Category 4 or Category 5: One Step on the Scale, Nothing Like One Step in Damage
A Category 4 hurricane has sustained winds of 130 to 156 mph. A Category 5 starts at 157 mph. On paper that gap is one mile per hour wide at the boundary, and the categories look like steps on a staircase.
They are not steps on a staircase. Damage does not rise in proportion to wind speed, it rises far faster, which is why a scale that looks evenly spaced describes outcomes that are not evenly spaced at all.
And there is a second problem with the number, which matters more than most coverage admits. The Saffir-Simpson scale measures wind and nothing else. It does not measure surge, it does not measure rainfall, and it does not measure how large the storm is. Every one of those has killed more people than wind.
What the categories actually describe
The official descriptions are worth reading, because they are more specific than the shorthand.
At Category 4, the National Hurricane Center describes catastrophic damage. Well-built framed homes can lose most of the roof structure and some exterior walls. Most trees snap or are uprooted. Power poles come down. Fallen trees and poles isolate residential areas, power outages run weeks to possibly months, and most of the area is uninhabitable for weeks or months.
At Category 5 the language shifts from severe damage to destruction. A high percentage of framed homes are destroyed, with total roof failure and wall collapse. The rest of the description, the isolation, the outages measured in weeks or months, the area being uninhabitable, is the same.
Read those two side by side and the difference becomes concrete. Category 4 is a house that has lost its roof and probably some walls. Category 5 is a house that is not there. Both leave the region without power for a very long time, which is why the recovery timeline is similar even when the structural outcome is not.
One more thing about Category 5: it has no upper bound. The scale stops. A storm at 160 mph and a storm at 190 mph are both Category 5, and the category alone tells you nothing about which one you are getting.
That open top is why you periodically see arguments for adding a Category 6. The case for it is that the strongest storms now being recorded sit far above the threshold where the scale gives up, and lumping them together removes information. The case against it is that the practical advice does not change: at Category 5 the guidance is already to leave, and there is nothing stronger to tell people. Whichever side is right, the useful takeaway for a reader is simply that "Category 5" is a floor rather than a description.
It is also worth knowing that the categories are based on sustained wind, not gusts. Sustained means averaged over one minute at a standard height, which is a deliberately smoothed number. Gusts run well above it. So a Category 3 storm is not delivering 120 mph continuously and nothing more; it is delivering something around that on average with peaks considerably higher, and it is usually the peaks that take the roof.
Why the jump is bigger than it looks
Wind does not damage buildings in a straight line. The force wind exerts rises with the square of its speed, and the damage that force produces rises faster still, because structures fail at thresholds rather than degrading smoothly.
That is the physical reason a modest increase in wind speed produces a disproportionate increase in destruction. A roof holds, holds, holds, and then goes. Once the roof is gone the walls lose their bracing and the building is open to rain. A storm that is fifteen percent stronger does not do fifteen percent more damage.
It also explains a pattern people find confusing after the fact, which is that two towns hit by the same category can report very different outcomes. Local wind speed varies with terrain, exposure and which side of the storm you were on, and if one town sat just below a failure threshold and the next sat just above it, the reports will not look like they describe the same storm.
So the practical reading of a category is not a percentage of severity. It is a rough band, and where you fall inside that band matters as much as the band itself.
The number does not include what usually kills people
This is the part that gets lost every season.
The Saffir-Simpson scale is a wind scale. It was designed to describe wind damage, and the National Hurricane Center has been explicit for years that it does not capture the other hazards. Storm surge, inland flooding from rainfall, and tornadoes spun off by the system are all outside it.
Historically, water causes the large majority of deaths in tropical cyclones, not wind. Surge drowns coastal areas, and rainfall floods inland ones that never saw dangerous wind at all. A storm can weaken from Category 4 to Category 1 before landfall, generate headlines about being downgraded, and still deliver a surge and a rainfall total that kill more people than a stronger, faster, drier storm would have.
The last twenty years of American storms make the point better than any argument. Katrina in 2005 came ashore in Louisiana as a Category 3, having been far stronger out over the Gulf. By category alone it was the weakest of the storms people remember from that decade. It killed roughly eighteen hundred people, and the mechanism was water: surge, and the failure of the levees that were supposed to hold it back.
Harvey in 2017 made landfall in Texas as a Category 4, which is the number people quote. Then it stalled. What destroyed Houston was not the landfall wind, it was rain that kept falling for days, with totals passing sixty inches in the worst-hit spots. The category described the first few hours of a storm whose damage was mostly done in the following four days.
Sandy in 2012 was not even a hurricane when it reached New Jersey. It had transitioned to a post-tropical system, which meant it carried no Saffir-Simpson number at all at landfall. It pushed water into New York Harbor, flooded subway tunnels and left large parts of two states dark. A scale that reported nothing described one of the most expensive storms in American history.
And Michael in 2018 is the counterexample that shows what the scale does measure well. It hit Mexico Beach in Florida at Category 5, and the wind did precisely what the Category 5 description says wind does. Buildings were not damaged, they were removed. When the number is high and the storm is compact and fast, the number is telling you the truth about your structure.
Size is missing from the number too. A compact Category 4 and a sprawling Category 2 can deliver very different amounts of total water, because surge depends heavily on how much ocean the storm has been pushing and for how long. The category tells you about the peak wind near the core. It says nothing about the footprint.
So how should you read the number. Use the category for one thing: deciding whether your structure is likely to survive intact. That is what it was built to tell you, and it tells you that reasonably well.
For everything else, read the specific products instead. Storm surge warnings and inundation forecasts tell you about water coming in from the sea. Flash flood watches and rainfall forecasts tell you about water coming down from the sky. Those are separate products for a reason, and in most storms they are the ones that determine whether you should leave.
And do not treat a downgrade as good news on its own. A storm dropping from 5 to 4, or from 4 to 2, has changed its wind. Whether it has changed the water depends on its size, its speed, the shape of the coast and the tide, none of which appear in the category.
The scale is useful. It is just answering a narrower question than most people think it is.
National Hurricane Center scale definitions: https://www.nhc.noaa.gov/aboutsshws.php
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