In reading books and articles written about a future in which we have to adapt to a certain amount of climate change, one is struck by the consistency of using New Orleans and Hurricane Katrina as an example that illustrates many of the social considerations that future planning must incorporate. This make sense, since we wouldn’t care about the effects of climate change if it were happening elsewhere (i.e., we don’t care about how the climate is or isn’t changing on the planet Xenu, nor how it is affecting what may be living there). This is analogous to the underlying meaning behind the question, “If a tree falls in the woods, and no one is there to hear it, does it make a sound?” (The question never made sense to me, because I grew up with the “strange” notion that sound is a byproduct of the motion of air, and so it doesn’t matter who may – or may not – be in proximity of any object that makes a sound; it will happen. Considering the implication of the question to be something more than a physical description didn’t dawn on me until my 20s.)
In the last days of August, 2005, the city of New Orleans was hit by the eleventh Atlantic hurricane, named Katrina. It had – prior to landfall – reached a “Category 5” condition, but by the time it made landfall, had diminished to “Category 3.” Still, due to several factors – the greatest of which were undoubtedly the catastrophic failure of the levees, the lack of a city-wide evacuation order, and the systematic loss of buffering wetlands and barrier islands – the city suffered widespread flooding, and the death toll shocked the nation and the world. Many of us know these basic facts about Hurricane Katrina; many of us know additional facts. However, my most recent reading of these facts piqued a question: “In a world that is warmer, how strong might the hurricanes get?” From that, as series of other questions, including, “Why is ‘Category 5’ the strongest category?” or, to put it in another way, “Why are there only five categories?”
Looking online, it is easy to discover the basic facts of the “Category” system (called the Saffir-Simpson Hurricane Scale, or SSHS for short): there are five enumerated categories, as well as additional classifications of “tropical depression” and “tropical storm” that below the “Category 1” hurricanes; that Category 1 hurricanes have wind speeds of 74-95mph, Category 2 of 96-110mph, Category 3 of 111-130mph, Category 4 of 131-155mph, and Category 5 of >155mph; and that the system is used by the US (other places may use other systems). Doing a query for the fastest wind speed ever recorded in a hurricane, I found that Hurricane Camille (1969) maxed out the meter at around 205mph; 50mph faster than the minimum for a Category 5 hurricane, which is a much greater interval difference than any other category. True, Hurricane Camille-strength equivalents do not often show up in recorded history, making them a statistical rarity, and it would therefore make sense that their consideration might not have been deemed “necessary” at the time when the SSHS was conceived in 1971 – even though it was derived after Camille.
As a side note, looking at the historic hurricane data from NOAA, there have been 77 weeks – in the corrected data record – during which there has been a hurricane with wind speeds at or exceeding 155mph, 48 weeks of which exceeded 160mph, 12 weeks of which exceeded 170mph, and 3 weeks of which exceeded 180mph. As of the writing of this entry, the NOAA site has records for 1949-2010 for the Eastern North Pacific and for 1851-2010, with possibly some years missing (i.e., check the website for the most up-to-date stuff). For example, I cannot find the data for Camille in the NOAA data that I used to get these hurricane wind speed frequencies. However, even looking at the historic hurricane record of the waters near the US, it is pretty clear that there have been times when the winds have been significantly stronger than 155mph. In these conditions, wouldn’t it be better to have other categories in order to compare and discuss differences between hurricanes?
Now back to my original train of thought.
If we are going to be approaching a world in which hurricanes will be stronger, then wouldn’t it be likely that Camille-class hurricanes will be more likely? If so, then why not extend the range into Category 6 and possibly beyond? I mean, just because one has not yet encountered such a hurricane doesn’t mean that one cannot set aside a classification for it. Helpfully, the Wikipedia page for the HHSH presents a formula for deriving the maximum wind speed for Categories 1 through 4 (remember that Category 5 has no stated maximum) as: 83x10^(c/15) mph, where c is the category number, rounding the final number to the nearest multiple of 5. (One thing to note is that if you plug in c=4, then you will get a value of 147.8mph, which, even when rounding to the nearest multiple of 5, will only get you 150mph, which is kind of screwy. In my calculations, I will use the calculated value of 150mph, instead.) The resulting Categories 1-7 are:
Category 1: <95mph
Category 2: 96-110mph
Category 3: 111-130mph
Category 4: 131-150mph
Category 5: 151-175mph
Category 6: 176-205mph
Category 7: 206-235mph
If we assume that Camille did, indeed, exceed the 205mph that was recorded before the measuring device broke, then it would place that hurricane just inside Category 7; two categories higher than what the current system can allow, and thus making it possible to show just how much stronger those winds were. Furthermore, using 150mph as the corrected break point between a Category 4 and Category 5 hurricane, this would mean that there were 114 weeks of hurricanes at or exceeding 150mph! (Recall, too, that some data are – at this point – missing from the historical analysis, so it is likely that the true number is larger.) Of these 114 weeks, 5 of them would be classified as Category 6, and – because of the incomplete data set – none exist in Category 7 (save anecdotally for Camille).
Being able to say that there had been at least one Category 7-hurricane as well as at least five weeks of Category 6-hurricanes presents a very different description of hurricane activity than saying that they are all Category 5s. Furthermore, with wind speeds of future hurricanes expected to increase due to increased sea temperatures, we may well need to start adding higher categories as a normal course of action.
UPDATE: Looking at Wikipedia, someone had also listed a Category 6 criticism, with a quote from a former NOAA hurricane center administrator and co-inventor of the SSHS that, "there is no reason for a Category 6 on the Saffir-Simpson Scale because it is designed to measure the potential damage of a hurricane to manmade structures. If the wind speed of the hurricane is above 155 mph (249 km/h), then the damage to a building will be 'serious no matter how well it's engineered'." However, this sounds like the wrong logic. Why not have a Category 6, 7, 8 or more, since the metric isn't potential damage to structures (which might be expressed as a percent), but is rather wind speed. The implication of a wind speed is that there will be concomitant structural damage, but wind speed is not a measure of structural damage. Furthermore, the presumption is that it is impossible to build a structure to withstand the occasional 155mph sustained wind. Once people build structures that can withstand 155mph of sustained wind, the whole premise of the "argumentation" falls apart, since now there's a structure that is well-enough engineered to withstand the high winds. Hey, granddad! Technology progresses! I know that you invented the SSHS, but don't hobble us with your generational thinking and personal biases!
UPDATE: The Taipei 101 tower is a 101-floor skyscraper that was designed to withstand sustained 136mph winds. The Gateway Arch in St. Louis is designed to flex 18 inches in sustained 150mph winds. Furthermore, according to one site, "Most steel buildings that are engineered to resist hurricane damage can withstand wind forces up to 170 mph." (This claim can be seen elsewhere on the interwebs.) Sorry, Bob, what were you saying about buildings not being able to withstand 155mph winds? Tell us again, using your incomplete and now falsified reasoning, as to why we don't need a Category 6?