The Saffir/Simpson Hurricane Scale describes five distinct categories for Hurricanes. The name is of course rooted in who it all came from. Back in 1971, Herbert Saffir and also Robert Simpson created it as a simple way to communicate the impact that a Hurricane was about to have and so assigning a specific category set expectations regarding the potential damage that would be inflicted when the storm hits land.
Initially the scale included wind speed, storm surge and flooding, but in 2009 the US National Hurricane Center simplified it to just describe wind speed because including that turned out to not be very meaningful.
What was the reason for the creation of this scale?
During an interview in 1991, Robert Simpson laid out the thinking behind it as follows ..
The problem of evacuating people and getting warnings out that are understood and which will evoke a response in the people who need to move has always been a difficult one. When I first came down to the Hurricane Center in 1967, I tried to come to grips with how we could do a better job of communicating. And that’s very difficult; scientists communicate with each other very easily, but a scientist trying to communicate with a person who is a non-scientist on a technical problem is very difficult at times.
So it occurred to me if we could find some means of expressing the gradations of risks that people have in a hurricane, it would help people like the American Red Cross and the Emergency Management people to decide how best to make their decisions and to deal with the people they were responsible to. So I was talking to Herb Saffir (in 1968) about work that he had been doing and had just completed for the United Nations. He had completed something in the way of a summary of what you could expect in the way of ornamental damage and basic damage to structures with winds of different strengths. I said this is probably, put in a different suit of clothing, exactly the type of thing we need but we’ll have to add the storm surge to it and a few other things. So I took on the job of working with him to get this thing put up in a new suit of clothing that we could then distribute to people, like the American Red Cross, who have to provide disaster relief when it’s all over.
It was used that way for a couple of years before I left the Hurricane Center in 1974. Then the year after that when Neil Frank became the director, the pressure was put on him to distribute this to the public. I often felt that it was a little bit premature to put the scale out without perhaps improving it a little bit, and at least educating the people as to what it meant a little bit more. But politics and the situation was such that when people want something they want something, they’re going to get it whether they know how to use it or not. So, I think that through the years it served a very good purpose for a lot of people. It’s been misinterpreted, misused in a lot of places, but almost any device which is technical is. And the main difference in making it a equally useful thing to everybody is education, and telling them what it amounts to.
The scale as devised, expresses what the extreme conditions can be expected from a hurricane of a certain type and a certain category. It doesn’t mean that everyone that a hurricane moves over, and the worst part of that hurricane, is going to receive that kind of damage or that kind of hazard. In other words, it’s a study in probabilities-the probability of being hurt. And why is that? It’s a great big storm, why isn’t there a uniform amount of damage that you get? And if you’ve ever surveyed damage after a hurricane you know that one block of houses may be almost totally destroyed, and two blocks to either side there will be little damage at all.
It’s almost like a tornado. It’s not a tornado, but what is happening is it’s not a uniform bowl of pudding that’s circulating around here. It’s something that has lots of streaks in it, and the streaks are made by the cumulus clouds that are embedded in this great big storm. And as these cumulus clouds circulate around, they’re relatively small. Some of them are no more than a couple of kilometers across and maybe four of five kilometers long. That means that just a few blocks to one side or to the other side of where this cumulus cloud is providing the extreme wind, you have much less than the extreme, and therefore get no damage at all that’s comparable on either side of it. So, there’s several problems. The problem is first, expressing to the people who have to leave that it’s a matter of probabilities, but if they don’t believe that they’re going to be in the worst sector and receive the worst damage or hazard, then they’re playing Russian Roulette. They have to assume the worst and act accordingly. Others are engineers who brag about the fact that the house or building that they engineered received no damage, and another engineer whose building received a lot of damage tries to explain why it did, because he knows he engineered it right. There isn’t that understanding, and it’s difficult to understand that it’s the difference in the hurricane, not the difference in the engineering that caused the difference in the amount of damage received.
What are the Five Categories?
The official National Hurricane Center has the following descriptions.
The scale basically estimates potential property damage. Hurricanes reaching Category 3 and higher are considered major hurricanes because of their potential for significant loss of life and damage. Category 1 and 2 storms are still dangerous, however, and require preventative measures. In the western North Pacific, the term “super typhoon” is used for tropical cyclones with sustained winds exceeding 150 mph.
Category 1 – Very dangerous winds will produce some damage – 74-95 mph
Well-constructed frame homes could have damage to roof, shingles, vinyl siding and gutters. Large branches of trees will snap and shallowly rooted trees may be toppled. Extensive damage to power lines and poles likely will result in power outages that could last a few to several days.
Category 2 – Extremely dangerous winds will cause extensive damage – 96-110 mph
Well-constructed frame homes could sustain major roof and siding damage. Many shallowly rooted trees will be snapped or uprooted and block numerous roads. Near-total power loss is expected with outages that could last from several days to weeks.
Category 3 – Devastating damage will occur – 111-129 mph
Well-built framed homes may incur major damage or removal of roof decking and gable ends. Many trees will be snapped or uprooted, blocking numerous roads. Electricity and water will be unavailable for several days to weeks after the storm passes.
Category 4 – Catastrophic damage will occur – 130-156 mph
Well-built framed homes can sustain severe damage with loss of most of the roof structure and/or some exterior walls. Most trees will be snapped or uprooted and power poles downed. Fallen trees and power poles will isolate residential areas. Power outages will last weeks to possibly months. Most of the area will be uninhabitable for weeks or months.
Category 5 – Catastrophic damage will occur – 157 mph or higher
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.
What exactly do those wind speeds mean?
To precisely measure the category, the sustained wind speed needs to be determined. This should be measured at a height of 33 ft and is the average wind speed over a one minute period.
Should we have a Category 6?
Given the observation that Hurricane Irma has sustained wind speeds of 185 mph which is well beyond the Category 5 definition of greater than 157 mph then the question of having a Category 6 definition in the scale is once again popping up.
Many will have different opinions on this, however, the best response perhaps comes from one of the originators of the scale.
When Robert Simpson was asked this very question back in 1991 during an interview, he replied as follows …
DI: Dr. Simpson, in your opinion, since the Saffir/Simpson scale is an open ended scale, do you think that hurricane windspeeds could become a category 6 or 7?
RS: I think it’s immaterial. Because when you get up into winds in excess of 155 miles per hour you have enough damage if that extreme wind sustains itself for as much as six seconds on a building it’s going to cause rupturing damages that are serious no matter how well it’s engineered. It may only blow the windows out, but on the other hand, it can actually rupture the stairwells, the elevator wells and twist them, and it’s happened in many buildings so that you can’t even use the elevators after they’ve experienced this. So I think that it’s immaterial what will happen with winds stronger than 156 miles per hour. That’s the reason why we didn’t try to go any higher than that anyway.
The categories exist to give a heads up regarding what is coming.
Wind speeds of 157 mph or greater for about six seconds or more will result in significant damage or even total destruction of most buildings. Advising people that a hurricane is Category 6 and not just a 5 adds no additional information to the warning.
Once you get to a 5 then catastrophic damage is certain.