Deciphering the Enhanced Fujita Scale
When tornados break out, people always want to know, how strong were they? But making that assessment is an in-depth process. It requires site surveys by National Weather Service (NWS) meteorologists, who use damage indicators and degrees of damage to estimate tornado strength. Following the assessment, a tornado gets a rating using the Enhanced Fujita Scale, or EF scale. The scale rates tornadoes from 0 to 5, as follows:
EF0: 65-85 mph (29-38 m/s)
EF1: 86-110 mph (39-49 m/s)
EF2: 111-135 mph (50-60 m/s)
EF3: 136-165 mph (61-73.8 m/s)
EF4: 166-200 mph (74-89 m/s)
EF5: 201+ mph (90+ m/s)
EF0 and EF1 tornadoes are considered “weak”, EF2 and EF3s are “strong” and EF4 and EF5 tornadoes are considered “violent”. While EF4 and EF5 tornadoes are the most devastating, they only make up 2% of all tornadoes.
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How are wind strengths measured?
Unlike the Saffir-Simpson Scale, which rates the intensity of hurricanes based on observed or measured wind data, the Enhanced Fujita scale estimates a three-second wind gust. They use this to determine the strength of a tornado at different points of damage along the suspected path.
The scale we use now is an update from the original Fujita, or F-scale, which tornado scientist Ted Fujita introduced in 1971. But the original scale had some drawbacks. It focused solely on damage from the tornado. What if the tornado passed over a field where there was no structural damage to assess? And it did not distinguish between a mobile home versus a masonry-constructed building.
So meteorologists and engineers updated the original F-scale to better reflect damage from tornadoes in relation to wind speed based on how structures are designed. It also looks at damage to vegetation. The EF update became operational in 2007.
The survey process
The National Weather Service spends hours, and sometimes days, surveying the paths of tornadoes. Meteorologists with the NWS follow the path and identify damage indicators (DI) from a list of 28 different indicators, which include things like damage to barns and outbuildings, schools and trees.
Within each of the 28 damage indicators, there is also a degree of damage (DOD). For example, the damage indicator for a one- or two-family home has ten degrees of damage ranging from “visible damage” to “destruction of engineered and/or well constructed residence; slab swept clean.”
Within those degrees of damage there are also bounds: expected, lower bounds and upper bounds. Meteorologists evaluate multiple damage indicators along the path of the tornado before they use all the data collected to determine the final EF rating. And they repeat this process for every sustained tornado damage path. (If you want to look at the DOD breakdown of each 28 DI, visit this NWS page and click on the correlating number to each DI.)
The history of the Enhanced Fujita Scale
Dr. Tetsuya Theodore (Ted) Fujita of the University of Chicago developed a scale to categorize a tornado’s intensity in February of 1971. This was the Fujita scale, or F-scale. Like the Enhanced Fujita scale we use now, the original Fujita scale also categorized tornadoes from F0-F5.
Fujita hoped his scale would help categorize tornadoes by their intensity. But he also wanted to estimate the wind speed of the tornado by assessing the damage. Just three years later, Fujita and his team would survey every tornado from the Super Outbreak of April 1974. This event solidified the Fujita Scale as the pillar of tornado ranking, until its reevaluation decades later.
While the Fujita scale was groundbreaking, by the late 1990s, meteorologists started to realize that the original wind speed estimates were too high. With the F-scale, an F3 tornado could have wind speeds of 200 miles per hour (90 m/s). But in the current Enhanced Fujita Scale, wind speeds of 200 miles per hour (90 m/s) indicate an EF5.
Meteorologists also determined that the evaluated damage with the F-scale could be too subjective, disregarding construction material. Fujita supported the reevaluation of his original F-scale. However, he died in 1998 and therefore was not alive to see the new scale put into practice in February 2007. It kept the original concept of the F-scale in mind, with modifications to include more damage indicator points.
The most recent EF5 tornado
It’s been more than 10 years since an EF5 tornado hit in the United States. On May 20, 2013, an EF5 tore through Moore, Oklahoma, and surrounding communities. The National Weather Service surveyed the damage and found most of it earned an EF4 rating. But there were a few areas, including at a local elementary school, that were consistent with EF5 damage. The tornado was on the ground for 40 minutes and 14 miles (23 km). At its largest it was 1.1 miles wide (1.8 km). Sadly, 24 people were killed in this tornado, and more than 200 were injured.
Bottom line: National Weather Service meteorologists use the Enhanced Fujita Scale to rate a tornado’s strength. It’s a long process! It requires National Weather Service meteorologists to go out and survey damaged areas, seeking multiple damage indicators and determining the degree of damage of those indicators. They then determine the tornado rating based on these data.
Via NOAA Storm Prediction Center
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