March 16th, 2020
by International Code Council

It is nearly impossible to measure the actual wind speed inside tornadoes as they can destroy just about any unprotected weather instruments placed in their path. While storm chasers and engineers have developed vehicles and instruments capable of withstanding tornadoes, the intensity of tornadoes is primarily measured after their impact using the Enhanced Fujita Tornado Intensity Scale (EF-scale).

The original Fujita Tornado Damage Scale, also known as the Fujita-Pearson Scale, was introduced in 1971 by Dr. Theodore Fujita, a severe storms researcher at the University of Chicago, in collaboration with Allen Pearson, then head of the National Severe Storms Forecast Center (now the Storm Prediction Center). The scale was named in honor of Dr. Fujita for his groundbreaking work in tornado intensity classification. Dr. Fujita’s scale, which ranges from F0 to F5, classified tornadoes by the type and severity of damage the tornado inflicted on human-built structures and vegetation.

The official Fujita Scale category was determined based on the following:

  • Damage surveys: Conducted either on the ground or through aerial observations.
  • Ground-swirl patterns: Analyzed for indications of tornado strength.
  • Weather radar data: Used to track storm activity and tornado signatures.
  • Witness testimonies: Accounts from people who experienced the tornado.
  • Media reports and damage imagery: Visual documentation from news outlets and photos/videos of the aftermath.
  • Photogrammetry or videogrammetry: Measurements derived from analyzing photographs or video footage.

At that time, there were very few actual measurements of tornado wind speeds that could be related to the damage, but Dr. Fujits used available data, along with a lot of educated insight, to devise approximate wind speed ranges for each damage category.

Campo, CO, tornado seen in sharp contrast with yellow wildflowers and clear skies

The Fujita Scale At a Glance

The Fujita Scale was updated in 1973 to include considerations for path length and width. In the United States, tornadoes began receiving ratings shortly after their occurrence starting that year. Additionally, the Fujita Scale was applied retroactively to tornadoes reported between 1950 and 1972 in the National Oceanic and Atmospheric Administration’s National Tornado Database.Dr. Fujita rated tornadoes from 1916 to 1992 and Tom Grazulis of The Tornado Project retroactively rated all known significant tornadoes (F2–F5 or causing a fatality) in the U.S. back to 1880.Below is a brief description of the Fujita Scale.

Fujita Tornado Intensity Scale

F-0: Gale tornado (40 – 72 mph). Light damage. Some damage to chimneys; branches broken off trees; shallow-rooted trees pushed over; sign boards damaged.

F-1: Moderate tornado (73 – 112 mph). Moderate damage. Peels surface off roofs; mobile homes pushed off foundations or overturned; moving autos blown off roads.

F-2: Significant tornado (113 – 157 mph). Considerable damage. Roofs torn off frame houses; mobile homes demolished; boxcars overturned; large trees snapped or uprooted; light-object missiles generated; cars lifted off ground.

F-3: Severe tornado (158 – 206 mph). Severe damage. Roofs and some walls torn off well-constructed houses; trains overturned; most trees in forest uprooted; heavy cars lifted off the ground and thrown.

F-4: Devastating tornado (207 – 260 mph). Devastating damage. Well-constructed houses leveled; structures with weak foundations blown away some distance; cars thrown and large missiles generated.

F-5: Incredible tornado (261 – 318 mph). Incredible damage. Strong frame houses leveled off foundations and swept away; automobile-sized missiles fly through the air in excess of 100 meters (109 yds); trees debarked; incredible phenomena will occur.

The Establishment of the Enhanced Fujita Scale

In subsequent years, structural engineers and meteorologists examined damage from many tornadoes. They used knowledge of the wind forces needed to damage or destroy various buildings and their components to estimate the resultant wind speeds. The engineers and scientists eventually determined that the original F-Scale wind speeds were greatly overestimated (too high) for categories F3 and higher.

In particular, homes are not constructed to withstand extreme winds of 250 to 300 mph. In many areas, building codes may only require roofs to endure winds up to 70 mph. Once a roof is torn off, the walls lose critical support, leading to structural failure. In other cases, the house frame may not be properly bolted to the foundation, it can then begin to slide off the foundation, perhaps partly lifted by strong upward winds in the tornado.

To read more about the creation of the first tornado-resistant building code, click here. 

By the early 1990s, even Dr. Fujita recognized these shortcomings and recommended adjusting the assessments. Though each damage level is associated with a wind speed, the Fujita scale is essentially a damage scale, and the wind speeds associated with the damage listed are not rigorously verified. The Enhanced Fujita Scale was devised by a panel of meteorologists and wind engineers convened by the Wind Science and Engineering Research Center at Texas Tech University. On Feb. 1, 2007, the Fujita scale was decommissioned, and the more accurate EF-Scale was introduced in the U.S.

Enhanced Fujita Scale for Tornados. The enhanced Fujita scale (EF), introduced in 2007, provides estimates of tornado strength on damage surveys. The original scale was developed by Dr.Theodore Fujita and implemented in 1971

The meteorologists and engineers who designed the EF-scale believe it more accurately matches estimated wind speeds to the severity of related damage caused by the tornado and is an improvement on the F-scale on many counts:

  • It accounts for different degrees of damage that occur with different types of structures, both man-made and natural.
  • The expanded and refined damage indicators and degrees of damage descriptions standardize what was somewhat ambiguous.
  • It is thought to provide a much better estimate for wind speeds and sets no upper limit on the wind speeds for the strongest level, EF-5.
  • It better reflects examinations of tornado damage surveys to align wind speeds more closely with associated storm damage.
  • The new scale has more to do with how most structures are designed.

Below is a brief description of the Enhanced Fujita Scale.

Enhanced Fujita Tornado Intensity Scale

EF-0: Light damage  (65 – 85 mph). Peels surface off some roofs; some damage to gutters or siding; branches broken off trees; shallow-rooted trees pushed over. Original Fujita Scale estimated wind speeds: less than 73 mph.

EF-1: Moderate damage (86 – 110 mph). Roofs severely stripped; mobile homes overturned or badly damaged; loss of exterior doors; windows and other glass broken. Original Fujita Scale estimated wind speeds: 73 to 112 mph.

EF-2: Considerable damage (111 – 135 mph). Roofs torn off well-constructed houses; foundations of frame homes shifted; mobile homes completely destroyed; large trees snapped or uprooted; light-object missiles generated; cars lifted off ground. Original Fujita Scale estimated wind speeds: 113 to 157 mph.

EF-3: Severe damage (136 – 165 mph). Entire stories of well-constructed houses destroyed; severe damage to large buildings such as shopping malls; trains overturned; trees debarked; heavy cars lifted off the ground and thrown; structures with weak foundations blown away some distance. Original Fujita Scale estimated wind speeds: 158 to 206 mph.

EF-4: Devastating damage (166 – 200 mph). Whole frame houses Well-constructed houses and whole frame houses completely leveled; cars thrown and small missiles generated. Original Fujita Scale estimated wind speeds: 207 to 260 mph.

EF-5: Incredible damage (Over 200 mph). Strong frame houses leveled off foundations and swept away; automobile-sized missiles fly through the air in excess of 109 yards (100 meters); high-rise buildings have significant structural deformation; incredible phenomena will occur. Original Fujita Scale estimated wind speeds: 261 to 318 mph.

According to the National Weather Service, the EF-scale remains a system of wind estimates based on observed damage. It uses three-second gust estimates at the point of damage, determined by assessing eight levels of damage across 28 indicators. These indicators include 23 types of structures, such as barns, mobile homes and schools, as well as five additional objects like trees, towers and poles.

Wind speed estimates are then provided for each structure and type of damage. The three-second gust is not the same wind as in standard surface observations. Standard measurements are taken by weather stations in open exposures, using a directly measured, “one-minute mile” speed. Guidelines in the original scheme lumped together homes, schools, mobile homes, vehicles and trees in one short description of damage for each F-scale category.

To read more about how increased knowledge of tornado risks leads to better building codes, click here. 

The National Weather Service is the only federal agency with the authority to provide “official” tornado EF-scale ratings. An EF-scale category is assigned based on the highest wind speed that occurred within the damage path.

When assessing tornado damage, trained National Weather Service personnel use one or more of the 28 designated damage indicators to evaluate the destruction. These indicators are matched to the type or construction of the building or structure in question. The observed damage is then aligned with one of the eight degrees of damage defined by the EF-scale.

Evaluators estimate wind speeds by judging whether the damage corresponds to wind speeds above or below the expected range for that degree of damage. This process helps refine the estimate of the tornado’s likely wind speed. Ratings, ranging from EF-0 to EF-5, are assigned based on evaluations of multiple structures, ensuring a comprehensive assessment before a final EF rating is determined.

A PDF file explaining the development and makeup of the EF-scale is available here

To learn more about tornado safety and recovery, click here. 

The Fujita Scale had many problems arise when trying to evaluate areas affected by a tornado. Thus, the EF-scale was created to better correlate wind speed and damage to certain structures.

The EF-scale estimates tornado wind speeds based on observed damage. The National Weather Service uses 28 damage indicators, including residences, commercial buildings, schools, professional structures, metal buildings, poles, and vegetation, to determine a tornado’s rating. Each damage indicator is assigned degrees of damage, and survey teams match the observed destruction to these degrees to estimate wind speeds. When conducting a survey, the pattern of damage, rather than the amount, is crucial in confirming whether a tornado occurred.

A tornado is officially confirmed and given a rating only after the National Weather Service completes its damage survey.​

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About the Author
International Code Council
The International Code Council is a nonprofit association that provides a wide range of building safety solutions, including product evaluation, accreditation, certification, codification and training. It develops model codes and standards used worldwide to construct safe, sustainable, affordable and resilient structures.