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Enhanced Fujita Scale
Developed in 1971 by Tetsuya "Ted" Fujita (1920 - 1998) "Mr. Tornado"  of the University of Chicago
The Fujita Scale is based on structural strength and engineering research done by Ted Fujita and a host of others. This research created the Fujita scale as a measurement of a tornadoes strength or intensity. Simply put, how intense would a wind have to be to move a cardboard box or to move bricks?  One questions to ask would be:  How fast or intense would the winds have to be to knock over a brick wall,? Obviously those winds would be significant.  Through extensive research the Fujita Scale was developed based on damage, not wind speeds. Based on the extent of this damage conclusions are drawn as to what the winds would likely have to be to cause that type of damage. Since two wooden structured homes can be vastly different in quality and strength, there can be some misleading conclusions from the damage. Because of this and other overly general conclusions, storm researchers came up with the Enhanced Fujita scale to better reflect a tornado’s strength.
Below is the new Enhanced Fujita scale started February  2007.

Fujita Scale Derived EF Scale Operational EF Scale
F Number Fastest 1/4-mile (mph) 3 Second Gust (mph) EF Number 3 Second Gust (mph) EF Number 3 Second Gust (mph)
0 40-72 45-78 0 65-85 0 65-85
1 73-112 79-117 1 86-109 1 86-110
2 113-157 118-161 2 110-137 2 111-135
3 158-207 162-209 3 138-167 3 136-165
4 208-260 210-261 4 168-199 4 166-200
5 261-318 262-317 5 200-234 5 Over 200

Enhanced F Scale Damage Indicators

NUMBER (Details Linked) DAMAGE INDICATOR ABBREVIATION
1 Small barns, farm outbuildings SBO
2 One- or two-family residences FR12
3 Single-wide mobile home (MHSW) MHSW
4 Double-wide mobile home MHDW
5 Apt, condo, townhouse (3 stories or less) ACT
6 Motel M
7 Masonry apt. or motel MAM
8 Small retail bldg. (fast food) SRB
9 Small professional (doctor office, branch bank) SPB
10 Strip mall SM
11 Large shopping mall LSM
12 Large, isolated ("big box") retail bldg. LIRB
13 Automobile showroom ASR
14 Automotive service building ASB
15 School - 1-story elementary (interior or exterior halls) ES
16 School - jr. or sr. high school JHSH
17 Low-rise (1-4 story) bldg. LRB
18 Mid-rise (5-20 story) bldg. MRB
19 High-rise (over 20 stories) HRB
20 Institutional bldg. (hospital, govt. or university) IB
21 Metal building system MBS
22 Service station canopy SSC
23 Warehouse (tilt-up walls or heavy timber) WHB
24 Transmission line tower TLT
25 Free-standing tower FST
26 Free standing pole (light, flag, luminary) FSP
27 Tree - hardwood TH
28 Tree - softwood TS

Enhanced Fujita Scale article has been completed by the Wind Science and Engineering Center at Texas Tech University.  Click here for the (pdf) document that details how the new scale is derived.  Discussed in the article is the need for revising the current rating system used that was created by Dr. Ted Fujita.
Thanks to http://www.spc.noaa.gov and http://www.wind.ttu.edu for the new information.
OLD FUJITA SCALE BELOW

Intensity Phrase  Type of Tornado ........ Estimated Wind speeds
F0...................Gale tornado.............. winds of 40-70 mph
Some damage to chimneys; break branches off trees, pushes over shallow- rooted trees-, damages sign boards.
Fl................... Moderate tornado.........winds of 73-112 mph
The lower limit is the beginning of hurricane wind speed- peels surface off roofs; mobile homes destroyed.
F2...................Significant tornado........winds of 113-157 mph
Considerable damage. Roofs torn off frame houses- mobile homes demolished; boxcars pushed over; larger trees snapped or uprooted-, light object missiles generated.
F3...................Severe tornado.............winds of 158-206 mph
Roof and some wall torn off well constructed houses trains overturned; most trees in forest uprooted
F4...................Devastating tornado........winds of 207-260 mph
Well-constructed houses leveled-, structures with weak foundations blown off
some distance; cars thrown and large missiles generated.
F5..................Incredible tornado...........winds of 261-318 mph
Strong frame houses lifted off foundations and carried considerable distances to disintegrate- automobile sized missiles fly in excess of 100 meters; trees debarked; steel reinforced concrete structures badly damaged.
F6.................Inconceivable.................winds of 319-379 mph
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
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