The original Fujita or F- Scale of tornado strength was published in 1971 by Dr. Theodore Fujita (revised 1973, Fujita and Pearson). Its purpose was to provide a method for estimating the intensity of a tornado by looking at the damage afterward. Certain levels of damage were believed to correlate with particular wind speed ranges; however, this was never proven scientifically (or even tested). Tornadoes move so quickly (average forward speed is 30 mph) and last such a short time (4 minutes is the average time on the ground) that it is virtually impossible to determine actual wind speeds. Also, since the damage descriptions in the original scale were general and vague, the scale was arbitrary and dependent on interpretation by the observer.
Clearly, meteorologists needed a better scale. Development of the Enhanced Fujita Scale began in 2001 at Texas Tech University’s Wind Science and Engineering Center. First, 28 damage indicators (DIs – different types of buildings, structures, and trees) were chosen. Next, a set of degrees of damage (DODs) was identified for each indicator, increasing in intensity from slight damage to total destruction.
In order to determine the wind speed range most likely correlated with each degree of damage (DOD), a technique called “expert elicitation” was used. A panel of experts in the field of meteorology was chosen, and each expert gave his or her estimation of the wind speed needed to cause that DOD. The experts’ figures were then averaged to give a range of wind speeds. Thus, for each of the 28 indicators, wind speeds can be estimated based on damage.
The assignment of intensity categories in the Enhanced Fujita Scale, just as in the original scale, is derived from observed damage and associated wind speeds. The category of a tornado is never assigned from only one damage indicator. Instead, several DIs are examined.
The original Fujita Scale listed hypothetical tornado intensities from F0 to F12, but in practice it was discovered that the extreme upper limit was F5. Only 1% of tornadoes fall into that category. The new Enhanced Fujita Scale does not go above F5, so it has only six categories, as follows:
EF0 – The lowest tornado intensity has estimated 3-second wind gusts from 65-85 mph. Damage at this level might include loss of less than 20% of shingles on a house, shattered glass in windows and doors of a fast food restaurant, and broken skylights or atrium walls in a large shopping mall.
EF1 – At this level, 3-second wind gusts are estimated at 86-110 mph. An EF1 tornado might cause the collapse of the chimney or garage doors on a house, loss of the roof covering on a fast food restaurant, and loss of rooftop heating and air conditioning equipment on a high rise building. A single-wide mobile home would roll over and the roof and walls might be destroyed as well, depending on their construction.
EF2 – Wind gusts of 111-135 mph are estimated to occur. The entire roof of a house might be lifted off, leaving most of the walls still standing. The canopies or covered walkways of a fast food restaurant would be destroyed and some outer walls might fall. The roof structure of a large shopping mall would most likely be lifted off or collapse. A single or double wide mobile home would be completely destroyed.
EF3 – At this intensity, 3-second wind gusts of 136-165 mph can be estimated. Damage might include collapse of most exterior walls of a house. In a fast food restaurant, only closely spaced interior walls would remain, and it might be totally destroyed. High rise buildings would have broken glass, loss of roofing material, and exterior damage.
EF4 – Wind gusts of 166-200 mph are expected from an EF4 tornado. Many homes would experience total destruction; only small interior rooms such as closets might remain. Fast food restaurants would definitely be destroyed, as would sections of strip malls and large retail buildings.
EF5 – The highest level of tornado has winds of greater than 200 mph. Houses, fast food restaurants, strip malls, and large shopping malls would be totally destroyed. Mid-to-high rise buildings (more than 5 stories) would suffer significant structural damage.
The new scale was implemented on February 1, 2007. Since that date, all tornadoes in the United States have been rated using EF categories. However, historical tornadoes are still categorized with the original Fujita scale.