Technical Hurricane questions

I saw the other day that the hurricane’s eye had the lowest measured pressure ever. 912 millibars. Normally, we see a pressure reading in the 29 range. How do these numbers reconcile? Would the 912 be a 9 on the same scale?

Also, they say Cat 5 is the most powerful they have. Is it basically impossible for a storm to get more powerful? For instance, if it just sat over a warm spot on the ocean for 3 days straight, could it even get that much more energy?

I was thinking there might be a hurricane “terminal velocity”. . .a point where the forces acting to drain power from it counteract the forces acting to strengthen it.

I know that Pacific typhoons can get a good bit stronger and meaner than Atlantic hurricanes, but ours are bad enough, frankly, I wouldn’t want to see worse!

Also, Cat 5 simply doesn’t have an upper range. I guess when the storms get that bad, there’s no point. I’m assuming there’s no real difference in damage between a storm with, say, 170 MPH winds and one with 190 or 200. Any way, you’re pretty much screwed once you get into a 5, at 155.

And I have another Hurricane question.
On the way up the scale to full-fledged hurricane, they are called tropical depressions tropical storms. Does one of the have to form in the tropics? That is, if a storm system formed off the East Coast in the Atlantic Ocean, directly east of the Outer Banks of North Carolina, could we still call it a tropical storm/depression (depending on strength) if it met all the other criteria?

Apparently so.

And thanks, Trunk, I was wondering to myself if 912 was the lowest ever. To lazy to Google it.

The ‘29’ number you’re used to seeing is the atmospheric pressure repesented in inches of mercury. Millibars is just another scale (like the difference between Fahrenheit and Celcius). You can convert them as follows:

Millibar - a common metric unit of atmospheric pressure, equal to 0.001 bar, 100 pascals, 1000 dynes/cm2, about 0.0295 inches (0.7501 millimeters) of mercury, or about 0.014 504 lb/in2. A millibar is the same thing as a hectopascal (hPa), and some weather agencies have replaced the millibar with the hectopascal in an effort to conform with the SI. However, many meteorologists resist this change and continue to use millibars. In fact, the appropriate SI unit for atmospheric pressure is the kilopascal (10 millibars).


answers one question. 912 millibars is 26.93 inches on the scale I was talking about.

Whiterabbit – I know there is no category above 5 but there has to be a practical limit on the strength of a hurricane. Pressure CAN’T be zero. Winds CAN’T blow at “infinity”.

I thought 160 sustained was somewhat equivalent to “120” for terminal velocity, but damned if I can find a cite.

Also, it appears that my info on the “912” was incorrect. cite. Still, that would place it 3rd.

A bar as a measurement of barometric pressure is “one atmosphere” – and the usage is important in working with high-pressure issues like the conditions on Venus or Jupiter, in the Marianas Trench, etc. For meteorological usage, the metric subdividers are customarily used – particularly “millibars” – 912 millibars is 91.2% of normal median atmospheric pressure at sea level. Considering 1000 millibars as equal to 30 inches of mercury on the Anglo-American scale is a reasonable approximation.

I seem to recall some material on theoretical meteorology using the term “hypercane” that suggested an upper limit on Cat. 5 hurricanes, beyond which windspeed additional phenomena invoking atmospheric physics that are not present in even the strongest present-day hurricanes begin to be invoked. But I’m not competent enough in such extrapolations to begin to explain it – and it is, of course, purely theoretical, an exercise in “What would happen if…”

Ivan got down in the lower nine-hundred-and-teens while rounding Jamaica, though exactly what the lowest reading was, I don’t know. I do recall seeing that he was the sixth lowest barometric pressure ever in hurricanes (tornadoes, of course, will go lower still).

Sean F, the character of a “tropical storm system” is not that it must geographically develop in the tropics, but that it has characteristics which normally occur in storms that develop in the Inter-Tropical Convergence Zone. Gaston and Hermine both developed off the coast of South Carolina, well out of the tropics, because weather conditions favorable to producing tropical storm formation were present there in August. In other words, what it takes to build a hurricane normally occurs in the tropics during the “summer” half of the year for that tropic, can occur outside the tropics under the right conditions, and will not occur in, e.g., the North Sea or Lake Superior in October or March.

Typhoon Tip in 1979 holds the world record for lowest atmospheric pressure, with 870 millibars.

That’s what I thought (empirically, just watching the summer storms develop over the years down here,) but I had no one back that with the voice of authority. Muchos gracias, senor.

That is not entirely clear, since there are different techniques and standards used for recording data… Different standards for measuring sustained winds, gusts, etc.

That’s what I was taught too, way back in 1943 but apparently things have changed just a tad. It looks like the use of one bar as precisely one standard atmosphere isn’t used any more. One bar is a pressure of 10[sup]6[/sup] dynes/cm[sup]2[/sup]. It is almost equal to an atmosphere being equal to 0.9869 atmospheres (scroll down to the tan colored table).

Eight of the first nine tropical cyclones in the Atlantic basin originated either in the Gulf of Mexico, off the coast of SC, or in the Caribbean. The hurricane season in the Atlantic extends from June 1 to November 30, and it is not until August that most of the TC’s begin near Africa, but usually north of the ITCZ. Tropical cyclones are initiated by rotating thunderstorms in sea surface temperatures of at least 80 degrees F, to provide the latent heat energy. Beginning in August, storms develop in Africa, flow westerly, and when they reach the Atlantic, they have an environment favorable for the development of a TC. Those storms don’t develop in Africa until August.

That is true to a degree. Worldwide tropical cyclone warning agencies measure winds as a 10-minute average, while the US agencies (National Hurricane Cetner, Central Pacific Hurricane Center, and the Joint Typhoon Warning Center (which has US interests’ warning responsibility for the entire globe outside the Atlantic Ocean)) use a 1-minute average. As such, wind speeds for US agencies seem higher, but just because of the different scale. Wind gusts remain the same though for both scales.

That said, satellite imagery is the main tool to determine intensity of tropical cyclones nowadays, except when planes are flying into them in the Atlantic, and that can be compared across basins. Planes used to fly into West Pacific storms years ago, but no longer do.

However, a storm with Ivan’s intensity of 912mb in the Atlantic, would have a lower pressure in the West Pacific, with the same wind speeds. Reasons why this is the case are not fully understood.

Finally, Trunk’s cite is the pressure readings at landfall, so lower pressures were recorded in the open sea.

Viscera, while (I hope) I have your attention again, you said in another thread that air in the eye rises. This site says it sinks:

So, which is correct?

Speaking of which, an interesting photo of Ivan from the International Space Station, with their solar panels in the foreground is available

(I find that a fascinating site in general. It has a new photo every day, showing some astronomical feature of the universe. Mostly real interesting.)

Sorry barbitu8, I just found the older thread where you asked a question, and didn’t see that you wanted a reply.

The air inside the eye is slowly sinking, which is why it is clear (sinking air, compresses and warms, which decreases the relative humidity, and dissipates the clouds), as I mentioned in a few links in the old thread, linked here.