An F5 tornado approaches Oklahoma City on May 3, 1999. Image credit: NSSL NOAA via Flickr | CC BY-ND 2.0

Tornadoes are a terrifying part of life in the United States. After all, the country is located in the most active part of the world for these devastating storms. Despite the fact that we usually see more than a thousand tornadoes every year, it’s still almost impossible for us to measure the winds in the average tornado.

A brilliant meteorologist by the name of Tetsuya “Ted” Fujita overcame this limitation by coming up with an ingenious way to use the damage a tornado leaves behind to estimate how strong its winds were, and thus the Fujita Scale was born. Here are 10 facts about the Fujita Scale, which helps us better understand the strongest storms nature can produce.

1. ZERO TO FIVE

Dennis Mersereau

Both the original Fujita Scale (1973–2007) and the current Enhanced Fujita Scale rate tornadoes from zero to five, with a five indicating the most destructive tornadoes. While images of mile-wide twisters tearing across Oklahoma are burned into our minds, the vast majority of tornadoes are relatively tiny and weak, and wind up on the low end of the scale. Out of the 60,114 confirmed tornadoes recorded between 1950 and 2015, a full 80 percent of them were rated an F0 or F1 on the old scale or an EF-0 or EF-1 on the new scale. Compare that to the 60 F5 or EF-5 tornadoes recorded since 1950.

2. SURVEY SAYS …

Meteorologists assign ratings to tornadoes by conducting ground and aerial surveys of the damage they leave behind to measure width and path length. They also look at the debris pattern to determine if there was a tornado at all (as opposed to straight-line winds), and inspect the damage to homes, businesses, and vegetation to estimate how strong the tornado’s winds were at a particular location.

3. THE SCALE WAS "ENHANCED" IN 2007.

The National Weather Service used the original scale devised by Fujita for more than 35 years (and retroactively rated tornadoes back to 1950), but meteorologists and engineers found that the scale overestimated a tornado’s strength. The new Enhanced Fujita Scale includes building quality as a factor in determining ratings, and for good reason. A tornado that destroyed a home built in 1940 likely had weaker winds than a tornado that destroyed a similar home that was built in 2015. Factoring in construction standards gives meteorologists a much better idea of a tornado’s true strength.

4. ENGINEERING PLAYS A BIG ROLE IN THESE RATINGS.

The Enhanced Fujita Scale is as much about engineering as it is about the weather. Meteorologists teamed up with engineers to figure out just how strong winds have to be to cause certain levels of damage. They use 28 different categories to survey damage to objects ranging from trees and barns to sturdy buildings like schools or prisons. For example, a tornado collapsing the walls at a box store like Walmart would likely make the tornado an EF-3 with winds near 140 mph.

If a tornado struck a hospital and deformed the entire structure, the expected wind speeds would be in excess of 200 mph, making the tornado an EF-5. This happened in Joplin, Missouri, in May 2011 when a mile-wide twister hit the St. John’s Regional Medical Center, deforming the structure of the nine-story hospital and compromising its foundation. Nearly 160 people were killed in the Joplin area alone. 

5. THE SCALE ONLY MEASURES DAMAGE.


An EF-5 tornado blew over these street signs in Moore, Oklahoma, on May 2013. Image credit: US Air Force via Flickr

The Enhanced Fujita Scale only measures the damage left behind by tornadoes. A huge tornado that tears through fields in Kansas could receive an EF-0 rating even if its winds were really much stronger—if it didn’t hit structures or trees, we’d have virtually no way to estimate its strength.

6. IT'S INCREDIBLY HARD TO FIND F5/EF-5 DAMAGE.

Meteorologists can have a hard time finding damage severe enough to rate a tornado an EF-5. There aren’t many structures strong enough to withstand winds anywhere close to 200 mph, and the clues that can tell you if winds got that strong can easily get buried in debris.  

7. SOME RATINGS ARE CONTROVERSIAL.

The rating assigned to a strong tornado can sometimes generate disagreement as to whether or not it was stronger or weaker than originally thought. Many of the controversies stem from the fact that EF-5 damage is so hard to find.

The widest tornado ever recorded touched down in El Reno, Oklahoma, in May 2013, and the 2.5-mile-wide monster received one of the most controversial ratings in recent history. A nearby mobile Doppler radar recorded winds within the tornado of nearly 300 mph. This measurement was initially used to give the tornado an EF-5 rating, but despite the scientific measurements, the National Weather Service later downgraded the tornado to an EF-3 because it didn’t cause any EF-5 damage.

8. THERE IS NO EF-6.

There’s always a discussion in the aftermath of a violent tornado about whether or not we should include an EF-6 designation for the worst of the worst tornadoes. Much like the Saffir-Simpson Scale that we use to categorize hurricanes, the top of the Enhanced Fujita Scale is open-ended. The level of damage produced by an EF-5 is so complete and devastating that there’s no need for a higher rating.

9. TWO DEADLY DAYS, HUNDREDS OF TORNADOES

An aerial view of the damage left behind by the EF-5 tornado that moved through Hackleburg, Alabama, on April 27, 2011. Image credit: NWS Birmingham

One violent tornado is horrific enough, but there have been some outbreaks where we’ve seen multiple devastating tornadoes touch down on the same day. Two days in modern history stand out more than any other. The first was the Super Outbreak of April 3, 1974, in which 148 tornadoes touched down across the Midwest, 23 of which were rated F4 and seven of which were rated F5.

The second outbreak occurred in the southeast on April 27, 2011, when a record 219 tornadoes—including 11 EF-4s and four EF-5s—swept through the region on just that one day, killing more than 300 people and injuring thousands.

10. FUJITA'S SEVERE WEATHER RESEARCH WENT FAR BEYOND TORNADOES.

If you’ve ever had to fly somewhere during bad weather, you can thank Fujita for getting to your destination safely. In addition to his eponymous tornado scale (and many other research efforts), Fujita’s work on microbursts was a critical step in improving aviation safety. Microbursts are sudden, violent downward bursts of wind from a thunderstorm. Some of the worst airplane crashes in U.S. history happened because of microbursts. Fujita’s research efforts helped aviation experts build technology to detect and avoid these deadly phenomena.