Why Are Hurricanes Classified by Category?

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Robert Simpson was just a kid in 1919 when a devastating hurricane hit his home of Corpus Christi, Texas. It was a Sunday, so he was at home with his family when the storm flooded the roads with water six to eight feet above street-level.

“The family had to swim—with me on my father’s back—three blocks in near hurricane force winds to safe shelter in the courthouse,” Simpson recounted in 1989. “A lot of what I saw frightened me, but also supplied a fascination that left me with a lifelong interest in hurricanes.”

Simpson went on to become a meteorologist and director of the National Hurricane Center from 1967 to 1973. However, today he’s best known for developing the Saffir-Simpson Hurricane Wind Scale that we still use to designate a storm as a Category one through five.

Developed in the early 1970s, this widely used scale measures hurricane wind speed. A Category one starts out with a wind speed of 74 to 95 miles per hour. A category two ranges from 96 to 110 miles per hour; category three storms have winds at 111 to 129 miles per hour; category four extends from 130 to 156 miles per hour. Anything above 157 miles per hour is a Category five storm.

The scale does not currently account for rainfall or storm surges, which has led some to wonder in the wake of powerful natural disasters if we need to develop a different way to categorize hurricanes. After all, it wasn’t the wind speed that made Hurricane Harvey so dangerous—it was the storm surges and 50-plus inches of rain.

Despite the scale’s inadequacies, it is a lot better than what came before. Prior to its adoption, meteorologists would describe a hurricane’s anticipated strength by comparing it to past storms that had swept through the area. But Simpson wanted to give the public an easier way to understand the strength of the hurricanes barreling toward them, says Neil Frank, the meteorologist who seceded Simpson as director of the hurricane center in 1974.

“He was very sensitive to being able to communicate to the public in meaningful terminology,” Frank says. Simpson thought a scale would allow people to make judgments about how they should respond to a hurricane based on how powerful it was.

Satellite image of Hurricane Irma along the coast of Cuba as it heads toward South Florida as a Category 5 storm. (Credit: Planetpix/Alamy Live News)
Satellite image of Hurricane Irma along the coast of Cuba as it heads toward South Florida as a Category 5 storm. (Credit: Planetpix/Alamy Live News)

Simpson began by adapting a scale that Miami engineer Herbert Saffir had developed in 1969. “Originally, it was designed to describe damage to buildings—it had nothing to do with wind,” Frank says. Simpson assigned each of Saffir’s categories a certain range of wind speed and level of storm surge to make a hybrid scale, called the Saffir-Simpson Hurricane Scale.

But storm surges don’t actually correlate to wind speed, and scientists removed storm surge measurements from the scale around 2010. Phil Klotzbach, an atmospheric science researcher at Colorado State University, says that Hurricanes Charley (2004), Katrina (2005), and Ike (2008) probably contributed to this change, since their storm surges didn’t line up with their wind speeds in the way the scale said they should.

A hurricane won’t create the same amount of flooding everywhere it goes because storm surge levels depend on specific geographic factors like how high a city sits above sea level. Since storm surges are extremely dangerous, critics of the current Saffir-Simpson scale say that we should alter or amend it with new categories that will let people know how bad the storm surge is expected to be in their area.

In 2012, Weather Channel meteorologist Carl Parker speculated about what this might look like: “If, for example, you have a Category one hurricane that has a surge value that is a four out of five, I think the public can really draw something from that.” Other meteorologists, like Frank, believe changing the current system might cause confusion—many people already focus more on the category designations than on other factors that may impact a city.   

“There’s kind of this break in Category two and three,” where everything at three and above is designated as a “major hurricane,” says Klotzbach.

“People will say, ‘Oh, it’s only a category two, therefore it’s not a major hurricane,’” he continues. “Well, any hurricane can do massive amounts of damage regardless of the category. And so I think sometimes you have to kind of break these things down.”

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