Flaws in the EF rating system

[rdale]

That’s what the EF scale revision group is working on.

 

[KenMcWatters]

I feel that there is a flaw in the EF-scale. The graph below shows the windspeeds in mph of the F-scale and EF-scale from 0 to 5. Note that the EF-scale doesn't have an upper bound for EF-5, but DOW measurements indicate real tornado wind speeds at ground level top out close to 300 mph, so I made that the maximum for EF-5.



What this shows is that EF-5 has a larger range (about twice as large) in values (200-300) vs. F-5 (260-318). Kinetic energy varies with the square of the wind speed (KE = ½mv²). Therefore a high-end EF-5 tornado would theoretically be more than twice as damaging as a low-end EF-5. The difference in damage in surveys should be readily apparent. This leaves the door wide open for the need to add an EF-6 rating at the high end of the EF scale. I'm not saying I want an EF-6 rating added, but the large range in wind speeds for EF-5 would certainly seem to make it advisable.

 

[rdale]

The "F" scale was not made based on science, Dr Fujita simply divided the speed of sound by 12. Since you noted there probably is a max ground wind speed under 300mph, and total destruction in EF5 means you couldn't determine a "6" anyways, there's no valid reason for a 6.

 

[Todd Lemery]

One factor that makes me lean away from using wind speeds, even if they were accurately measured at a given height, is that it would neglect to account for the time on target. A tornado traveling at 45-50 mph does considerably less damage than one traveling at 10-15 mph, when they have similar wind speeds. I obviously don’t have any data to support this, but it isn’t too hard to believe a slow moving, mile wide tornado with 90 MPH winds spending a full minute over a home would do more damage than a skinny tornado with 120 MPH winds flying by and only impacting a house for a few seconds. Wind speeds alone wouldn’t reflect that.

 

[Brandon Skalniak]

I've always found it really interesting to compare violent tornadoes with their speed, size, etc. It's just truly amazing to marvel at. With violent tornadoes, especially EF-5's, it doesn't seem that size or speed or time spent over a structure would matter. With a massive, very slow tornado such as Jarrell or even Moore '99, it would produce some more pronounced instances damage (i.e. granulation of debris, extreme vegetation damage, etc.), one should take into account tornadoes such as Phil Campbell-Hackleburg or Smithville, which were both very wide but had incredible forward speed (70+ mph). Smithville took only a few seconds to wipe away well-built, 2-story brick homes, as well as being able to shred and wipe away shrubs and bushes. Phil-Campbell was similar, being able to wipe numerous homes and businesses from their foundations in mere seconds, not even taking into account the impact on the jeans factory.