When Hurricane Harvey hit the coast of Texas in 2017, its ferocious winds made it a Category 4 storm—the second-highest ranking in the five-category scale based on wind speeds. But it wasn’t the 130-mile-per-hour winds that did most of the storm's estimated $125 billion in damage at the time. That came from the five feet of rain that Harvey dumped on some areas after it had weakened to a mere tropical storm.
Harvey is just one of the many storms that have underscored the limitations of the Saffir-Simpson Hurricane Wind Scale, the system that has been used for decades to officially mark the strength of hurricanes in the Atlantic and northeastern Pacific Oceans. Wind can be incredibly damaging, and wind speeds are an important part of what distinguishes hurricanes as a meteorological phenomenon. But experts say that focusing too much on a hurricane’s category can leave people unaware of greater dangers posed by particular storms. This is especially the case as climate change exacerbates many hurricane hazards.
With the 2024 hurricane season brewing—and early forecasts that it could be a blockbuster—experts are considering other metrics to warn those in harm’s way and are advising the public to look beyond any given storm’s Saffir-Simpson ranking. “It’s a tool, but it’s, like, 1 percent of the story,” says Kim Wood, an atmospheric scientist at the University of Arizona, of any given hurricane’s category.
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The Saffir-Simpson scale’s simplicity is both its strength and its weakness. The system was originally developed by a structural engineer to set categories based on the damage that winds do to trees, buildings and infrastructure. Category 1 storms, with peak sustained winds between 74 and 95 miles per hour, can cause some damage to homes and trees, for example, with a couple days of interruption to local power systems. When facing Category 4 winds of up to 156 miles per hour, meanwhile, affected regions should expect “catastrophic damage,” with well-built houses losing roofs or walls or both, and fallen trees and power poles cutting off residential areas from aid.
But hurricanes are much more than their most powerful sustained winds, which only ever occur in a small portion of the storm very close to the central eye. Hurricanes unfold in time and space, and the damage they cause is shaped by local geography, the surrounding atmospheric landscape and, of course, human choices in where and how to build cities and towns. A very large part of the danger hurricanes pose is tied to water, from both the sky and the sea: some 90 percent of hurricane fatalities between 2013 and 2023 came from water hazards.
“Hurricanes aren’t just the wind; they’re not just the surge; they’re not just the rain,” says Phil Klotzbach, an atmospheric scientist at Colorado State University. “All these threats are coming from the same system.”
And that’s the real shortcoming of the Saffir-Simpson scale, many experts say: by only looking at one small aspect of a storm, it ignores everything else about a hurricane. “It’s impossible to boil the threats of a hurricane down to one number,” says Brian McNoldy, a hurricane researcher at the University of Miami.
Still, some scientists are trying to find a new single indicator of danger because they recognize the communicative power of simplicity. Klotzbach argues that the minimum pressure at the heart of a hurricane is both easier to measure and more reflective of the overall strength of a storm than peak wind speed, making it a promising candidate for a single-value scale. “It’s more of an integrated metric of how much juice the hurricane has,” he says. Other experts are developing scales that evaluate the whole suite of threats from a particular storm, then amalgamate them into one overall ranking.
Even as scientists wrestle with how best to evaluate different hurricanes, climate change is retuning the overall picture. It’s not clear yet whether climate change is increasing the number of tropical storms worldwide each year, but it does seem to be poised to lengthen the Atlantic hurricane season beyond its traditional six months.
And experts agree that climate change appears to be making storm hazards worse. Some of that is because of background factors: as climate change causes sea levels to rise, coastal communities see more flooding from a given level of storm surge. In addition, climate change is making heavy rainfalls more common in all kinds of storms, which increases flood risks—all beyond the scope of the Saffir-Simpson scale. “The hazards that a storm produces do seem to be getting worse,” says Robbie Berg, warning coordination meteorologist at the National Oceanic and Atmospheric Administration’s National Hurricane Center.
There is some evidence that climate change is leading toward more of the most intense storms, and some scientists have proposed that a “Category 6” be added to the Saffir-Simpson scale. But others note that Category 5 already indicates “catastrophic damage” that leaves most of an area “uninhabitable for weeks or months,” according to the National Hurricane Center definition. “I don’t see why you would need anything more than that,” Klotzbach says.
One particular aspect of increasing wind threats is that climate change is leading more hurricanes to undergo what experts call rapid intensification, in which peak sustained winds increase by at least 35 miles per hour within 24 hours. When rapid intensification occurs close to land, communities can be left with less time to evacuate. And although these storms’ winds sometimes weaken just as rapidly, the storms can grow larger during the process, which lets them drop more rain on more people.
For now NOAA is sticking with the Saffir-Simpson scale but putting emphasis on the specific hazards of each individual storm rather than its category. For example, in recent years the agency began to issue formal storm surge watches and warnings to emphasize the threat of water pushed inland when a storm hits the coast. This August the National Hurricane Center will start using new graphics that emphasize inland risks in its map of where a storm is headed, rather than only coastal ones.
“It’s about the impacts, not the category,” National Weather Service director Ken Graham said at a May 23 NOAA briefing held to unveil the agency’s early outlook for the 2024 Atlantic hurricane season, which formally runs from June 1 to November 30. It’s a daunting forecast, with between 17 and 25 expected named storms, the most the agency has ever predicted this early in the year. Between eight and 13 of these are expected to become hurricanes.
“All the ingredients are definitely in place to have an active season,” Graham said, adding that scientists are particularly concerned by the combination of abnormally warm ocean temperatures and the planet’s expected transition into a La Niña climate pattern in the coming months. During La Niña, waters in the tropical Pacific Ocean are cooler than normal, which changes where heat is released into the atmosphere and alters weather patterns, including in ways that tend to favor more hurricanes in the Atlantic basin.
That said, the prediction is just that—a prediction, not a destiny. “The background conditions are indicating a high likelihood of activity, but any one storm depends on the conditions on that day and what that disturbance is doing,” the University of Arizona’s Wood says.
Even when a storm does form, the threat it poses depends on where—and whether—it makes landfall. But because that is impossible to know more than a few days out, experts warn people to focus now on their own preparations for hurricane season. “You don’t really care if you heard it’s going to be an inactive season if you get slammed,” says Jennifer Collins, a hurricane expert at the University of South Florida.
And if a lower-category hurricane does approach, don’t write it off. “Look beyond the category,” Berg says. “Dig deeper into the story to really understand what your risk is for that particular storm.”