A scale used since February 2007 for categorizing tornados by estimates of wind speed based on the damage done. This scale took the place of the Fujita scale, on which it was intentionally based in order to preserve comparability with existing tornado databases. Values on the Enhanced Fujita Scale are stated as “EF numbers,” from EF0 to EF5. Each value is associated with a range of wind speeds as shown in the table below. To avoid giving the appearance of unjustified precision, the developers of the new scale rounded off wind speeds to the nearest speed divisible by 5 (unlike the Fujita scale). (In converting the miles per hour values to km/h and m/s for the table, we've made slight fudges to avoid gaps created by rounding.)
|Enhanced F Scale||Fujita Scale
(obsolete, for comparison)
|Fastest ¼ mile wind|
|miles per hour||km per hour||m/s||mph||km/hr||m/s|
|EF5||over 200||over 322||>89||F5||261-318||420-512||117-142|
Some of the problems with the Fujita scale:
1. J. E. Minor, J. R. McDonald, and K. C. Mehta.
The tornado: An engineering oriented perspective.
NOAA Technical Memorandum, ERL NSSL-82.
Norman, OK : National Severe Storms Laboratory, 1977.
L. T. Phan and E. Simiu.
The Fujita tornado intensity scale: a critique based on observations of the Jarrell tornado of May 27, 1997.
NIST Technical Note 1426.
Gaithersburg, MD: U.S. Department of Commerce, 1998.
The first step in revising the scale was creating classes of clearly and narrowly defined objects a tornado might damage. These are called Damage Indicators. The first version of the scale has 28 such indicators, including such things as automobile showrooms, motels, one or two family residences, high schools, transmission line towers and hardwood trees¹.
For each Damage Indicator, experts defined degrees of damage (DOD). The DOD's represent visible steps from scarcely observable damage to utter destruction. Different Damage Indicators have different numbers of degrees of damage; 5 for hardwood trees, for example, and 10 for one- or two-family homes. For each Degree of Damage three wind speeds are given: the expected speed, a lower bound and an upper bound. Here, for example, are the Degrees of Damage (DOD) for single-wide manufactured homes.
|1||Threshold of visible damage||61||51||76|
|2||Loss of shingles or partial uplift of one-piece metal roof covering||74||61||92|
|3||Unit slides off block piers but remains upright||87||72||103|
|4||Complete uplift of roof; most walls remain standing||89||73||112|
|5||Unit rolls on its side or upside down; remains essentially intact||98||84||114|
|6||Destruction of roof and walls leaving floor and undercarriage in place||105||87||123|
|7||Unit rolls or vaults; roof and walls separate from floor and undercarriage||109||96||128|
|8||Undercarriage separates from unit; rolls, tumbles and is badly bent||118||101||136|
|9||Complete destruction of unit; debris blown away||127||110||148|
An observer at the site can assign any speed between the upper and lower bound, if circumstances warrant it. For example, suppose a trailer is found on its side. That is DOD 5, with an expected wind speed of 98 mph. But suppose the inspector discovers the trailer had unusually substantial ground anchors. He or she might then assign a higher speed, anything up to 114 mph. On the other hand, if the ground anchors were weak or nonexistent, he or she might assign a speed of 90 mph or even lower, down to 84 mph.
In assessing a tornado, the inspectors try to find more than one Damage Indicator. These can be averaged and compared to the table at the top of the page to assign an EF number to the tornado.
Ideally, the correlations between wind speed and degrees of damage would be done by, for example, testing structures in a wind tunnel. That was far beyond the means of those revising the scale, and so instead they resorted to a Delphi-like process in which they polled panels of tornado-watching experts.
1. For a complete list of the damage indicators, see www.spc.noaa.gov/faq/tornado/ef-scale.html
The key document, with color photographs depicting some degrees of damage for some of the damage indicators:
A Recommendation for an Enhanced Fujita Scale (EF-Scale). Revision 2.
Lubbock (TX): Wind Science and Engineering Center, Texas Tech University, 10 October 2006.
Available on the web at www.depts.ttu.edu/nwi/Pubs/FScale/EFScale.pdf
NOAA has a web page devoted to the Enhanced Fujita Scale.
The website of the National Wind Institute at Texas Tech, Fujita’s final home base, has more information on the scale and its application than anywhere else. Unfortunately, as far as the Fujita scales go, it is poorly organized and requires heavy use of the site's search function.
A video on development of panels to resist lumber flung by EF5 winds:
Thoughts on improving the EF scale:
Roger Edwards and Harold E. Brooks.
Possible impacts of the enhanced Fujita scale on United States tornado data.
Preprints, 25th Conference on Severe Local Storms (2010)
available online at http://www.spc.noaa.gov/publications/edwards/eftrends.pdf
Roger Edwards, James G. LaDue, John T. Ferree, Kevin Scharfenberg, Chris Maier and William L. Coulbourne.
Tornado intensity estimation. Past, present, and future.
Bulletin of the American Meteorological Society, vol. 94, no. 5, pages 641-653 (May 2013).
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Last revised: 5 May 2014.