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An F6 Tornado?

Some food for thought regarding wind speeds at varying levels close to the surface...

During several hurricane research trips I was associated with, we used two meteorological towers... one at 2m and 10m. It was quite impressive in the difference of wind speeds throughout the duration of the storm. Take Hurricane Rita for example. The towers were deployed just west of Johnson’s Bayou, LA (the landfall site of Rita) in a maritime environment (no significant obstacles to obscure winds). During the climax of the storm, the sustained and wind gusts varied by nearly 35mph stronger at 10m than 2m. This is significant when considering a 35mph difference in only a 26ft depth.

To relate towards the subject at hand... the difference of wind speeds from 2m (6.5 feet) versus winds at 30m (98.4 feet) would be unbelievably significant. This would imply the May 13th Knox tornado might have had winds near the surface (damage region) of weak F2 caliber, instead of the official moderate F3 rating (2m~120mph versus 30m~177mph). It would appear this tornado was probably overrated in regards to winds found at the surface where damage would have existed. (Now vertical velocities may induce more damage than what’s represented here by just considering horizontal winds... but overall the vertical velocities should be relatively negligible when compared to the horizontal.)


Rob wrote:
As kind of an aside for Scott...nice picture in the June Storm Data...and it kind of ties in with the tornado database...SD has it listed as an F0 with a 30 mile path, but the narrative says "four"different" tornado touchdowns over a 30 minute period northwest of Ekalaka Mt....the tornadoes came from different parts of a large mesocyclone" "at times the tornadoes were multiple vortex" Note they say "tornadoes"...so this brings me to another problem with database accuracy...and maybe this should be a different thread...can you give a better account of what happened there? Was it 1 "long tracked tornado" Or 4 tornadoes? The vagueness of SD submissions is most frustrating.

Yeah, I was somewhat surprised to see Storm Data list all four tornadoes into one tornado in the official database. And you’re right... this is a perfect example of inconsistency when related to the entire national tornado database. This is especially tough when considering the lack of tornado reports in such a sparsely populated region of southeastern Montana.

There were four separate tornadoes in the course of a 45 minutes. The first tornado was a rope and touched down east of the wall cloud. The second tornado was the longest duration and developed in association with the wall cloud. This tornado actually marry-go-rounded across the entire cusp of the wall cloud for a total duration of ~10 minutes. The last two tornadoes lasted ~3 minutes each and developed in-time well separated from each other.

My only guess why it was imputed into Storm Data like this was not enough detailed information was available at the time to the WCM on when each tornado began and ended. If there were only quick gaps between each “tornado/condensed vortexâ€￾, then it would make plenty sense to call it one tornado with intermittent gaps. However, this was not the case on 6/6/05.

Scott Blair
http://www.targetarea.net/
 
To relate towards the subject at hand... the difference of wind speeds from 2m (6.5 feet) versus winds at 30m (98.4 feet) would be unbelievably significant.

The difference is not this bad - you should be relating the 30 m obs to 10 m height, not 2 m, as 10 m is the meteorological standard used for the F-scale. Nevertheless, the winds are probably stronger at 30 m than at 10 m, though that said, nobody really knows that for sure and if so how different are they. There is no reliable analytical model for relating tornado wind speeds at any one height with those at another (we struggle even doing this in an ordinary boundary layer with any consistent accuracy), and nor is there a reliable tool for accounting for centrifuging effects of debris sampled by the radar that dominates the doppler velocity signal. Also, anyone that's looked at closeup tornado video knows there can be significant vertical velocities even very near the surface, so how important is this (unmeasured) wind speed in damage production? I think it was a mistake to 'upgrade' the Knox county tornado, not that I wouldn't want a more accurate database of wind intensity within tornadoes, but because you have to have a consistent means of classification within a database to be able to derive meaningful statistics from it. If I were using storm data for tornado intensity distribution estimates, I'd have to pull this ob out.

Glen
 
Yes, I’m aware 10m is the standard meteorological height. However, in this example it was important to show the differences between winds at varying levels from 2m, 10m, and 30m where damage to my first floor of the house, the top of my roof, and the top of my tall pine tree respectively. There will likely be stronger winds at 30m versus 10m and an even greater difference of wind from 30m to 2m. And agreed, the Knox County tornado was a poor decision based on the aforementioned wind differences at varying levels and the inconsistency with classification.

Scott Blair
http://www.targetarea.net/
 
The biggest problem facing us by using DOW data to justify a specific rating without damage is the simple climatological inconsistency that will result.

Also, for those who may not be up on the latest literature, there has been an effort to try and 'calibrate' winds measured with the DOW's with damage occuring at the ground with the Spencer, SD tornado - (See Wurman and Alexander, MWR 2005). Additional studies such as this one could some day be used to estimate the relationship between radar measured winds and near surface wind damage - but many more studies (~30 for statistical significance) need to be completed. Regardless, I agree the DOW observations shouldn't be used to justify a higher tornado rating - as it diminishes the value of the database. Instead, it might eventually offer a more realistic climatology of typical wind speeds in tornadoes with all of the cases they keep accumulating.

Also Scott, I'm sure you were aware of the 10 m height, but most readers here are not, and your post I thought could easily be misinterpretted.

Glen
 
Exactly... An F6 will NEVER occur - even if they record winds well in excess of 318MPH, it's still an F5. Why? Because the Fujita scale is purely a damage scale... You can only destroy something so much until there is nothing left to destroy (an F5 would do that).

If this is the case, then the idea of "F6 to F12" is ridiculous. Why bother if what you say is true? That just seems silly. :roll:
 
Go to the middle of page 4 of this thread. Fujita defined it as both wind and damage. There can be an F-6 tornado.
 
I keep hearing the same arguments of why there can be a F6 with no logic. It seems some people would be devastated themselves if they accept what is so obvious. Perhaps this will be another thread that goes in circles, not because there is real evidence a F6 could exist, but because some just cannot admit science and the basis of the scale does not support such a fictional rating.

In attempt to summarize to those that ‘still believe’ in the F6 to come...

1) you have read the last few posts regarding near-surface wind speeds and frictional effects.

2) you understand that the modern day F-scale ranges from F0-F5.

3) The F-scale is based off damage to estimate wind speeds, and total destruction cannot exceed total destruction. F5 is the highest rating possible.

4) Recent research has shown the estimated wind speeds in the current scale are too high.

5) No surface wind speed measured has ever come that close to a high end F5. Again, the fastest wind speed “recordedâ€￾ in a tornado was May 3, 1999 found between 300-318mph. However... this wind was sampled at 50-100m above the surface!!! This is 160-300 feet above the ground!!!

If you still believe in the F6, fine... but just know science and the basis of the scale itself doesn’t support it.

Scott Blair
http://www.targetarea.net
 
Scott,

Re: Point 2. Are you saying Fujita didn't have the right to define his own scale? Are you saying that 1992 isn't "modern day"? I guess I don't understand your point. Fujita (see my post on pg. 4 for this thread) CLEARLY defined F-6.

Point 3. The Fujita scale is defined as both wind and damage. Read the reference I cited from Fujita himself.

Fujita's damage examples for f-5 were based on homes and similar structures. As indicated earlier, an f-5 struck a skyscraper and left it standing. If a skyscraper (for example) were toppled, it might be damage-based evidence of f-6 conditions.

We don't know what the winds were in the Tri-State Tornado, the Woodward Tornado, Xenia and others. All we know is the damage. I don't know whether winds in a tornado have ever exceeded 319 mph and neither does anyone else.

Fujita clearly defined an F-6. Lets see what science measures the next decade or so.
 
Let me make this real simple for you Mike...

1) F-scale rating is based off damage. Wind speeds are derived from this.

2) F5 is THE highest damage possible. It does not matter if every downtown building in some arbitrary city and their puppies are swept clean. It’s still F5 damage.

If you can understand these two simple points, then you can accept there will not be a F6. Period.

I can’t make it much more simpler than this... time to go play in the near-blizzard conditions outside :)

Scott Blair
http://www.targetarea.net
 
Scott,

What is your source for your contentions?

I have referenced Fujita's own work.

Mike
 
What makes an F5 the final rating in determening damage? thier have been many stuctures such as skyscrapers that have although heavily damaged remained intact after an assualt from an F5. Fujita indicated in his scale that the maximum alloted damage in an F5 is 318 mph. I have heard numerous points about the Fujita scale being strictly a damage scale, if this is truly the case why then did he include an F6 rating. Perhaps it is because he knew that an F5 cannot cause absolute destruction. Many structures have survived without total annhiolation and that's why we need to accuratly measure windspeeds so far as technology advances the process beyond all error then all will have to see an F6.
 
"if this is truly the case why then did he include an F6 rating."

I'd suggest reading this thread... It's been made quite clear.

"Perhaps it is because he knew that an F5 cannot cause absolute destruction."

Then why did he say F5 is absolute?
 
I would like to try this again, so I am re-posting from page 4 of this thread:


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.

He is absolutely clear on the point: An F-6 is possible and he does not rule out an f-6 (although he did not define what the damage would look like).

A number of people posting on this thread have stated unequivocally that a f- or F- 6 is "impossible." Please reference where Fujita explictly states that it cannot occur. Opinions don't count.
 
3) The F-scale is based off damage to estimate wind speeds, and total destruction cannot exceed total destruction. F5 is the highest rating possible.

I agree with just about everything you've covered, but just FWIW, I cannot find the words "total destruction" in any of the Fujita scale references I've studied. All of them simply state "Incredible Damage" and go on to describe strong frame houses lifted off of foundations and carried considerable distances to disintegrate, automobile sized missles fly through the air in excess of 100 meters, etc, etc...

The references that do document how Fujita described F6 damage state "Inconceivable damage. These winds are very unlikely. The small area of damage they might produce would probably not be recognizable along with the mess produced by F4 and F5 wind that would surround the F6 winds. Missiles, such as cars and refrigerators would do serious secondary damage that could not be directly identified as F6 damage. If this level is ever achieved, evidence for it might only be found in some manner of ground swirl pattern, for it may never be identifiable through engineering studies."

So while I agree that we won't likely ever see a tornado rated F6, the door appears to have been left slightly open for that to happen. Just because we can't conceive something doesn't mean it isn't possible. =)

Scott
 
I would like to try this again, so I am re-posting from page 4 of this thread:


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.

He is absolutely clear on the point: An F-6 is possible and he does not rule out an f-6 (although he did not define what the damage would look like).

A number of people posting on this thread have stated unequivocally that a f- or F- 6 is "impossible." Please reference where Fujita explictly states that it cannot occur. Opinions don't count.

Good points... I've been reading this discussion since my original post, and I am starting to change positions. After reading your post, it makes complete sense - the lowercase "f" is purely for damage, and the upper case "F" is purely for wind. Why has the scale been twisted so much? Now EVERY rating is an uppercase "F", but not for wind. And, Fujita DID say that the only way to identify (yep, identify) and f6 (damage) would be some sort of ground swirl pattern. So, if a DOW clocks a wind speed of 400MPH and that ground swirl is present - then what?
 
A number of people posting on this thread have stated unequivocally that a f- or F- 6 is "impossible." Please reference where Fujita explictly states that it cannot occur. Opinions don't count.
It is generally accepted in the modern "community" of storm damage experts that F5 is the highest rating given, and that the Fujita scale is a damage scale and not a wind scale, regardless of what Fujita initially desired. Chuck Doswell's comments sum that up very well.

But, certainly, it is possible that a tornado could exceed the the 318 mph speed limit that Fujita arbritrarily applied to the rating scale. But recall that those numbers are quickly becoming moot in light of present day engineering analyses and research. Furthermore, if and when the Enhanced Fujita Scale is applied, there will no longer be any designation above EF5.

Yes, the F5 designation is for complete destruction to well-built single-family residences. But if an engineered steel and concrete structure (like an office high rise) were to be swept clean someday, I'm sure the engineers will study that profusely and might consider a higher rating. But for now, I wouldn't expect any F6+ or EF6+ ratings from official survey teams.


greg
 
Greg,

Thank you. At last, we get to the crux of the matter.

I don't dispute your statement about what the "community" might think or believe regarding wind speeds in tornadoes. I also don't dispute that Fujita's wind speed estimates may be high.

But, my point is that the Fujita Scale is the Fujita Scale. The "community" has no standing to change Ted's scale and still call it the Fujita Scale.

So, if the "community" wants to create an "Enhanced Fujita Scale" and redefine damage and wind speeds, knock yourselves out. I am confident it will be an improvement. But, it should be called the "Enhanced Fujita Scale" (or whatever), not the Fujita Scale.

Ted allowed for F-6 and f-6. That is a fact. Whether we will ever be able to detect a tornado of this intensity is another matter... if and when one should occur.
 
Originally posted by Saul Trabal
^

For winds higher than 318 mph, you'll have to go to other planets in our solar system.

:)

or be in the vicinity of a nuke explosion, lol. :roll:

I read that that was the reson for f6+.
 
Fujita's wind speeds associated with the damage were a guess. Given that it was a scientifically researched guess, but still, a guess. There were no methods to use to actually measure wind speed associated within a tornado during the creation of the F scale.

Tim Marshall has shown through his research that damage to structures that appear to be from the F-0 to F3 range can actually occur without the tornado actually striking the structure. So...just how do we accurately rate a tornadoes strength?
 
I'm somewhat puzzled by the many adamant proclamations that the Fujita scale is just purely a damage scale? The Fujita scale, even if it's wind speed estimates are off, still correlates damage and strength... a tornado that levels and sweeps away a well-anchored and well-built home is still stronger than a tornado that just tears some shingles off. If Fujita had intended it to be purely a damage scale, why did he ascribe wind speeds to each category? He actually began the scale with the wind categories, and then assigned a damage level to each...
Fujita established categories, each representing arange of wind speed. He then described damage thatwould likely be observed for each range of wind speed.
 
Michael: in your case it is straight forward, but lets consider a tornado that takes a roof off a home. Whether the house had tornado/hurricane straps or not can make a big difference. Did the tornado hit a lumber yard before? Debris in a torando can also induce different severities of damage. How long was the tornado over the location of the house? Lots of factors go into damage. That's why it is so hard (operationally) to determine a tornado's true strength (in wind speed) and relate it to the damage scale.

Aaron
 
fscale.jpg



Fujita created a scale which groups tornadoes into categories based on how much damage the caused. He came up with categories by connecting the Beaufort wind scale with the speed of sound in twelve steps. For each category he estimated how strong the wind must have been to cause the damage.

Estimated. Now again...estimated. Not measured. Not calculated using photogrammetry. Not using scientific methodology in the study of damage patterns and computer modeling. Estimated.


Before 1971, there was no way for scientists to rank tornadoes by their strength. The visual size really had no bearing on how strong it was. Because they're so intense, you can't measure the wind speed or pressure in a tornado like you can in a hurricane. T. Theodore Fujita, a professor at the University of Chicago, came up with a system to rank tornadoes according to how much damage they cause.

The above quotes and image are courtesy of UCAR: http://www.windows.ucar.edu/tour/link=/ear...ado/fujita.html

Now this from the National Climatic Data Center:

The Fujita scale bridges the gap between the Beaufort Wind Speed Scale and Mach numbers (ratio of the speed of an object to the speed of sound) by connecting Beaufort Force 12 with Mach 1 in twelve steps. The equation relating the wind velocities (V in mph) with the F scale (F) is V = 14.1 * ((F+2) to the 1.5 power).


So basically, he divided by 12 to get estimated wind speeds. An estimation. Granted, a scientifically thought out estimation, but still an estimation.

Now..from Texas Tech, and their research on the Fujita Scale Enhancement Project:

http://www.wind.ttu.edu/F_Scale/default.htm

Dr. Theodore T. Fujita invented the Fujita Scale in 1971. The purpose of the scale was to classify tornadoes by intensity, i.e. to distinguish between weak ones and strong (intense) ones. He defined six categories F0 to F5, with F0 being weak and F5 being the most intense expected. Fujita then envisioned the type of damage he would expect in each of the six categories.

Fujita also assigned wind speed ranges to each category, which are non-overlapping and increase in magnitude from F0 to F5. The ranges also are shown in \"Original Fujita Scale\". The wind speeds in the table have been converted to three-second gust speeds at 10 m height in flat open terrain. There were no attempts at a definitive correlation between appearance of damage and wind speed. Fujita used his best judgment as the world’s leading expert on tornadoes at the time. Both the meteorological and engineering communities immediately accepted the Fujita Scale because in 1971 there was a critical need for it. Among other needs, a growing nuclear power industry was struggling to design tornado-resistant facilities.

This information is from the team attempting to re-design the F-Scale into an enhanced version. I would venture to guess that they most likely have pretty accurate information about the origons of the Fujita scale, and the processes that were used to define it. The members of the research team include:



1. Timothy P. Marshall, P.E. (18 years experience conducting wind damage surveys; has conducted approximately 65 surveys.)
2. Timothy A. Reinhold, PhD (Associate Professor of Civil Engineering Clemson University)
3. Douglas A. Smith, PhD, P.E (Associate Professor of Civil Engineering, Texas Tech University)
4. Greg Forbes, PhD (PhD from the University of Chicago; Studied under Dr. Ted Fujita)
5. Thomas Lee Smith, AIA, RRC (Author of a number of papers on performance of roof systems in high wind.)
6. Don Burgess, Chief: Warning Research & DevelopmentDivision, National Severe Storm Laboratory



Hmmm...sorry, but I have to go with these guys.

Several studies indicate that the Fujita Scale overestimates wind speeds in categories F3, F4 and F5.

So..if this is true, then the theory of "319=F6" is history.

And lastly, from the storm prediction center:

*** 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.

So there it is...feel free to believe whatever you choose to believe, but personally I am more likely to go with that the scientist and engineers (including one that studied under Fujita) that have worked on this subject for years have to say on the subject. They didn't just read his book...they lived it.
 
...... And he said an F6 might be identifiable by a ground swirl pattern. So if it's possible that it could be identifiable, then it's possible that it could exist - or at least that would be the first logical thought.

I'm not arguing that the Fujita scale isn't based primarily on damage, I'm just arguing that an F6 would be possible based on what Fujita said (as Mike Smith quoted).

I think the only way an F6 rating would be assigned is if the following occured:

1) Widespread extensive F5 damage
2) DOW wind speed in excess of the current known F5 wind rating
3) An extensive ground swirl pattern (perhaps the tornado created a trench in the ground) is evident, unlike any seen before

Obviously it's all hypothetical, but that's how it'd probably go down.
 
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