I didn’t believe it until
@A Guy produced a Fujita document proving it. He really did consider F6. Believe it or not, it wasn’t based on all the slabbed homes on the west side of town. The F6 consideration in Xenia was actually based on cycloidal scouring in farm fields as the tornado exited the east side of town. Cycloidal marking analysis has been used to come up with some pretty ridiculous wind speed estimates (Charles City, IA), and I haven’t seen it used in the modern era of tornado damage analysis. As a result, I question its validity.
Do you have a source on that being the reason for the F6 rating for Xenia? It's not in the Fujita paper that I showed had the rating, and I don't recall reading about him making an estimate from ground markings at Xenia elsewhere. He has papers that say '
probably 250-305 MPH' and
one that implies up to 290 MPH. There's also the 265 MPH figure that was mentioned in one of those documentaries Grazulis made, but I'll be danged if I can keep track of the source. These were all from photogrammetry.
I've explained the ground markings before but will again. The Charles City measurement, and I think other very high ones, was based off a method Edgar van Tassel published
in 1955, who applied it one of the Scottsbluff tornadoes (and got 484 MPH). He assumed the markings were approximately closed ellipses (
not cycloids) which seems to be the source of the very high speeds. I actually can't conceive how you'd get a closed, regular course.
Fujita in his
Palm Sunday paper, made the sounder assumption that the marks are cycloidal (they obviously
resemble cycloids in well-formed examples). As cycloids are open they accomodate more translation, so rotation (tangential velocity) can be slower. It relies the predictable variation of a cycloid's shape with the ratio of rotation to translation. He initially describes that this ratio can be gotten from the
width of a loop, but then goes on to use loop
spacing to determine the rotation
time and to calculate windspeed based on that. It's not clear to me why the second step is necessary, except to obtain an average tangential velocity, as wind speed is finally produced by summing the tangential and translational velocities.
The one tornado in the paper he applied it to was L2, south of Swayzee. In this he uses multiple sets of marks to obtain rotational times at seven locations. Assuming the translational speed equalled the average speed of tornado family L, he got speeds of 172, 176, 173, 180, 180, 173 and 166 MPH. The Greentown tornado is known on here as one of the more intense ones from the outbreak, so low-end EF4 might surprise.
This assumes the mark is created from a point (which he assumed to be a suction vortex) that remains at constant angular velocity and central distance. In his real case he concludes the distance variation is probably not important (even though this means the markings aren't true cycloids). This is analogous to assumptions made in analyses of treefall patterns, and to single-POV photogrammetry (I once saw someone say that one reason photogrammetry declined in popularity was uncertainty over the actual shape of the trajectory. Now we have some videos with two or more angles filmed simultaneously it would be worth revisiting).
The method wasn't definitively disproven, but doesn't seem to have been very popular in published literature (it requires the tornado to have made marks, and getting rectified aerial photographs). A PhD thesis exploring markings only came up with a few citations, though I suspect there's more unpublished instances (e.g the measurement used to rate the 1990 Goessel tornado F5 wasn't formally published). Interest in direct measurements seems to have dried up, with photogrammetry disappearing by the mid 80s. I think the adoption of the Fujita scale was a factor, as it provided seemingly satisfactory estimates without the effort of, say, tracing film frames. Of course damage proxy measurements have turned out to be a minefield of assumptions and subjectivity.
The only people who've explored surface markings recently are David Lewellen and Michael Zimmerman, mainly in semi-formal literature like conference papers. Zimmerman's
thesis found marks (cycloidal and otherwise) are created by irregularities in the corner flow region of the tornado depositing debris, with the type and strength of the marks depending on a few factors interacting in a complex manner. He and Lewellen seem more interested in what the qualitative nature of the marks says about the vortex properties. But Zimmerman does attempt a couple of image classification techniques, finding the marks too variable to get good results with his methods. So it appears not possible to do fairly objectively at the moment (Fujita effectively eyeballed his paths, having to assume they were fairly regular and continuous).
Practically no work of significance has been done since, and many other explorations of direct measurements are in the same boat. There just doesn't seem to be the interest to work these things out from basics to field. Unfortunately without fresh sets of eyes exploring each possible method the same issue is going to keep coming up where EF scale measurements are clearly problematic but there's nothing else.