Interesting Comparison: Beaufort, Torro, and Fujita Scales

[Justin Turcotte]

Originally posted by Chris Nuttall
The F scale is a damage scale. Nothing more, nothing less. Yes, it could be used to estimate winds inside a tornado. But nothing more than that.

I do not know very much about conducting damage surveys. Maybe our techniques are good enough to make better determination of wind speeds. I think this is why the NWS was looking to revise the Fujita scale.[/b]

Ok, Fujita devised his scale on the basis of wind speed. The F-scale is really just a mathematical equation that connects the Beaufort scale to the Mach scale with an F12 being Mach 1.

Since we usually arn't able to directly measure wind speed within a tornado we are forced to infer it through observed damage. So yes, the F-scale is being utilized as a damage scale but it was mathematically devised as a wind speed scale.

 

[Bill Tabor]

Originally posted by Chris Nuttall
I've read in several places that it is believed that the in the strongest F5 tornadoes, particularly multi-vortex tornadoes, that F6 damage has likely been produced. However, with the debris leftover and the fact that an F5 pretty much wipes out everything, F6 damage is not descernible from F5.

I've heard and read that from different sources as well. IMO though I think you could potentially determine F6 if there was a structure strong enough for F6 that could be measured afterwards. This is just a guess of course. F scale goes all the way up to Mach 1 at F12 I believe.

I would like to add the footnote that (from what I understand) it is true that winds above ground are typically higher than those at ground level due to frictional effects however obstacles (such as overpasses) can focus tornadic winds and using the Bournoulli Effect accelerate winds to higher speeds perhaps matching or surpassing those above ground level.

 

[Stuart Robinson]

I will chip in briefly here.. While the F scale is adequate for rating big tanking USA tornados.. The finer resolution of the T-Scale is more suited for world wide tornadoes as almost all of them are less than F2.

While I don’t want to get in brawled in a F/T scale debate (perhaps I already have?) we need to look at a scale that can be applied to tornados world wide.

Over all, and not wishing to stir up a flame war. How long is Fahrenheit going to be used for USA surface Obs when the rest of the world uses Celsius. An international scale is required – an one that fits all locations. So are we looking at a change here (Also might be time to go to M/S wind speed!) – International Athletics already do it during there running events.

BTW I have to declare that I am a member of TORRO and they are the body that devised the T- Scale back in Early 1970’s so I guess that I am some what biased on this subject but still I keep a very open mind on the way forward).

Any way great debate.

 

[Scott Olson]

Originally posted by Jeff Snyder+--><div class='quotetop'>QUOTE(Jeff Snyder)</div>

Originally posted by Scott Olson+--><div class='quotetop'>QUOTE(Scott Olson)

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I would like to see the DOW measurements be taken into account as part of our scale to rate tornadoes.

That's a good idea in theory, but we all know how hard it is to storm chase. I don't think we could get a DOW on every single tornadic storm, even though I wish we could. Imagine how much more we could learn both in real-time and in later analysis. But I digress.... Even the DOWs succomb to the limitation of Doppler radar. Their measurements, although closer to the ground, are still not necessarily at the surface.

A perfect example is the 5/3/1999 measurement of 318mph in the Bridge Creek/Moore tornado. Contrary to popular belief among the public, that wind speed was not at ground level.

Just to clarify, I didn't mean to actual build a fleet of DOWS and have them chase after tornadoes. I just meant for tornadoes that the DOW scans and when it contradicts with the damage assessment by a good degree. I think it is more accurate than a scale that is built on damage assessment. Verification of ground speeds using Samaras probes will be used and then they can come up with a formula to bring the normal low-level scan mode to represent a best guess ground speed.[/b]

I think consistency is desired for any scale... What to do if the damage assessment indicates higher wind speeds than a DOW measurement? What do you do if there is a small-scale eddy not sampled by a radar that has winds >30mph above the "ambient" tornado winds. Do you take radar-sampled winds when the tornado is in a field and does no damage? That means that a tornado may be given an F3 rating when there is no damage to support it. Overall, this may not be too bad, but it leads to inconsistency that can cause disruptions with climo IMO[/b][/quote]

I would tend to say to go with the higher of the two. With a greater emphasis on the DOW measurement when less reliable damage indicators are present. The idea I was presenting actually was to take both into account not to nearly throw away the damage assessment and base it only on the DOW. I definately see your point though regarding inconsistency as DOW assisted ratings would likely skew the ratings system since it is not across the board on all tornadoes. But the Fujita scale doesn't seem terribly consistent to me, in some cases there isn't resources to have lengthy investigations into the quality/building codes of a structure and If not done throughly one could overlook details that would greatly affect the rating.Dr. Rasmussen presented an example of this in one of his papers. Perhaps based on the resources we have the way it is now is the best we can hope to do.

But with the DOW's and a much better understanding of damage/wind speed coreleation I think the new scale could be valueable. In the least I would hope that Doppler measurements could be used as a indication. For instance, if I go out and do a damage survey in an open field and can only find indicated damage to support F1 but the DOW measured a 50m windspeed that when computed to 10m supports a F3 windspeed range. It seems just as silly to me to ignore the DOW measurements as it would be to ignore the fact that their is only F1 damage.

 

[Chris Nuttall]

Originally posted by Stuart Robinson
Over all, and not wishing to stir up a flame war. How long is Fahrenheit going to be used for USA surface Obs when the rest of the world uses Celsius.

Probably the same time that the U.S. switches to S.I. units. There's been a movement ongoing for sometime. As easier as it would be, I just don't see it realistically happening. It may be the best thing to do scientifically, but until there is a real pressing need, it probably won't happen.

A lot of people have that "...when you pry it from my cold, dead fingers" attitude about it I think. :roll:

 

[Stuart Robinson]

Chris I agree - these days I also have become quite good at mentally converting F to C or MPH to Knots or M/S - so much so that it is no longer an issue for me - but still a pain.

However we are going off the point here...

As far a tornado scale rating goes - then I believe that the time is right to go to a F point scale or the T scale - I already see reports written of a weak F2 or a strong F3 so already the scale can be sub divided.

Over all my gripe is that there is some difference between a weak F2 and a strong F2 - perhaps F2.25 or F2.75 that would be T3 or T4. But and the major point is world wide the F scale currently is too course for tornado rating.

Way off topic here .. Does anyone care to guess the number of tornadoes world wide that we gat a year?? 3000 perhaps?? I really dont know