University of Chicago meteorologist Ted Fujita devised the Fujita Scale, the internationally accepted standard for measuring tornado severity. His goal was to create categories that could separate weak tornadoes from strong ones. The F Scale also met a need to rate both historical and future tornadoes according to the same standards.
The version used today—the Enhanced Fujita Scale—ranges from EF0 tornadoes with winds of 65 to 85 miles an hour, to EF5 tornadoes with winds exceeding 200 miles an hour. The U.S. National Weather Service has rated tornadoes according to the Fujita Scale since 1973.
Tornadoes are extremely dangerous; if you are in a warning zone, take shelter immediately and do not wait to find out what category it is.
How was the Fujita Scale created?
Tornadoes have been seen on every continent, but due to its geography the mainland U.S. typically records more tornadoes per year than anywhere else in the world. Before the mid-twentieth century, our understanding of how they worked, why they formed, and how to predict them, was very limited. They can be devastating, but they strike with relative infrequency and without much warning—all of which makes them difficult to study.
In a 1971 publication titled “Proposed Characterization of Tornadoes and Hurricanes by Area and Intensity,” Fujita laid out his scale to measure tornado intensity by linking estimated wind speeds to the Beaufort Wind Scale and to the degree of damage that they caused. Fujita based the latter on the hundreds of tornado-damage surveys that he had conducted on the ground and from the air.
For example, his scale said that tornadoes with winds of 207 to 260 miles an hour, causing devastating damage, would be called classified as F4. Fujita described the damage that F4 tornadoes cause as: “Whole frame houses leveled, leaving piles of debris; steel structures badly damaged; trees debarked by small flying debris; cars and trains thrown some distances or rolled considerable distances; large missiles generated.”
The F Scale quickly impacted the field of meteorology. Today, a version of that scale is still used to categorize every tornado in the U.S.
What is the Enhanced Fujita (EF) Scale?
After three decades, the Weather Service decided to rework the scale in order to better define damage indicators, to account for construction quality, and definitively correlate damage with wind speed.
A team of meteorologists and engineers issued the Enhanced Fujita Scale. Among the experts who devised the new scale were two of Fujita’s former students: severe-weather experts Greg Forbes, SM’73, PhD’78, now retired from The Weather Channel, and Roger Wakimoto, PhD’81, of the University of California, Los Angeles.
Adopted on Feb. 1, 2007, the EF Scale remains the standard of the U.S. National Oceanic and Atmospheric Administration (NOAA). Though it was devised according to American construction practices, other nations now use adapted versions of the EF Scale.
How are tornadoes classified?
The EF Scale primarily classifies tornadoes based on the damage left behind. By surveying the damage, scientists assign a wind speed that is likely to have caused that damage.
There are 28 categories that range from flagpoles and power lines all the way up to schools and high-rise buildings. This is because scientists need to take into account how sturdy different structures are; a barn will sustain catastrophic damage at much lower speeds than a brick house.
Each category contains its own “degree of damage” scale, starting with the threshold of visible damage at the low end to destruction at the high end. For example, damage to a motel can range from shingles stripped off, to collapse of exterior walls, to “total destruction of entire building.”
Multiple damage indicators are considered before an EF Scale rating is assigned to a tornado.
With the EF Scale as a metric, the National Weather Service now can consistently rate the severity of all tornadoes. The U.S. experiences about 1,200 tornadoes a year.
See the full scale on the NOAA website.
What are examples of F5 tornadoes?
EF5 tornadoes are the most violent tornadoes according to the EF Scale. They have winds of more than 200 miles per hour and are capable of damaging or destroying even large buildings. They can crush large trucks or carry them long distances. The worst damage is often caused by trees, cars and other objects that are lifted off the ground and become projectiles.
Topping the list of the nation’s deadliest tornadoes is the one that killed 695 people along a extraordinarily long path through Missouri, Illinois, and Indiana on March 18, 1925. Known as the “Tri-State Tornado,” this storm is classified as an EF5. It lasted for a record-setting 219 miles and is estimated to have destroyed 15,000 homes.
As of 2022, the most recently recorded EF5 tornado occurred in Moore, Oklahoma, on May 20, 2013. It stayed on the ground for nearly 40 minutes and grew to a mile wide at times, causing tremendous damage—including to several elementary schools—and 25 deaths.
After the F Scale’s introduction, meteorologists came to realize that F3, F4, and F5 tornadoes account for only about 9 to 12 percent of all tornadoes, but they produced 88 percent of tornado fatalities.
Experts recommend familiarizing yourself with the best locations to take shelter before a storm arrives, and to stay updated with alerts when severe weather may occur. More tips are available at Ready.gov.
Why do some tornadoes destroy one building but leave the one next door unharmed?
Scientists believe that what appears to a single tornado can contain multiple smaller vortexes that circle around each other, known as “subvortices.” This explains why a tornado may wipe one house off its foundation while leaving the one next door untouched.
(When Fujita first proposed this in the 1970s, meteorologists scoffed—but it is now widely accepted.)
Noboru Nakamura
—Prof. Kunle Odunsi
