What’s the difference between F4 and EF4 tornadoes?

(WJET/WFXP) — A tornado outbreak recently slammed the Midwest, and now the National Weather Service is tasked with giving each twister a rating. Preliminary ratings for those tornadoes are expected to be finalized in the coming weeks. But you may have seen two different phrases for tornado “ratings,” EF and F.

But what is the difference?

The differences between the scales are significant. The EF scale, better known as the Enhanced Fujita Scale, was adopted by the National Weather Service (NWS) in 2007. This scale is a revised version of what was known as the Fujita Scale, or F-Scale.

The difference becomes clear when using a recent tornado as an example. On April 27, the town of Marietta, Oklahoma, was hit by a tornado. That tornado was given a preliminary rating of EF4. Its approximate peak winds clocked in at 165-170 mph. If we used the original Fujita scale, it would have been classified as an F3 tornado, which is associated with severe damage, including roofs and walls torn off, trains overturned and trees uprooted.

What is the Original Fujita Scale?

As the decades have passed, research has been constantly ongoing into tornadoes. Each scale relies on a slightly different methodology for assessing the damage and approximating the wind speeds.

The original Fujita scale, named after its developer T. Theodore Fujita of the University of Chicago, looked at approximate winds and general damage approximations. F-scale winds were also not meant to be used literally, and precise windspeed numbers were purely guesses and not based on any science or engineering.

The issue began to arise that different wind speeds may cause similar-looking damage from place to place and sometimes from building to building. The scale also failed to consider differences in construction and was based only on the worst damage, even if it was only one building or home. This made it almost impossible to gauge the actual speeds needed to cause the damage without thorough engineering analysis.

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Later, in 1992, Fujita released a “modified” Fujita scale that sought to factor in different building materials and structure types, but it still overestimated wind speeds greater than an F3.

After a May 3, 1999, tornado in Moore, Oklahoma, it was brought to the forefront that the wind estimates may have been too high in the F-scale. Meteorologists, emergency managers and engineers met in Moore to study the weaknesses in the structures destroyed by the tornado of May 3, 1999 (as documented in FEMA 342.)

According to engineers, they claim that many homes are rated to withstand winds of 100 mph. This raised the question of if a tornado has winds over 200 mph, how can the structure reveal this estimate when much of it is gone?

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Two years later, 26 experts met in Grapevine, Texas, in 2001 to identify issues and develop strategies to improve or replace the Fujita scale. The first revision of the EF-scale proposal was released in 2004 and suggested altering the methodology of how tornado damage is ranked and interpreted. The proposal would see one more revision in 2006 before being officially adopted on Feb. 1, 2007.

What is the Enhanced Fujita Scale?

While the original Fujita scale is based more on approximate winds and general damage, the Enhanced Fujita Scale is based on several factors.

While it is still a set of wind estimates based on damage, it also uses various damage measurements to estimate its three-second gust speed.

The damage is based on a judgment, at the point of damage, of eight levels of damage to the 28 indicators, which can be viewed here. These indicators each have an associated damage type, known as the degree of damage, and each one has different wind gust estimates based on the damage of each indicator.

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During the damage assessment, NWS survey teams canvas the area documenting damage and ranking the damage using one of the 28 indicators. The personnel then assess multiple structures using their best judgment to classify the damage and identify upper and lower bounds for wind speeds. Using the results from the survey, the estimated three-second gust speed can be derived and estimated. At this point, the preliminary rating can be determined.

Compared to the Fujita scale, the EF scale tightened the wind speed criteria for each rating. On the Fujita scale, for a tornado to be classified as an F-5, its peak wind speeds must exceed 261 mph. For a tornado to be classified as an EF-5 on the EF-Scale, the estimated three-second gust speed must exceed 200 mph.

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