Wednesday, May 7, 2014

The EF Scale.

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.
If you want to read about Dr  Fujita, Tim Marshall wrote  a tribute to this remarkable man. You can find it here. 

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

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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