For decades, Americans have made life-or-death choices based on one number: the hurricane category. It’s the number you see plastered across your screen when a storm is on the way – up to Category 5.

It feels straightforward. But what seems simple can be deceptive, and in this case, dangerously so.

The current warning system, known as the Saffir-Simpson Hurricane Wind Scale, only considers wind speed. The system completely ignores storm surge and rainfall, which actually cause most hurricane-related deaths.

The disconnect between how we rate storms and how they truly behave is confusing, and confusion during a hurricane can be fatal.

Hurricane numbers are misleading

Most people assume a lower-category storm isn’t a big deal. Category 1 sounds harmless and Category 2 seems manageable. But some of the deadliest storms in U.S. history weren’t high on the wind scale at all.

In 2005, Hurricane Katrina registered as a Category 3 when it came ashore. However, the majority of its 1,800 fatalities and $125 billion in damage were due to flooding and storm surge – neither of which has to do with wind.

In 2018, Hurricane Florence made landfall as a Category 1. Yet the storm caused devastating flooding that killed 55 individuals.

Edward Rappaport is a former deputy director of the National Hurricane Center. In a 2014 study, he reported that wind kills only eight percent of hurricane victims. Storm surge kills 49 percent, and rain kills 27 percent. The current system doesn’t capture the full story.

Hurricane scale with real warnings

Now, a team of researchers has come up with a better way to rate hurricanes. It’s called the Tropical Cyclone Severity Scale (TCSS), and it gives each storm a score based on three factors: wind, storm surge, and rainfall.

Each hazard is scored on a scale from 1 to 5. The scores are then combined to produce an overall rating. If one hazard dominates, the rating reflects that. But when multiple hazards are severe, the scale can climb as high as Category 6 to capture the total threat.

So, for instance, if wind and rain are both a 2, but storm surge is a 4, the storm is at least a Category 4 overall. And if two out of the three hazards are a Category 3 or higher, the overall score increases even more.

The new scale isn’t just about recalculating the data – it’s about conveying risk more clearly and effectively.

How people react to hurricane warnings

The Tropical Cyclone Severity Scale was co-created by Jennifer Collins, a hurricane researcher and geosciences professor at the University of South Florida.

In collaboration with international partners from the University of Amsterdam and Tilburg University, Collins has spent years studying how people react to hurricane warnings.

“Frequently, people use the storm’s category to decide whether to evacuate,” Collins said. “That’s incredibly dangerous because if they hear it’s only a tropical storm or Category 1, too often no alarm bells go off, and they see no cause for concern.”

Testing the new hurricane scale

The new system was tested in a major online experiment. Thousands of residents from coastal areas were shown mock hurricane warnings. Some were based on the old wind-only system, while others were based on the new scale.

The results were clear. People who got the TCSS warnings were more likely to understand the real danger – and more likely to evacuate if they saw high rainfall or storm surge threats.

“We were interested in whether better identification of the main hazard would improve precautionary measures, and whether the overall category difference would increase evacuation intent for more dangerous storms,” noted the researchers.

“The higher category is important,” Collins said. “According to my evacuation research, many people base their decision to evacuate on that number, not just on the details of the hazard.”

The old scale needs an upgrade

The original hurricane scale was introduced in 1971. Back then, it included wind, storm surge and pressure. But by 2012, it had been stripped down to just wind speed, and renamed the Saffir-Simpson Hurricane Wind Scale.

That change may have simplified things. But it also made the scale less useful and more deceptive.

“Without satisfactory and complete information, individuals are likely to miscalculate their personal risk or even be moved to inaction,” said the researchers.

A more realistic hurricane scale

Collins, who moved to Florida in 2005, started focusing on hurricane evacuation behavior after major storms like Matthew in 2016, Irma in 2017, and Ian in 2022.

More recently, hurricanes Helene and Milton in 2024 gave her team fresh data on how people respond to storm warnings.

In 2017, Collins connected with co-author Nadia Bloemendaal in the Netherlands. Their shared interest in human behavior and hurricane risk led to the creation of the TCSS.

The researchers published their first paper in 2021. Now, after years of testing, they’re preparing to take the idea to the National Hurricane Center.

“Change is hard for any institution that’s been doing the same thing for years,” Collins said. “But I’m fairly optimistic that now is the time.”

“We now know many people make decisions based on the category messaging, so we need to ensure that we are communicating with a scale which is more realistic of the severity of the hurricane.”

The full study was published in the journal Scientific Reports.

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