded Category 5 wind speeds. Instead, some argue that communicating hurricane risks in a warming world may require rethinking the scale entirely, while others believe the existing system should remain unchanged.Jennifer Collins
A professor in the School of Geosciences at the University of South Florida who co-developed an alternative to the SSHWS.
The current SSHWS—as the name implies—is just based on wind. Regarding that scale, my thoughts on a Category 6 is that it is not needed when a Category 5 on that scale would lead to total destruction anyway. There was a lot of discussion about this in the scientific community about a decade ago, and I believe that to be the general consensus.
Our newly proposed scale, the Tropical Cyclone Sensitivity Scale (TCSS), considers that wind generally only accounts for 10% of fatalities. Storm surge accounts for roughly 50% and rain about 30%. Our scale includes all three of these hurricane hazards, assigning each one a category between 1 and 5. Then, it gives an overall category which can never be lower than the highest category given to the hazards.
For example, Hurricane Florence in 2018 would be a Cat 1 at landfall for wind, a Cat 4 for storm surge, and a Cat 5 for rainfall. So, its overall score would be a Cat 5. If you consider the flooding and loss of life, I believe people who lived through it would agree that calling it a Cat 1—which the SSHWH did—does not adequately reflect the other hazards they experienced. People underestimate the risk of a low-category hurricane, or even a tropical storm, when they consider their evacuation decision, according to my previous evacuation research.
The proposed TCSS also reflects the high potential risk of two or more hazards. We consider a hazard high risk when its category is classified as a 3 or higher (equal to the definition of a Major Hurricane on the SSHWS). Whenever at least two high-risk hazards have the same category and the third hazard has a lower category, this bumps the hurricane’s overall category up by 1. So, a tropical cyclone with a Cat 3 score for both wind and storm surge, but a Cat 1 score for rainfall, would be classified as a Cat 4.
As such, a high-risk tropical cyclone can be classified as a Cat 6 on the TCSS in two scenarios.
