I've been fielding a few questions on the EF scale. So I
thought I would do a little write-up about the the EF scale. I will briefly discuss the F and EF scale, discuss
why the Vilonia, Arkansas tornado wasn't an EF 5, and discuss a weakness in the
EF scale.
Outbreaks such as the recent April 27-30 2014 tornado outbreak, often sparks a lot of controversy in social
media and weather discussion sites revolving around the damage left behind. The conversation is normally about why was a
tornado rated a particular way?
Admittedly, I know it's hard not to look at pictures and video of
severe damage left in the wake of a tornado. We try to understand the kind of force it takes to reduce towns and
cities to churned rubble. A violent tornado destroys in seconds, things
we build, things that we think are powerful and strong. I've seen the devastation up close and
personal. I've stood there amid devastation, looking in every direction and
seeing nothing but desolation, seeing a landscape that looks straight out of
the gates of hell. I too can only imagine the amount of force brought to bear
on the world of humankind.
This was the reason
University of Chicago meteorologist Tetsuya "Ted" Fujita, in
collaboration with Allen Pearson director of the National Severe Storms Forecasting Center, developed the original Fujita scale. To try to
understand the inconceivable power of the tornado. The Fujita scale, or F-Scale, a system of
classifying tornado intensity based on damage to structures and vegetation. The F-scale used a scale of F0 to F5, with the
least intense tornadoes rated "F0" and the most intense rated
"F5.". It was used by the
National Weather Service from 1973 until 2007, when the Enhanced Fujita
scale (EF scale) was officially adopted.
Here is a video about Dr. Fujita.
Why change the EF Scale?
Over the years, the F-Scale revealed some weaknesses. The
estimation of wind speed in relation to damage was difficult to apply with no
damage indicators. It made no allowance for difference in construction. Often the assessment was based on the worst of
the damage in an area, even if it was just one building or one house. Another
weakness of the Fujita scale, was that it overestimated wind speeds greater
than F3. The old F-scale also placed too much emphasis on estimated wind speeds.
Because of these flaws a change was needed . The EF scale was formulated
by a team of meteorologists and engineers called together by the Wind Science
and Engineering Research Center at Texas Tech University. The main point of the
enhanced scale, was that it would continue to maintain and support the original
database. There was to be conformity between the original Fujita scale and the
Enhanced Fujita scale. A few additions were to be made as well. The Enhanced
Fujita scale would have a more consistent assessment of damage. It would include more detail on damage, , this
would include photos and examples, not only structures, but vegetation as
well, the EF scale would allow for
different types of construction, note the mean and maximum damage width, the latitude
and longitude of the starting and ending point of the damage path, who was on
the survey, time spent conducting the
survey, a PC-based expert system, and
enhanced training materials.
It should be noted that the Enhanced F-scale is a set of wind
estimates (not measurements) based on damage. It uses three-second estimated
gusts estimated at the point of damage. Important: The 3 second gust is not the
same wind as in standard surface observations. Standard measurements are taken
by weather stations in open exposures, using a directly measured, "one
minute mile" speed. These estimates vary with height and exposure.
Forensic meteorologists use 28 damage indicators and up to 9 degrees of damage
to assign estimated speeds to the wind gusts.
The 28 Damage Indicators used in applying the Enhanced Fujita
Scale:
1: Small barns, farm outbuildings
2: One- or two- family residences
3: Single-wide mobile home
4: Double-wide mobile home
5: Apartment, condo, townhouse (3 stories or less)
6: Motel
7: Masonry apartment or motel
8: Small retail building (fast food)
9: Small professional building (branch bank)
10: Strip mall
11: Large shopping mall
12: Large, isolated retail building
13: Automobile showroom
14: Automotive service building
15: School- 1 story elementary (interior or exterior halls)
16: School- jr. or sr. high school
17: Low-rise (1-4 story) building
18: Mid-rise (5-20 story) building
19: High-rise (over 20 stories) building
20: Institutional building (hospital, university)
21: Metal building system
22: Service station canopy
23: Warehouse
24: Transmission line tower
25: Free-standing tower
26: Free standing pole (light, flag)
27: Tree- hardwood
28: Tree- softwood
1: Small barns, farm outbuildings
2: One- or two- family residences
3: Single-wide mobile home
4: Double-wide mobile home
5: Apartment, condo, townhouse (3 stories or less)
6: Motel
7: Masonry apartment or motel
8: Small retail building (fast food)
9: Small professional building (branch bank)
10: Strip mall
11: Large shopping mall
12: Large, isolated retail building
13: Automobile showroom
14: Automotive service building
15: School- 1 story elementary (interior or exterior halls)
16: School- jr. or sr. high school
17: Low-rise (1-4 story) building
18: Mid-rise (5-20 story) building
19: High-rise (over 20 stories) building
20: Institutional building (hospital, university)
21: Metal building system
22: Service station canopy
23: Warehouse
24: Transmission line tower
25: Free-standing tower
26: Free standing pole (light, flag)
27: Tree- hardwood
28: Tree- softwood
Each one of these indicators have a description of the typical construction for that category of
indicator. Then, the next step is to the determine the degree of damage. Each degree of damage in
each category is given an expected estimate of wind speed, a lower bound and
upper bound wind speed.
Here's a link to the 28 damage indicators so you can see how all of this is applied. The Storm Prediction Center website.
Why wasn't the Vilonia, Arkansas tornado rated an EF5?
The damage in Vilonia was very severe and extensive. Many buildings
were damaged or destroyed. In the emails
and instant messages I received, Several
noted that the severe damage in Vilonia was indicative of
EF 5 damage, some even saw places where slabs were all that was left. And that is very true. So if the houses were completely swept away,
then the assessment teams must have gotten it wrong. Isn't slabs left where houses once stood EF
5 damage? the quick answer is yes. BUT the not so quick answer is maybe.
When the damage assessment teams out of NWS Little Rock looked at the damage close up,
they found poor construction in several
buildings. Some of these buildings had even been built
after the Super Outbreak of 2011. what the teams found was homes secured to the
foundation with cut nails instead of anchor bolts, or they had anchor bolts, but no washers or nuts holding the house in place.
Photo on left shows the foundation was held down with cut nails. The photo on the right, shows the lack of washers and nuts holding down on the Sill Plates of the house. Photo Credit NWS Little Rock.
Without anchor bolts the buildings just didn't have the strength
to resist the sliding and lifting loads that result from strong winds. Therefore the lack of anchor bolts meant much
less wind force was needed to sweep complete
houses away. Because of this the tornado
was rated EF 4 instead of EF 5.
Under the old F scale the Vilonia, Arkansas tornado would most likely
have been rated an F5. But because the new EF scale pays attention to quality
of construction it caught this oversight and gave the tornado an appropriate
rating. This is why you can't rate
tornado damage from pictures or video.
Problems with the EF Scale.
IMO, the EF scale focuses to much on structure damage. For trees, it only allows for soft wood or hard wood.
There is no doubt that rural tornadoes strike far more trees than
human made structures. So if the NWS wants to assign the most realistic wind
speed estimates along the damage path, it only makes sense that trees make up
part of the damage indicators. However out of the 28 damage indicators only two
allow for tree damage. This is much too
generalized to capture variability in tree strength across different species.
Examples of hardwood trees would be oak, birch, maple, anacua,
cherry, elm, ash. Examples of softwood trees would be pine, fir, spruce, cedar,
hemlock and cypress.
I've walked several damage paths.
I can tell you, I've seen many examples of how tornado damage differs
among different types of trees. Each species has different root structures,
leaf design, and type of wood grain. Therefore , it only makes sense that each
would react to strong winds differently.
A softer hardwood would be more susceptible to breakage and debarking, than a harder type of
softwood., types of roots and leaf structure would have a lot to say about why
a tree became uprooted, different types of trees have different types of bark characteristics,
each becoming debarked in different ways.
Because of all of these things, tree species has a lot of potentially very
useful information to pass along. Understanding and noting damage to various tree species would not only
be useful in rating rural tornadoes. It would be one more clue to help assign a
rating in more urban areas, and residential neighborhoods.
Well that should give everyone a better understanding of the EF
scale. If you enjoyed this post, here is another you might like. Tornadic winds versus Straight line winds.
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