Thursday, September 07, 2017

Hurricane Irma, warm oceans, and expanding the Saffir-Simpson Hurricane Scale

Back in 2011, I wrote about the current five-category hurricane system that the US uses (known as the Saffir-Simpson Hurricane Scale), noting that the foundational logic of the scale was based on structural engineering questions:
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'."
The current scale tops out at a "Category 5," which is any sustained wind speed above 155 mph. However, if one uses the threshold values for Categories 1 through 5 to develop a regression equation, it is possible to extend this relationship ever outward. Specifically, a revised category scale would be something like this:
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
Back in 2011, Hurricane Camille had sustained wind speeds of 175 mph, which is what prompted me to write that post. Currently, Hurricane Irma is reported as having sustained wind speeds of 185 mph, making it the strongest Atlantic hurricane in recorded history. However, based on the current hurricane scale, both Camille and Irma are classified as Category 5, even though Irma is obviously far stronger than Camille (which was - itself - a massive hurricane).

Indeed, the current system is fundamentally limited and fundamentally limiting, since one loses any sense of comparative scale once you enter "Category 5." And what would it hurt to look at adding a "Category 6," especially if warming waters are known to lead to stronger and more sustained hurricanes? Indeed, with warming oceans, hurricanes that will reach sustained wind speeds between 175 and 205 mph will not be theoretical. Indeed, Hurricane Irma is proof-positive that such hurricanes can and will form.

But so what? Why would that matter?

Well, in the US, the SSHS is a widely known and used shorthand for hurricane strength. It's something that people latch on to when discussing preparedness measures and when making comparisons against past events. But if the maximum scale is effectively open-ended, the designation "Category 5" will be shared by a hurricane with wind speeds of 155 mph and another with winds speeds of 185 mph (like Hurricane Irma). And the simple fact is that wind speeds of 185 mph are fundamentally different than wind speeds of 155 mph, and placing both in the same open-ended category will not help with making short-hand comparisons that would be equivalent to comparing a Category 4 hurricane against a Category 3 hurricane.

The way we categorize natural phenomena is important, since it structures the way that we view and respond to the world, and if we continue to use a hurricane classification system whose comparative utility declines into a future that is expected to have stronger hurricanes, that can impact the type of public response given to future storms.

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