Why there no Category 6 hurricanes

Hurricane Irma captured over the Leeward Islands and Puerto Rico.
Image credit: NASA Goddard MODIS Rapid Response Team
Date:8 September 2017 Tags:, , , , ,

As Hurricane Irma barrels towards the United States mainland with 297 km/h winds that put it squarely in Category 5, reports of its record-breaking strength have coalesced into a rumour. Scientists are scrambling for a new classification, the story goes, and Irma could be the first Category 6 storm. Despite Irma’s unprecedented strength the rumour is false. There are no Category 6 hurricanes for a reason – it would be pointless to make the distinction.

By Eric Limer

Hurricane strength is rated using the Saffir–Simpson scale, first developed in 1971 by Herbert Saffir and Robert Simpson, a civil engineer and meteorologist respectively. While flooding can account for much of the lasting damage a hurricane can cause, the Saffir-Simpson scale is concerned solely with windspeed. It uses the max speed of sustained winds to organise hurricanes into the five established categories.

Hurricane categories:

  • Category 1: Very dangerous winds will produce some damage. 119 – 152 km/h winds.
  • Category 2: Extremely dangerous winds will cause extensive damage. 154 – 177 km/h winds.
  • Category 3: Devastating damage will occur. 178 – 207 km/h winds.
  • Category 4: Catastrophic damage will occur. 208 – 251 km/h winds.
  • Category 5: Catastrophic damage will occur on a large scale. 252 km/h or higher winds.

The potential for a Category 6 hurricanes seems obvious. Hurricane Irma’s 297 km/h winds are much higher than a Category 5’s lower bound, after all. It’s a moot point however, because the Saffir-Simpson scale is not designed to arbitrarily classify storms into tiers based on wind speed or some sort of abstract power level. The Saffir-Simpson scale is designed to reflect the damage a given storm will cause. The damage is measured against buildings and other man-made structures in its path. Category 5 is widespread, catastrophic damage. There’s not really anything worse than that.

Why there are no Category 6 hurricanes, explained

Dr. Simpson explained the lack of Category 6 (or 7!) storms in a 2001 interview with the Sun-Sentinel by putting it this way:

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 (251 km/h). That’s the reason why we didn’t try to go any higher than that anyway.

As the climate continues to warm, creating circumstances that lend themselves to ever-stronger hurricanes, maximum wind speeds seem likely to increase. This alone, however, would not necessitate the need for a Category 6 classification.

The only thing that could make a sixth designation useful, so far as the purpose of the Saffir-Simpson scale is concerned, would be an abundance of buildings constructed with advanced engineering that would let them withstand winds above 251 km/h, but only to a certain point. We simply don’t have that kind of infrastructure right now. Though if this year’s hurricane season is any indication, we sure could use it.

 

From: PM USA