Fujita Scale Enhancement Project
Dr. Theodore T. Fujita invented the Fujita Scale in 1971. The purpose of the scale was to classify tornadoes by intensity, i.e. to distinguish between weak ones and strong (intense) ones. He defined six categories F0 to F5, with F0 being weak and F5 being the most intense expected. Fujita then envisioned the type of damage he would expect in each of the six categories. See "Original Fujita Scale" table for descriptions of damage."
Fujita also assigned wind speed ranges to each category, which are non-overlapping and increase in magnitude from F0 to F5. The ranges also are shown in "Original Fujita Scale". The wind speeds in the table have been converted to three-second gust speeds at 10 m height in flat open terrain. There were no attempts at a definitive correlation between appearance of damage and wind speed. Fujita used his best judgment as the world’s leading expert on tornadoes at the time. Both the meteorological and engineering communities immediately accepted the Fujita Scale because in 1971 there was a critical need for it. Among other needs, a growing nuclear power industry was struggling to design tornado-resistant facilities.
The job of classifying tornadoes has fallen to the National Weather Service. NWS personnel go to the field following a tornado event and identify what they believe to be the most intense damage produced by the storm. They then assign a Fujita-Scale category based on the appearance of damage.
A number of limitations on the Fujita Scale were soon recognized. The concept does not recognize differences in construction quality; the damage descriptions consist of a limited number of damage indicators; assessments are subject to bias of the person making the Fujita Scale assignment. Several studies indicate that the Fujita Scale overestimates wind speeds in categories F3, F4 and F5. In some instances the damage indicators are oversimplified. “Foundations swept clean” is frequently cited as basis for F5, regardless of the type of structure or translational speed of the tornado.
Recognizing the need to enhance the Fujita Scale, personnel here at the Wind Science and Engineering Center agreed to take the lead in a project to improve and enhance the Fujita Scale. To date the following steps have been or will soon be taken:
Expert Elicitation Process
Following a protocol established by the Senior Seismic Hazard Advisory committee, we undertook an expert elicitation. After identifying 27 damage indicators consisting of buildings and other structures, varying degrees of damage were defined for each damage indicator ranging from no damage to total destruction. Experts were asked to assign an expected, upper and lower bound wind speed corresponding to each degree of damage. After the first round, we provided the experts with the tabulated results. Discussions were held to clarify some of the degrees of damage. We recently conducted a second elicitation round and tabulated the results. We provided the experts an opportunity to make changes in a third elicitation round, this resulted in few changes.
The Expert Elicitations include the data and results for each damage indicator. Each page contains the name and abbreviation of the building or other structure, a description of the construction, the degrees of damage and mean expected, upper and lower value of expert’s estimated wind speeds, and plots of the expert’s results. The "Expert Qualifications" includes the names and background of the participating elicitators. Additional background and information relating to the study is included in the 21st Severe Local Storms Conference paper.
One primary goal for this project is to gain a consensus of users of the
new Enhanced F-Scale.
Wind Science and Engineering
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Copyright © April 2002