The Enhanced Fujita Scale (EF Scale)
Go to http://www.wdtb.noaa.gov/courses/EF-scale/index.html
for more information regarding EF-Scale training by the WDTB.
To view the Enhanced Fujita Scale Document, go to
Dr. T. Theodore Fujita first introduced The Fujita Scale in the
SMRP Research Paper, Number 91, published in February 1971 and titled,
"Proposed Characterization of Tornadoes and Hurricanes by Area and
Fujita revealed in the abstract his dreams and intentions of the
He wanted something that categorized each tornado by
intensity and area.
The scale was divided into six categories:
- F0 (Gale)
- F1 (Weak)
- F2 (Strong)
- F3 (Severe)
- F4 (Devastating)
- F5 (Incredible)
Dr. Fujita's goals in his research in developing the F-Scale were
Dr. Fujita and his staff showed the value of the scale's application by
surveying every tornado from the Super Outbreak of April 3-4, 1974.
The F-Scale then became the mainstay to define every tornado that has
occurred in the United States.
The F-Scale also became the heart of the tornado database that
contains a record of every tornado in the United States since
- categorize each tornado by its intensity and its area
- estimate a wind speed associated with the damage caused by the tornado
Figure 1: Number of tornadoes per year, 1950-2004 (click image to enlarge)
The United States today averages 1200 tornadoes a year.
The number of tornadoes increased dramatically in the 1990s as the
modernized National Weather Service installed the Doppler Radar
The National Weather Service modernization also began
the Warning Coordination Meteorologist program increasing partnerships
with media and Emergency Management across the United States.
This program also initiated the training of storm spotters across the
County Warning Area of each Weather Forecast Office.
With more people trained to relay information on storm activity to the Weather
Forecast Office and improved communication and digital technology, more
tornadoes could be reported.
While the Super Outbreak of tornadoes was the spring board for the
F-Scale, it was the Jarrell, TX tornado of May 27, 1997 and the
Oklahoma City/Moore, OK tornado of May 3, 1999 that brought to the
forefront the problem that maybe the wind estimates were too high in
Meteorologists, Emergency Managers and Engineers convened on Moore,
OK to study the weaknesses in the structures destroyed by the
tornado of May 3, 1999.
The findings can be found in the document
FEMA 342, Building Assessment Report, Midwest
Tornadoes of May 3, 1999, Observations, Recommendations and Technical
That document can be found
Engineers claim that many homes are rated to withstand winds to 100
Therefore, the question was raised that if a tornado has
over 200 mph winds, how can the structure reveal this estimate when
much of it is gone?
|The Fujita Scale|
The Fujita Scale is a well known scale that uses damage caused by
a tornado and relates the damage to the fastest 1/4-mile wind at the
height of a damaged structure.
Fujita's scale was designed to connect smoothly the Beaufort
Scale (B) with the speed of sound atmospheric scale, or Mach
Fujita explains explicitly that "F-scale winds are estimated from
structural and/or tree damage, the estimated wind speed applies to
the height of the apparent damage above the ground."
Figure 1 shows graphically the relationship between the three scales.
Figure 2: Fujita's smoothly derived relationship of the F-Scale with the
Beaufort Scale and the Mach Scale as explained to the right.
The Beaufort Scale is defined by the Glossary of of Meteorology (AMS)
as a system of estimating and reporting wind speeds numerically
from 0 (calm) to 12 (hurricane).
The Mach scale is the speed of sound in the atmosphere.
(click image to enlarge)
From this graph, Fujita then released the following descriptions for
the F-Scale http://www.spc.noaa.gov/faq/tornado/f-scale.html.
Over the years, the F-Scale has revealed the following weaknesses:
- It is subjective based solely on the damage caused by a tornado
- No recognition in difference in construction
- Difficult to apply with no damage indicators
- if the 3/4-mile wide tornado does not hit any structures,
what F-scale should be assigned?
- Subject to bias
- Based on the worst damage (even if it is one building or house)
- Overestimates wind speeds greater than F3
And the F-Scale has had its misuses over the years:
- Too much reliance on the estimated wind speeds
- Oversimplification of the damage description
- Judge the F-scale by the appearance of the tornado cloud
- Unrecognizing weak structures
- mobile homes
- modified homes
Fujita recognized that improvement was necessary.
He published his memoirs called Mystery of Severe Storms
in 1992 updating the Fujita Tornado Scale to include an estimate
of f-scale damage then selecting the F-scale as
a combination of f-scales and types of structural damage.
Figure 3: The "Modified" Fujita
Scale. (click image to enlarge)
For example, if a tornado knocks down the walls of an area of homes.
If it is determined that the walls collapsed, then the damage assigned
If it is a brick home, then that lowers the damage to F2.
Then, according to the table since it was a brick structure,
then you +1 making the rating F3.
|The Enhanced Fujita Scale|
When the committee met to develop the Enhanced Fujita Scale (see
original document) one point was made very clear:
it must continue to support and maintain the original tornado database.;
In other word, there must be some conformity to that of the F-Scale
that is listed in the database.
Other ideas were agreed to including:
- Consistent Assessment of Damage
- enhance description of damage with examples and photos
- include not only structures, but also vegetation
- base damage assignment on more than one structure, if available
- develop a PC-based expert system
- develop training materials
- Data Collection
- maintain current tornado database
- surveys should include additional data
- mean and maximum damage path width
- basis for damage assignment
- latitude/longitude of where the path began and ended
- number of hours spent on the damage survey
- names of survey team member(s)
When using the EF-Scale to determine the tornado's EF-rating,
begin with the 28 Damage Indicators.
Each one of these indicators have a description of the typical
construction for that category of indicator.
Then, the next step is to find the Degree of Damage (DOD).
Each DOD in each category is given and expected estimate of wind speed,
a lower bound of wind speed and an upper bound of wind speed.
Let's take the earlier example, a tornado moves through a neighborhood
and walls are knocked down of an area of homes.
Here the Damage indicator would be #2, One or Two Family Residences (FR12).
The typical construction for this fits being a brick veneer siding
The DOD would be a 8, most walls collapsed in bottom floor.
Thus, the estimated winds would be 127 - 178 mph with the
expected wind speed of 152 mph.
Now, taking this number to the EF-Scale,
the damage would be rated EF-3 with winds between 136 - 165 mph.