- You may have seen two different expressions for tornado “ratings.” EF and F.
- The EF scale, 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 (F scale).
- Each scale relies on slightly different methodologies to assess damage and approximate wind speed.
(WJET/WFXP) — There have been tornadoes in the Midwest recently, and now the National Weather Service is tasked with giving ratings to each twister. Preliminary assessments of these tornadoes are expected to be determined in the coming weeks. But he may have seen two different expressions for tornado “assessment.” EF and F.
But what’s the difference?
The differences between scales are important. The EF scale, 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 (F scale).
The difference is clear when you look at recent tornadoes. On April 27th, a tornado hit the town of Marietta, Oklahoma. This tornado was given a preliminary rating of EF4. Approximate maximum wind speeds were 165 to 170 miles per hour. Using Fujita’s original scale, it would be classified as an “F3” tornado, which caused extensive damage, including peeling off roofs and walls, overturning trains, and uprooting trees.
What is Original Fujita Scale?
Even after several decades, research on tornadoes continues. Each scale relies on slightly different methodologies to assess damage and approximate wind speed.
The original Fujita scale, named after University of Chicago developer T. Theodore Fujita, considered approximate wind and general damage estimates. F-scale winds are also not meant to be used literally, and exact wind speed numbers are purely speculative and not based on science or engineering.
| scale | Estimated wind speed (MPH) | typical damage |
| F0 | < 73 | light damage. Chimneys were damaged, tree branches were broken, shallow-rooted trees fell, and signboards were damaged. |
| F1 | 73-112 | medium damage. Roofs were torn off, mobile homes were thrown off their foundations or overturned, and cars were blown off the road. |
| F2 | 113-157 | considerable damage. The roof of a frame house was torn off, a mobile home was destroyed, a boxcar was overturned, large trees were broken or uprooted, light missiles were fired, and cars were lifted off the ground. |
| F3 | 158-206 | serious damage. Parts of the roofs and walls of well-built houses were torn off, a train was overturned, most of the trees in the forest were uprooted, and heavy cars were lifted off the ground and thrown. |
| F4 | 207-260 | catastrophic damage. Well-built houses are torn down, buildings with weak foundations are blown away, cars are thrown, and large missiles are fired. |
| F5 | 261-318 | incredible damage. Sturdy frame houses collapse from their foundations and are swept away, car-sized missiles fly more than 100 meters into the air, tree bark peels off, and unbelievable phenomena occur. |
The problem began to arise that different wind speeds could cause similar damage from location to location and even from building to building. This scale also did not take into account structural differences and was based only on the worst damage, even if it was a single building or house. This made it nearly impossible to measure the actual speed required to cause damage without thorough engineering analysis.
Then, in 1992, Fujita published a “modified” Fujita scale that took into account different building materials and structure types, but still overestimated wind speeds above F3.
After the May 3, 1999 tornado in Moore, Oklahoma, the possibility that wind estimates were too high on the F scale was brought to the forefront. Meteorologists, emergency managers, and engineers converged on Moore to study the vulnerabilities of structures destroyed by the May 3, 1999 tornado (described in FEMA 342).
Engineers say many homes are rated to withstand winds of 160 miles per hour. This raised the question, if the tornado’s wind speeds exceed 200 miles per hour, how can the structure account for this estimate when most of the tornado has dissipated?
Two years later, in 2001, 26 experts met in Grapevine, Texas, to identify the problem and develop a strategy to improve or replace Fujita Scale. The first revised version of the EF scale proposal was published in his 2004 and proposed changes to the way tornado damage is ranked and interpreted. The proposal will be revised one more time in 2006 before being formally adopted on February 1, 2007.
What is Enhanced Fujita Scale?
While the original Fujita scale is based on approximate wind and general damage, the expanded Fujita scale is based on several factors.
This is still a set of damage-based wind estimates, but uses various damage measurements to estimate 3-second gust speeds.
| Operational EF scale | |
| EF number | 3 second gust (mph) |
| 0 | 65-85 |
| 1 | 86-110 |
| 2 | 111-135 |
| 3 | 136-165 |
| Four | 166-200 |
| Five | 200 or more |
Damage is displayed based on an 8-level damage rating for 28 metrics at the time the damage occurs. here. Each of these indicators has a damage type associated with it, known as a loss degree, which results in different wind gust estimates based on the damage for each indicator.
During a damage assessment, NWS survey teams survey an area, document damage, and rank damage using one of 28 indicators. Officials then use their best judgment to assess multiple structures, categorize damage, and identify upper and lower wind speed limits. The findings can be used to derive and estimate the estimated 3-second gust speed. At this point, a preliminary assessment can be determined.
Compared to the Fujita scale, the EF scale has stricter wind speed standards for each rating. On Fujita’s scale, maximum wind speeds must exceed 261 miles per hour for a tornado to be classified as an F-5. For a tornado to be classified as an EF-5 on the EF scale, the estimated 3-second gust speed must exceed 200 miles per hour.
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