What is the Enhanced Fujita Scale?

In order to understand what the Enhanced Fujita Scale is, you'll need to know about the regular old fashioned "Fujita Scale" (click here or for more in depth details). The Fujita scale (or F-Scale) was originally designed as a wind speed scale for tornadoes, where F0 is the weakest and F5 is the strongest. The Fujita scale got it's name from the man who created it in 1971, Dr. Theodore Fujita, a pioneer in the understanding of severe storms.

The key to understanding the Fujita scale is that although it was designed as a wind speed scale, in practice it is a damage scale. Think about it, there's just no way we can walk up to a tornado and measure the wind speed. It's difficult enough to be in the right spot when a tornado forms and even then you'll want to keep your distance.

Well, the Fujita scale is an "estimated" wind speed (estimated being the key word). The estimates come from looking at the damage a tornado has done after it passes. After tornadic events, a National Weather Service survey team will go to the area of destruction and come up with the F-Scale rating based on the damage. For example, based on wind science and structural engineering experiments it's estimated that if a tornado is able to tear a roof off a frame house, the tornado likely had winds from of 113-157 mph or an F2.

So, the survey team will look for things like roofs blown off houses, cars overturned, houses moved off foundations etc...and then assign that tornado a rating based on the Fujita scale.

But what if a really powerful tornado only twisted through corn crops and never hit a house or a town? That makes it impossible to compare the strength of that tornado to one that did hit a town. Also, not all houses are built equally. So how can you estimate wind speed based on destruction of a house knowing that the house may have been poorly constructed? The Enhanced Fujita Scale or EF-Scale addresses some of the shortcomings of the original F-scale and introduces improved science to more accurately estimate wind speeds based on damage. The EF-scale was developed by The Texas Tech University Wind Science and Engineering Research (WISE) Center and will be implemented as of Feburary, 1st 2007.

 

Original Fujita Scale	Estimated Wind Speed Fastest 1/4 mile wind		Enhanced Fujita Scale	Estimated Wind Speed 3 Second Wind Gust
F0	40-72 mph		EF0	65-85 mph
F1	73-112 mph		EF1	86-110 mph
F2	113-157 mph		EF2	111-135 mph
F3	158-207 mph		EF3	136-165 mph
F4	208-260 mph		EF4	166-200 mph
F5	261-318 mph		EF5	Over 200 mph

 

What is different about the Enhanced Fujita Scale?

-It Recognizes differences in construction
There are 28 damage indicators (click here), which not only make the damage survey more accurate, but also recognize the difference between types of housing. It even distinguishes between a double wide and single wide mobile home.

-It uses degrees of damage (DOD)
Within the damage of one structure, say a well built frame house, there are degrees of damage. For example, did the roof blow off, did the vinyl siding blow off, were the walls collapsed in all of the house or just the upper floor? Each of these will correspond to a certain wind speed range. The 28 damage indicators coupled with the 8 degrees of damage help to provide a lot more data to the damage survey and make estimating the wind speed a more accurate process.

-It is more comprehensive
The original Fujita scale made a classification based on the worst damage, even if it was only to one home. The EF-Scale will base it's ultimate classification on more than one structure (if available) and include damage done to vegetation. The damage surveys will also include the mean and maximum damage path and provide the latitude and longitude of where the path started and ended.

-It's new but different-
Although the scale will still be 0 to 5, the wind speeds correlating to these categories changed. Furthermore, the guidelines that lead you to the wind speed range are different. Thus, on the surface it would not be fair to say that an F-3 is an EF-3. In fact, the true EF rating of historical tornadoes likely will never be known.

If the EF-scale is better than the F-scale...I can't really attest to that. Because the way I see it, neither is perfect. Ideally, we would like a standardized way to measure the wind speed of the tornado, not the variable after effects as seen in the damage. However, you make do with what you got. One thing that will be helpful is the increase in the amount of data obtained and archived by the National Weather Service survey teams. The more info the better in understanding tornadoes.

 

 

photo's Courtesy of NSSL http://www.spc.noaa.gov/faq/tornado/torscans.htm