This entry is part 2 of 3 in the series Tornadoes

The Fujita Scale

The highest wind speeds on Earth come from tornadoes.

Needless to say, things that cause the highest wind speeds can produce massive amounts of damage to human-created infrastructure and natural surroundings. Though we have made progress in tornado science over the past century, much about the mechanisms and materializations of the storms remains unknown. This ignorance stems partially from the unpredictable nature of tornadoes. They can manifest quickly, and their paths can alter instantaneously, making the gathering of data from them difficult. Further, even if we could reliably prognosticate a twister’s path, its power precludes easy instrument deployment.

As such, for many tornadoes, we can’t even gather an accurate wind speed reading.

In 1971, a meteorologist at the University of Chicago, Ted Fujita, wondered about methods to quantify the intensity of a tornado, given the difficulties in procuring proper data.

He developed a system that “connected” the Beaufort scale – developed in the 1800s to describe wind speeds for ships – and the Mach scale – a method for comparing things to the speed of sound.

We fittingly call it the Fujita scale.

Starting at Level 12 on the Beaufort scale, the hurricane level, the Fujita scale theoretically had 13 levels, the last of which hits Mach 1, the speed of sound in Earth’s atmosphere. However, in practice, Fujita limited his scale to six levels: F0-F5.

Though wind speeds were assigned to each level, the valuation of a tornado’s intensity came from the damage it created. At the time, scientists knew little about the wind speeds required to produce specific types of damage. Structural damage seemed to be a more concrete method (pun intended) than the unpredictable wind speeds produced by each tornado. Experts could survey the damage afterward, instead of worrying about getting accurate measurements in a raging, unpredictable maelstrom.

The damage associated with each level, according to Fujita:

F0: Light damage.

Well-built structures are typically unscathed, though sometimes sustaining broken windows, with minor damage to roofs and chimneys. Billboards and large signs can be knocked down. Trees may have large branches broken off and can be uprooted if they have shallow roots.

F1: Moderate Damage.

Damage to mobile homes and other temporary structures becomes significant, and cars and other vehicles can be pushed off the road or flipped. Permanent structures can suffer major damage to their roofs.

F2: Significant damage.

Well-built structures can suffer serious damage, including roof loss, and the collapse of some exterior walls may occur in poorly built structures. Mobile homes, however, are destroyed. Vehicles can be lifted off the ground, and lighter objects can become small missiles, causing damage outside of the tornado’s main path. Wooded areas have a large percentage of their trees snapped or uprooted.

F3: Severe Damage.

A few parts of affected buildings are left standing. Well-built structures lose all outer and some inner walls. Unanchored homes are swept away, and homes with poor anchoring may collapse entirely. Trains and train cars are all overturned. Small vehicles and similarly sized objects are lifted off the ground and tossed as projectiles. Wooded areas suffer an almost total loss of vegetation and some tree debarking may occur.

F4: Devastating damage.

Well-built homes are reduced to a short pile of medium-sized debris on the foundation. Homes with poor or no anchoring are swept completely away. Large, heavy vehicles, including airplanes, trains, and large trucks, can be pushed over, flipped repeatedly, or picked up and thrown. Large, healthy trees are entirely debarked and snapped off close to the ground or uprooted altogether and turned into flying projectiles. Passenger cars and similarly sized objects can be picked up and flung for considerable distances.

F5: Incredible damage.

Well-built and well-anchored homes are taken off their foundations and they go into the air before obliteration. The wreckage of those homes is flung for miles and those foundations are swept completely clean. Large, steel-reinforced structures such as schools are completely leveled. Low-lying grass and vegetation are shredded from the ground. Trees are completely debarked and snapped. Very little recognizable structural debris is generated with most materials reduced to a coarse mix of small, granular particles and dispersed. Large, multiple-ton steel frame vehicles and farm equipment are often mangled beyond recognition and tossed miles away or reduced entirely to unrecognizable parts. Tall buildings collapse or have severe structural deformations. The official description of this damage highlights the extreme nature of the destruction, noting that “incredible phenomena can and will occur”.

The destruction of F5 tornadoes is hard to fathom. Entire towns can be erased.

Though Fujita ended the F5 classification at the upper bounds of what he believed tornado wind speeds might reach, he left open the possibility of F6 twisters. He dubbed F6 an “inconceivable tornado.” Early in the process, this “inconceivable” status went to two famous tornadoes. The first was the strongest from the 1974 Super Outbreak, which annihilated Xenia, Ohio; the second occurred in 1977 near Birmingham, Alabama.

Retroactively, both these tornadoes were “downgraded” to F5 status.

The Fujita scale revolutionized the way we viewed tornadoes. It lent some objectivity to a disaster notoriously hard to characterize.

However, the system wasn’t perfect. It could not really distinguish between construction techniques or workmanship. Damage to one house might seem equal to damage to another, but the houses might have had significantly different tolerances to wind. Though it was technically objective, subjectivity sometimes crept into the assessments afterward. Further, the original scale’s guesses about the speeds associated with each level overestimated how much wind it took to destroy structures. In other words, the speeds needed to wreak havoc were lower than we initially thought, a scary thought with how powerful these tornadoes can get.

To help with these issues, scientists developed a new system in 2007, called the Enhanced Fujita scale.

Filled with decades of extra data and expertise, meteorologists updated the scale to be more objective, based on 28 damage indicators. Additionally, the wind speeds associated with each level now reflect measurements in the field more accurately.

On the Enhanced Fujita scale, the levels received slightly updated damage titles, though the descriptions remained largely the same. A level beneath F0 was added.

EFU – Unknown – No surveyable damage
EF0 – 65–85 mph – Light damage
EF1 – 86–110 mph – Moderate damage
EF2 – 111–135 mph – Considerable damage
EF3 – 136–165 mph – Severe damage
EF4 – 166–200 mph – Devastating damage
EF5 – >200 mph – Incredible damage

The majority of tornadoes receive EF0 designation: 52.82%. Approximately another third – 32.98% – hit EF1. Only 8.41% of twisters reach EF2. Everything above is incredibly rare. Just over 2% are EF3s, while 0.45% reach EF4 level.

Thankfully, the truly monstrous cyclones border on unicorns: just 0.05% of tornadoes become EF5s. The most recent to receive the classification happened in Moore, Oklahoma, in 2013.

As we hone our construction practices and prediction models, the Enhanced Fujita scale continues to help us potentially save lives. By mapping tornadic aftermath to live storms, we can direct people to the proper amount of cover.

The next time you encounter a meteorologist or news anchor discussing a tornado, you’ll know the “F” in the name means Fujita!