This question is derived from Hurricane Melissa, whose strength is shocking given the late time of year.
I’m posting this in “Factual Questions” because there are numerous conditions that govern the strength of hurricanes, and I don’t know whether or not those conditions could produce a Category 6 hurricane.
What is the official definition of a Category 6 Hurricane?
Good question.
" Definition: A Category 6 would start at winds greater than 192 mph
This is not a meteorological question but a definitional one. The Saffir-Simpson scale has no upper limit to Category 5; it’s any hurricane with wind speeds of 157 mph or greater.
Maybe in the future if hurricane intensity increases due to much warmer sea surface temperatures caused by climate change, it might make sense to define a new upper category, but no category above 5 exists today.
Hypercanes with 500 MPH sustained winds could form given the right conditions, and may have actually formed in the past. (For instance, on a bad day 66 million years ago.)
Rather than trash the question, let’s set that hypothetical speed at 192 MPH. Could conditions on the earth allow for that? For example, I’ve read that straight line winds are limited by the rotational speed of the earth.
I’m interpreting this as, “Yes, but it would take extraordinary circumstances for it to happen.”
215 MPH:
Of course, hurricanes are not “straight line winds”. Hurricanes with wind speeds greater than 192 mph have already occurred (e.g.- Patricia in 2015). The real question is how often will these super-strength hurricanes occur, and the answer really hinges on sea surface temperatures, and so the question become one of how soon we can get climate change under control. Climate models tend to show hurricane frequency not necessarily increasing with warming SSTs, but hurricane energy dissipation increasing – i.e.- not necessarily more hurricanes, but stronger ones when they do occur.
Didn’t for some reason know about Patricia and its vast strength, and that’s the kind of fact I was hoping to get. That for sure satisfies my operational definition for this thread of what could be considered a Category 6 (192 MPH), and I agree that only time will tell what will be produced in the future.
Now that we’ve mostly addressed the direct FQ …
There is an ongoing debate in the NHC and emergency management circles about the whole “Category” system.
The short version being that wind speed is a really crappy indication of the hazard severity of a hurricane. Everyone, both sober expert commentators and sensationalist click-baiters, lurve to focus on peak wind speed. That’s not completely irrelevant as to hazard, but it’s a lousy proxy for it.
Storm surge height and extent, rainfall rate, speed of storm progress, and lateral size of storm system are all far, far more important to predicting how much stuff will be destroyed, how many people will be killed, etc.
So for public warning purposes the S-S Category system ought to be replaced. The challenge is coming up with a replacement that can be based in science, be agreed to, and can be predicted far enough in advance to give a useful warning time.
My own bet (assuming NWS is allowed to continue existing at all) is we’ll see a replacement multi-factorial hurricane severity-of-impact scale before we see an S-S Category 6 added on top of the current 5.
Great stuff, and thank you!
This being FQ, here’s a cite to get you started down the S-S replacement rabbit hole:
Here’s another:
With the right thermodynamic conditions there is no physical reason a hurricane can’t grow to any level of power and size short of turbulence conditions causing instability in the central vortex or exceeding the isothermal condition on the inflow. If the outflow at the eye approaches transonic conditions (about Mach 0.8, around 600 mi/hr or 960 km/hr) there would probably be loss of stability of the vortex that would cause a slowdown at the inflow.
Stranger
Seems theoretically reasonable so, even though I’m a lowly lay person weather wise, I have to agree.
I guess there is a physical limit to the strength of a hurricane?
After all, the air pressure at the center can’t go below zero. Though what it would take to create anything that strong is anyone’s guess…?
For very round numbers, normal surface atmospheric pressure is 30 inHg. A truly monster record-setting hurricane drops the surface pressure in the eye to about 26 inHg. So ~13% below normal.
There’s one hell of a lot of room for further pressure drop before zero becomes a limiting factor.
The real limiting factors are supersonic flow and the temperature difference between the lower stratosphere & the surface.
@Darren_Garrison’s cite upthread to the wiki on Hypercanes is informative if you haven’t read it yet. As is its reference section.
John Barnes’ Mother of Storms is a 1994 novel about a gigantic supersonic hurricane and its spawns menacing the entire world. My memory tells me he spent a lot of time trying to justify such a storm, so it might work as a primer for how superstorms might be possible.
As it happens I’m rereading that novel right now (read it back in the 'Nineties). Some of the specifics about how ‘Hurricane Clem’ (the megastorm that is spawning other, often larger megastorms) builds and progresses stretches credibility a bit (especially the storm winds reaching supersonic velocities) but it was based upon speculative storm modeling theory at the time. The “clathrate gun” hypothesis for the Late Glacial Interstadial heat anomaly upon which the inciting event for the store (that kicks off massive ocean heating) is been somewhat disproven but we are discovering other unsuspected sources of methane sinks in frozen tundra as well as newer heat-trapping and albedo reducing mechanisms that contribute to global heating today. The potential for year-round hurricanes and typhoons, once thought to be impossible without a massive shift in climate heating, is becoming more plausible, and the typhoon that recently struck the cost of western Alaska further north and such storms usually reach is a harbinger for things to come with climate heating.
What is even more interesting about the book, aside form the meteorology and atmospheric thermodynamics, are how accurately the speculations of a lot of the social dynamics and technologies came to pass. We don’t have ‘XV’ (essentially a brain-machine interface for entertainment) or moon bases established and abandoned by many countries, and some of the autonomous technologies and functional AI in the book are beyond the current state of the art, but the author guessed pretty well on a lot of things. (Fortunately Washington DC wasn’t nuked and substantial national autonomy ceded to the UN but the decade is still young.) I’m not a big fan of the third person omnicient narrative style with a continuous narrative not broken by chapters, and constantly creating vignettes developing minor characters only to kill them off a few pages later but the central core of the story is pretty compelling.
Stranger
I once participated in a trivia contest where a question about what an EF12 tornado might actually be. None of us knew the exact answer, but we all agreed that if such a thing existed, anyone in the way would be extra F’ed.
EF12 is Mach 1.