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An F6 Tornado?

The wind speeds assigned to the F-scale by Fujita were mostly abritrary, and were never calibrated. More recently, research by the Texas Tech wind engineering folks, and a committee of other damage experts (including Tim Marshall) are finding that the actual wind speeds that cause the described damage in the Fujita descriptions might be as much as 30-40% lower than what Fujita guessed. Also add to the fact that Fujita's descriptions were for well-built single-family residential homes only. Unfortunately, as we are discovering on almost every damage survey, very few homes hit by tornadoes are "well-built".

Regarding the comment about direct measurement of tornadoes by the Doppler On Wheels (DOW). This is not possible, because even at very close ranges (1-2 miles), there are still some horizon/clutter effects blocking any beam aimed near the ground. I think the closest to the ground that the measurements have been estimated is still 50-100 feet above ground level, which is still above the roofs of most single-family residential structures. There are rapid changes in vortex wind speed and acceleration in that lowest "skin" layer within 100 feet of the ground.

And yes - those like me who do damage surveys as part of our job are trained to know that the F-scale is a *damage intensity* scale, and that the distribution of wind speeds capabile of producing said damage actually have distributions that ovelap each scale category. There are many other factors to consider, such as exposure, angle to the wind, building construction, secondary damage caused by debris impacts, etc., such that a wind speed of a constant xxx mph could cause a range of damage to similar structures.


greg
 
"I will take the side that it was designed as a wind scale "

Let me check on copyright issues, but if it's okay I've got plenty of his early papers that I would love to scan and put online. His son is a prof at Michigan State Univ, and called me one day saying his wife wanted to clean out the basement so either I take boxes of Dr F's stuff or it went to the landfill... Easy decision, now they are sitting in my basement though ;>

- Rob
 
Originally posted by rdale
\"I will take the side that it was designed as a wind scale \"

Let me check on copyright issues, but if it's okay I've got plenty of his early papers that I would love to scan and put online. His son is a prof at Michigan State Univ, and called me one day saying his wife wanted to clean out the basement so either I take boxes of Dr F's stuff or it went to the landfill... Easy decision, now they are sitting in my basement though ;>

- Rob

He also brought boxes of old books and papers to the central Illinois AMS conference last month. I got a copy of his book on the DFW microburst and some other pamphlets/papers. Pretty cool stuff.
 
Originally posted by Mike Smith
There could be an F-6 tornado. On the rare occasions when tornadoes have struck downtown areas (Lubbock, Topeka, Ft. Worth, etc.) most engineered buildings have little (in relative terms) damage. If a skyscraper was knocked down or an engineered building completely blown away it would be F-6 or higher.

Hi Mike,

I wonder if there are any figures regarding the number of skyscrapers that have been directly hit by a tornado, regardless of intensity? I think a Dallas skyscaper may have been in 1957, but I'm not certain.

Anybody have any info on this?

Thanks

Pat
 
There was a well-photographed tornado in Dallas in 1957, but it passed north of downtown.


No there cannot, at least with the F-Scale, because Dr F defined F5 as total cleansing...

\"The only real way to distiguish an F6 from an F5 would be an actual wind measurement.\"

He described total cleansing of homes on slab foundations. Houses are not built as well as engineered buildings. While I am speaking semi-theoretically, if an engineered building suffered severe STRUCTURAL damage (i.e., more than broken windows, etc.) it would be an indication of a tornado stronger than F5.

On May 20, 1957, the Ruskin Heights (F-5) tornado struck the newly built gym at the Ruskin High School. If you Google "tornado ruskin high school" and click on "images" you will see a sepia-toned photo of the gym after the tornado hit. If the girders, which were placed in strong concrete foundations, were uprooted and/or completely twisted, that would be an indication of a tornado stronger than F-5.

Please note that I am not saying F-5's occur, just how we might detect one if it ever did occur.

Agree that if winds were stronger than the F5 threshold, that would meet the critieria, also
 
We have already discussed how the F scale is really based on damage, but I would almost be willing to bet if Dr. Josh Wurman gets on a tornado and the DOW clocks a 319 mph in a storm, they will say it's an F-6, just for the mere fact that an F-5 only contains wind speeds UP TO 318 mph. There might not be any more devastating structural damage but I bet it gets an F-6, if it ever happens, just because that high of a wind speed has never been detected in a storm that we know of.
 
Forget about my earlier skyscraper question...I forgot about the Ft. Worth storm:)

Pat
 
Originally posted by Andrew Khan
Cattle were Skinned, and there lungs were sucks out.....? It seems very odd, and unlikely for cows to actually have there lungs and skin ripped off...but anything is possible, that's sad, however. I thought in order for your flesh to be ripped of winds would need to exceed 500 mph....

There were documented cases of human corpses found by rescuers after the infamous "Labor Day hurricane" of September 1935 which were literally "sandblasted"; skinless skeletal remains found clad only in leather belts and shoes. This exceedingly small but vicious hurricane created a gruesome disaster scene in a narrow corridor of the middle Florida Keys.

This was an extremely small cat-5 hurricane; based on the known pressure gradient (central pressure of 892.3 mb/ 26.35") and size of the eye and eyewall, sustained winds were likely 190-200 mph....with peak gusts as high as 230 mph (200 kts) or more. I've seen references in my research which indicate the carnage in the core area was similar to the aftermath of a plane crash....or a violent tornado.

As it crossed the Florida Keys, the Labor Day hurricane was only moving an estimated 8-9 mph, so it's reasonable to theorize that max eyewall winds occurred for two hours or longer; wind speeds equivalent to a F4 tornado for two hours at a given location.....with catastrophic and very gruesome results.

PW
 
Interesting discussion.

Fujita scale is a scale of tornado wind speed. Just look at the scale that Justin posted. The problem is, that the only way we can assign a rough order number is the damage they leave behind. The only way we can REALLY get a handle on tornado wind speeds is by scientific measurements similar to what Bluestein, Wurman, and Samaras are doing.

At this point, let's drop the Fujita scale for a moment (for the sake of argument), and see if tornado wind speeds can exceed 319 mph.

There is some modeling efforts that have been done that suggest transonic wind speeds in tornadoes are possible based on work by Lewellen, et al:

http://eiger.mae.wvu.edu/papers/LXL2002_5_30.pdf

Based on his work it is entirely possible, although short lived.

If you were to ask me if wind speeds in a tornado can exceed 318 mph, my answer is yes. Probably in the corner flow region in the lowest 100 meters.

Can one estimate wind speeds > than 318 mph by damage? No.

It's only a matter of time before someone collects actual measurements in the field that would validate this.

Tim
 
Originally posted by Andrew Khan
If you were to exclude the damage part of the scale, would it then be possible to reach F6 wind speeds?
If using the enhanced Fujita scale, then Wurman's measurements of the May 3 1999 Bridge Creek tornado segment would be extroplated out to an EF7 or EF8. But what's the purpose of this? There was already complete destruction at the ground. This tornado was as strong as they probably get (but we really shouldn't jump to conclusions that it was the "strongest tornado ever" that I hear so often).


g

p.s. Wurman's measurement wasn't 318, but a range of samples surrounding about 300 mph with a peak in the distribution at 318, I believe). Plus, remember - it was about 100-200 feet above ground level.
 
Originally posted by Andrew Khan+--><div class='quotetop'>QUOTE(Andrew Khan)</div>
If you were to exclude the damage part of the scale, would it then be possible to reach F6 wind speeds?[/b]

And:

<!--QuoteBegin-Mike Smith

There could be an F-6 tornado. On the rare occasions when tornadoes have struck downtown areas (Lubbock, Topeka, Ft. Worth, etc.) most engineered buildings have little (in relative terms) damage. If a skyscraper was knocked down or an engineered building completely blown away it would be F-6 or higher.

No. What you are proposing here is impossible because, as stated numerous times before in this thread alone, the Fujita scale is strictly a damage scale. F5 damage is the maximum allowable on this scale.

Remember, the high wind mark and damage ratings are all based on research that did not include doppler radar windfield measurments, because the technology did not exist then. Yes, the Fujita scale is outdated and in serious need of updating, however, it is what is is...a damage scale, where the highest level of measurable damage is an F5. Do not try to make more out of it.

To quote NOAA:

http://www.spc.noaa.gov/faq/tornado/f-scale.html

*** IMPORTANT NOTE ABOUT F-SCALE WINDS: Do not use F-scale winds literally. These precise wind speed numbers are actually guesses and have never been scientifically verified. Different wind speeds may cause similar-looking damage from place to place -- even from building to building. Without a thorough engineering analysis of tornado damage in any event, the actual wind speeds needed to cause that damage are unknown.

And to quote the researchers themselves:



F6-F12: 319 mph-MACH 1 the speed of sound: The maximum wind speeds of tornadoes are not expected to reach the F6 wind speeds.

The researchers did not believe that windfield speeds in excess of 318 were reachable within a tornado.

Here is a nice Powerpoint presentation by Dam McCarthy:

http://www.spc.noaa.gov/wcm/The%20Fujita%2...files/frame.htm

Summary: Based on the only measurment scale that is available for regular use, F5 is the maximum sustainable damage that is allowable. Anything else is just the theoretical application of ideas based on, at best, partial research...and at worst "gut feelings". The only solution is for an accredited team of researchers to put together a viable project whose results will be recognized by NOAA and other applicable government bodies. Changing the mindset that has existed for 3 decades will not be an easy task, and it will take people who are very well know within the meteorological and government communities, very precise, accurate, and thorough documentation of the entire project, solution sets, findings, and research methodology, plus a good bit of political clout to make a change in the rating scale.
 
Originally posted by Greg Stumpf+--><div class='quotetop'>QUOTE(Greg Stumpf)</div>
<!--QuoteBegin-Andrew Khan
If you were to exclude the damage part of the scale, would it then be possible to reach F6 wind speeds?
If using the enhanced Fujita scale, then Wurman's measurements of the May 3 1999 Bridge Creek tornado segment would be extroplated out to an EF7 or EF8. But what's the purpose of this? There was already complete destruction at the ground. This tornado was as strong as they probably get (but we really shouldn't jump to conclusions that it was the "strongest tornado ever" that I hear so often).


g

p.s. Wurman's measurement wasn't 318, but a range of samples surrounding about 300 mph with a peak in the distribution at 318, I believe). Plus, remember - it was about 100-200 feet above ground level.[/b]

Hi Greg,

Is there any way to know what the approximate ground wind speed was with the Bridge Creed tornado? Say we have a wind speed measurement of 300 MPH at 100 feet AGL, is there any sort of mathematical formula to determine what this would equal say over water at the surface.

Thanks

Pat
 
Chris,

I don't understand your point. The Lubbock tornado was rated F-5. I struck a skyscraper. The skyscraper stood, although heavily damaged.

If a tornado struck a well-engineered skyscraper and levelled it, why wouldn't that be evidence of F-6 damage?

Mike
 
Originally posted by Andrew Khan
If you were to exclude the damage part of the scale, would it then be possible to reach F6 wind speeds?

Show me a reliable way to get wind measurements from the lowest 10m inside a tornado....that is usable 100% of the time on 100% of the tornadoes that occur. Then, we'll talk about excluding the damage part of the scale.

As stated earlier the F scale is a damage scale that was designed to correspond with specified wind speeds. These wind speeds are supposed to be estimates for construction standards. Specifically, the purpose was for designing nuclear power plants that would withstand extreme winds.

As it stands, with current technology, we do not have a way to regularly record the wind speeds in the lowest 10m of a tornado. I don't see us getting anything like that anytime soon. As stated before, some research has found that the F5 scale winds may be a little high...that could also correspond to better construction methods nowadays. In any case, we will most likely see a revised version of the F scale before we can ever base it on actual wind speeds in an actual tornado.

I mean, it's not like we can fly a tornado hunter aircraft into the center of every 'nader.
 
I think that what Chris is saying is that according the the parameters specific to the Fujita Scale at the time of its inception, there cannot be an F6 tornado. Chris mentions that the Fujita Scale is in part obsolete.

If the Fujita Scale is updated or replaced, we will be able to assign a rating to observed damage and derived windspeed that would be eqiuvalent to an F6. But we won't be calling it an F6; we'll be calling it an F(version 2.0)6. Or a Wurman W6. Or a Bluestein B12 (heh, B12, take your vitamins). Whatever. Perhaps the first definitive measurement of tornadic winds comfortably in excess of 318 will spur change. If there is no definitive measurement, then there will be less impetus for change.

One thing is sure: Nature will do what She wants whether or not we have a functional descriptive bureaucracy.
 
"If a tornado struck a well-engineered skyscraper and levelled it, why wouldn't that be evidence of F-6 damage? "

Becaus there is no definition for F-6! It's like asking what the word "justaphot" means. There's no such word, so you can't define it. Fujita did not define an F6, so it's not in the "damage dictionary" ;>
 
I guess a lot depends upon how important tornado climatology is. If you value the tornado climo., then it's probably not the best idea to start mixing measured wind speeds with the F-scale; it'd probably be a better idea to formulate a new scale which can differentiate damage-assessed tornadoes from a new wind-assessed tornado. I mean, in the end, a goal would be to ensure that an F5 from the 1950s would still be considered an F5 now. A new name, such as EF, would help avoid confusion. Back to climo... Our climatological record is based on damage assessments. Now that we're finding out that the wind speed attached to the damage expected by Fujita and others in the past may be significantly in error, the tornado climo database may get royally messed up if we start letting measured wind speeds be a reason for tornado F-scale classification. Would it be better to rate each scale on measured wind speeds? IMO, most definately. However, since tornadoes have always been rated on damage production, tornado climo will become nearly useless if "wind assessments" (let's assume that's even plausible right now) become the primary mode of assigning an F-scale to each tornado... Tornadoes in the field, which would otherwise receive F0-F2 based on a lack of damage could be rated F3-F5 based on measured / determined wind speeds. On the other hand, a relatively weak tornado that moves very slowly and churns over a housing development would be rated weaker on the wind-speed scale than would the damage-scale would indicate... Obviously, scientific truth is more important than climatological continuity, but just something to be aware of!

Wind obs would be a more true representation of the 'intensity' of a tornado than damage, for reasons that are obvious and/or have been discussed before, but, we also must realize that VERY few tornadoes are even sampled at the ground (maybe the couple of ASOS/AWOS obs that have been directly hit by tornadoes, though I'm not aware of any). Radars can sample tornadoes, but the data obtained are often several hundred meters about the ground. In addition, there are inherent errors in Doppler wind velocity estimates, which, IIRC, is the main reason Dr. Wurman changed his data from 5-3-99 Bridge Creek from 318mph winds to something like 301mph +/- 17mph. Radar measurements may be a better indicator of wind speeds than an application of a damage-wind speed relationship scale, but there are still plenty of errors in radar obs. In addition, how many tornadoes are caught by Doppler radars each year? A very small fraction of all tornadoes across the US each year, that's for sure!

Photogrammetry has been applied to tornadoes to help determine wind speeds, but even those are estimates (after all, there are complicating factors such as drag and centrifuging effects). I know Tim Samaras mentioned photogrammetry being applied to his media turtles, but I'm not sure if he's done any of that yet.

So, that leaves direct measurements... Those would be nice, but the most 'controversial' tornadoes are those that are in the upper range of intensity. It'd be difficult, I believe, to ensure that an anemometer withstands F4/F5 winds. In addition, we know that wind fields in tornadoes are quite heterogeneous (e.g. not axisymmetric!). There are subvorticies, etc, which may very well 'miss' the anemometers... Then what?

Again, I do think direct or remote measurements would probably be a more accurate tool in the determination of the strength of a tornado (relative to damage / wind relationships), but both methods can (usually) only be applied to a very small number of tornadoes each year.
 
Originally posted by Mike Smith
Chris,

I don't understand your point. The Lubbock tornado was rated F-5. I struck a skyscraper. The skyscraper stood, although heavily damaged.

If a tornado struck a well-engineered skyscraper and levelled it, why wouldn't that be evidence of F-6 damage?

Mike

This presupposes that the skyscraper was in a segment of F5 damage. Most F5 tornadoes do F5 damage in only a few places. Take the Bridge Creek/Moore tornado for example: the tornado tracked 38 miles and destroyed thousands of houses and yet only 17 locations along that entire path length were rated F5.

The Fujita scale assigns a rating of F5 to any tornado that levels well-constructed homes, tosses cars extreme distances, etc. As I understand it, Fujita did not include damage to industrial or commercial buildings (such as a skyscraper) in his rating system. Which begs the question, "If the skyscraper had been leveled, would the tornado have been rated an F6?" Nope.

First, because there is no way of knowing whether or not a skyscraper or high-rise building can withstand stronger winds than an ordinary dwelling space. Furthermore, it is logistically impossible to study the effects of wind speed on such heterogeneous structures. That is, there is (generally speaking) no national standard for building large structures. They come in all different sizes and shapes, are made with different materials, and are constructed differently.

Second, as has been said ad nauseum, there is no such thing as an F6 tornado in the real world. An F6 tornado is called "inconceivable" for a reason. Because the damage that an F5 tornado incurs is so complete, it leaves nothing further to damage. Fujita didn't leave room for it in his scale, and that's that. (Though I think a better damage scale could be created to better delineate between "weak" and "strong" F5 tornadoes...which is what I believe those working with the Enhanced Fujita Scale are trying to accomplish.)

Gabe

However,
 
Originally posted by Mike Smith
Chris,

I don't understand your point. The Lubbock tornado was rated F-5. I struck a skyscraper. The skyscraper stood, although heavily damaged.

If a tornado struck a well-engineered skyscraper and levelled it, why wouldn't that be evidence of F-6 damage?

Mike

Because there is no F6 rating.
 
Another question would be: do F-5 tornadoes really occur? With engineers finding that F-5 damage occurs at lower windspeeds, are past F-5's really deserving of that destinction? Maybe there haven't been any F-5's yet?
 
"r question would be: do F-5 tornadoes really occur?"

Yes...

"With engineers finding that F-5 damage occurs at lower windspeeds"

Not to pick on you - but why are windspeeds STILL being mentioned? Wind speeds have nothing to do with rating tornado damage. The F-Scale is a damage scale.

"are past F-5's really deserving of that destinction?"

Do they meet Fujita's definition of F5? If so - they are considered F5.

"Maybe there haven't been any F-5's yet?"

Not sure I follow.

- Rob
 
Sorry, folks. I am still not getting this discussion.

I went to Dr. Fujita's own discussion on page 31 of his book, "Mystery of Severe Storms" published in 1992 by the University of Chicago Press. The F-Scale chart clearly shows BOTH wind speeds and damage as F-Scale determinators.

If the F-Scale rating is made using damage then a lower case "f" is to be used before the numerical value.

If the F-Scale rating is made using wind speeds then an upper case "F" is to be used.

So, if one documented a tornado with 325 mph wind speeds, it would clearly be F-6. Fujita DID define the wind speeds associated with F-6 and even used an adjective, "inconceivable", to describe it. He just (at least at that time) didn't speculate how one would tell the difference.

I don't understand why this is being called "impossible." I don't think we know.
 
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