Hurricane category ratings can be misleading. Check out these deadly ‘minor’ storms.

On its way to the North Carolina coast, Hurricane Florence weakened, yet simultaneously meteorologists warned that it was becoming more dangerous. That might seem counterintuitive if you assume that the category system for hurricanes is based on severity. It’s not. It’s based on just one variable: wind speed.

But wind speed isn’t the only—or even the most important—variable in what makes a hurricane destructive. Part of the equation is, of course, where it hits. Plenty of hurricanes bluster through the middle of the Atlantic with little fanfare, since they’re far enough away that they don’t really affect us. And wind speed is, of course, a factor. But most of our most recent dangerous hurricanes have been all about water.

Storm surges—when the ocean rises rapidly due to changes in pressure around it—are, in part, driven by wind speed, but they’re more directly determined by how large the cyclone is. Some storms may have 135 mile per hour winds, enough to be a Category 4 system, but the area in which the wind reaches that speed may be small. Conversely, a hurricane with slower winds that covers a much larger area is likely to do more damage. Take Hurricanes Ivan and Dennis. These Category 3 storms hit roughly the same part of the Gulf Coast one year apart. Dennis, which struck in 2005, caused approximately $2.2 billion in damage. One year before, Ivan had caused $14.2 billion. When meteorologists analyzed the differences between these two storms, they found that Ivan caused more than six times the damage because it covered five times the area, and thus could last much longer over land. This means a greater swatch of the U.S. experienced hurricane-force winds and torrential rain for longer periods of time.

That’s reminiscent of what made Harvey such a destructive storm for Houston: instead of moving quickly, it stalled over the area, dumping unprecedented volumes of water. That’s also what meteorologists are predicting will happen with Florence.

Total destruction, both in terms of dollars and casualties, also has to do with the population density of the area and the infrastructure in place. Katrina, of course, became a much more deadly storm when the levees failed in New Orleans, which at the time had 450,000 residents. All these factors contribute to how loose the correlation is between hurricane category and destruction.

The graph above comes largely from a single report analyzing all hurricanes from 1900-2010, in which researchers at the National Oceanic and Atmospheric Administration worked out what made certain storms particularly destructive. They found that the vast majority of the deadliest cyclones were of at least Category 3, but only a third were at 4 or above. (Part of that has to do with the fact that Category 4 and 5 storms are more rare). They also discovered that a lot of the costliest storms in terms of damage occurred as a result of inland flooding due to rain, whereas high death tolls were mostly the result of large storm surges. Hurricanes like Katrina, which have storm surges around 10 feet or more, kill large numbers of people who simply cannot fight the rushing waters.

Troublingly, the researchers also noted how likely people are to ignore evacuation orders, thinking they can outlast the storm. “Sociologists estimate, however, that people only remember the worst effects of a hurricane for about seven years,” they wrote, and that even if they did remember, other researchers have estimated that “85% of U.S. coastal residents from Texas to Maine had never experienced a direct hit by a major hurricane.” This is largely because these coastal areas have experienced high population growth.

Inexperienced and experienced coastal-dwellers alike might do well to watch this clip from the National Weather Service, which shows what storm surges actually look like.

The latest iteration of our IMR group's work. This is what storm surge looks like. #Florence will make landfall in the next 36-48 hours and bring with it, 6-9 feet of potential storm surge. @parkertwc will show you what that looks like. @LocalNow @weatherchannel #NCwx #SCwx pic.twitter.com/mG9JjOOJeM

— Ryan Davidson (@RyanDavidsonWX) September 13, 2018

Unfortunately, researchers note that even increased awareness isn’t enough to prevent hurricanes from becoming more costly and deadly. “Continued coastal growth and inflation will almost certainly result in every future major landfalling hurricane (and even weaker hurricanes and tropical storms) replacing one of the current costliest hurricanes. For example, all three of the U.S. hurricane landfalls of 2008 made the top 30 list, despite none of them being major hurricanes at landfall.” As long as people build and live along the coast, we’ll have high death tolls and repair costs, especially since hurricanes are likely to become more intense as climate change progresses. Hurricanes are not just big, exciting rainstorms. They’re dangerous—even if they’re only a Category 1 or 2.