Hurricanes are categorized according to the Saffir-Simpson hurricane wind scale, which organizes tropical cyclones with winds greater than 74 mph into categories from 1 to 5. Each category comes with a description of what damage can be expected in an area that the center of the storm passes over. Occasionally, I see talk of a need to add a 6th category to the Saffir-Simpson scale. Some recent examples of this come after Hurricane Patricia (2015) managed to get itself to 215 mph winds (see this article / archive) and more recently as Hurricane Irma barrelled through the Lesser Antilles with 185 mph winds. Some of these posts are generated as click bait, while others are giving a legitimate argument to creating a 6th category. Well no, we do not need to add it, but before we get into that let me give you some background.
The Saffir-Simpson scale was created in the 1970s by a civil engineer (Herbert Saffir) and the then-current director of the National Hurricane Center (Robert Simpson). Saffir was on commission for the United Nations to work on a project that studied low-cost housing in regions prone to hurricanes. His scale originally just took wind speeds into consideration, but when presented to Simpson at the National Hurricane Center he added the effects of storm surge and flooding. The storm surge and flooding stipulations were dropped in 2009, bringing it back to just wind. Later, small modifications were made to the wind to make the categories more consistent when the NHC rounds the wind speed values and converts them to knots.
To help understand why we don’t need to add Category 6, I enlisted the aid of three meteorologists who all provided me their thoughts. Be aware that these statements do not necessarily reflect the view of their employers or their parent companies. These meteorologists are:
– Damon Lane (archive), chief meteorologist at KOCO-TV in Oklahoma City, Oklahoma
– Scott Sistek (archive), meteorologist at KOMO-TV in Seattle, Washington
– Charlie Phillips, meteorologist at Avangrid in Portland, Oregon (Charlie also runs his own WeatherTogether blog)
Going a little off track here, we measure tornadoes using the Enhanced Fujita scale. The EF-scale is similar to the Saffir-Simpson scale in that it categorizes tornadoes based on their wind: from EF-0 to EF-5. Also like the Saffir-Simpson scale, EF-5 doesn’t have a cap. An EF-5 is just any tornado with winds exceeding 200 mph. The EF-scale replaced the original Fujita-scale, which originally went clear to F-12, which was the speed of sound. When this scale was created, little was known about the winds inside a tornado. Fujita (the originator of the scale) marked the description of an F-6 as having inconceivable damage. The purpose of the scale was to estimate wind speeds that occurred within a tornado, which is the opposite of the Saffir-Simpson scale’s purpose.
Damon Lane offered me this description for a Category 5 hurricane:
“Catastrophic damage will occur: A high percentage of framed homes will be destroyed, with total roof failure and wall collapse. Fallen trees and power poles will isolate residential areas. Power outages will last for weeks to possibly months. Most of the area will be uninhabitable for weeks or months.”
As well as this description for an EF-5 tornado:
“Total destruction of buildings. Strong-framed, well-built houses leveled off foundations are swept away; steel-reinforced concrete structures are critically damaged; tall buildings collapse or have severe structural deformations; some cars, trucks and train cars can be thrown approximately 1 mile”
Reiterating what we’ve already stated, Phillips told me “The Saffir-Simpson scale was created to express the damage potential from a hurricane as a function of wind speed.”
All three meteorologists offered the same sentiments as far as damage categories are concerned. “There’s not much of a reason to go above this when Cat 5 has language as damaging as it is. It would be nearly impossible for a storm to produce anything higher than ‘catastrophic damage,'” Lane said in his email. Furthermore, Sistek told me this in his reply: “I’m not sure what emergency planners could or would do differently if told there was a Category 5 or Category 6.”
Phillips did offer me some criticisms of the scale, though. This criticism is not directly related to the Category 6 issue, can give further background to improvements that could be made in general. In his reply, he discussed how Hurricane Katrina (2005), a category 3 at landfall, was larger and more destructive than Hurricane Andrew (1992), which made landfall as a category 5. Andrew had incredible wind but was much more compact than Katrina was. Another criticism is that, unlike the Moment Magnitude Scale we use for earthquakes, the Saffir-Simpson scale is tiered. This means that with earthquakes, we can have ratings such as seen with the 2001 Nisqually earthquake, rated at a 6.8. Under the Saffir-Simpson scale, meteorologists can only use whole numbers. Phillips said, “there is a massive difference between a Category 4 storm with 130 mph winds and one with 155 mph winds, yet there is relatively little difference between a Cat 4 with 155 mph winds and a Cat 5 with 160 mph winds.”
Responding to the need for a more specific scale for hurricanes, Dr. Kantha, a professor at the University of Colorado at Boulder, developed two alternative scales to the widely used Saffir-Simpson scale. While my purpose in this blog post is specific to whether we should add a Category 6 to the current standard or not, readers might be interested in them. They are the Hurricane Intensity Index and the Hurricane Hazard Index and details on the argument behind using them can be found in this article. Coming back to the original topic, though, Phillips responded “Saffir and Simpson saw a Category 5 as ‘complete devastation,’ and adding a Category 6 wouldn’t make sense. That’s not to say there isn’t a massive difference between 157 mph winds and 195 mph winds… It’s just that 157 mph winds are all that is needed to completely devastate a community.”
Well, if we add a Category 6, that’ll give the public more urgency when it comes to preparing for and evacuation ahead of an incoming storm. I go back and forth on this one. Some people would take a Category 6 more seriously initially, but the novelty of the new category would probably wear off the general public within a decade. Beyond that, “if you’re foolish enough to think you’re going to ride out a Category 5, I’m not sure having a Category 6, Category 10 or Category Zillion will change your mind,” Sistek remarked. No matter the size of the storm, there will always be people who chose to stay for various reasons. The only thing that will change this is a change in mindset.
We live in a time were, at least in the United States, people move away from home frequently. In the Greater Miami area, about two million people have moved to their metro area since Hurricane Andrew ravished the area. Thousands of these are people from areas not prone to hurricanes. Adding a 6th category would almost be detrimental to these people. If a Category 6 is added, that diminishes the value of a Category 5 similar to how some people today say “oh, it’s only a Category 3.” No, rather than adding a new category to instill more urgency, those living where hurricanes can strike need to familiarize themselves with hurricanes and have a plan in the event that one comes their way. This goes for people living in places we think of as being “hurricane prone,” such as Miami and New Orleans, but this also goes for regions that rarely see hurricane strikes such as Boston and Nova Scotia.
I’ll leave you with this final thought from Lane: “There needs to be a limit and the science behind our current limits is sound and just fine. If you do decide to make an EF6 or a Cat 6 then do you stop there or make room for a Cat 7?” Whether your reasoning for adding a 6th category is because there seem to be stronger hurricanes than before, because you want more public urgency or some other reason the fact still stands that complete destruction is complete destruction whether it’s 157 mph winds or 200 mph.
Edit: After this article was published, the National Hurricane Center determined that Hurricane Irma made landfall with 180 mph winds, rather than the 185 that was initially reported by them and subsequently referred to in this post.
