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Fujita scale
Fujita scale
Fujita scale
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Fujita scale
Scale Speed Damage
mph km/h
F0 < 73 < 117 Light
F1 73–112 117–180 Moderate
F2 113–157 181–253 Considerable
F3 158–206 254–332 Severe
F4 207–260 333–418 Devastating
F5 261–318 419–512 Incredible

The Fujita scale (F-Scale; /fuˈdʒiːtə/), or Fujita–Pearson scale (FPP scale), is a scale for rating tornado intensity, based primarily on the damage tornadoes inflict on human-built structures and vegetation. The official Fujita scale category is determined by meteorologists and engineers after a ground or aerial damage survey, or both; and depending on the circumstances, ground-swirl patterns (cycloidal marks), weather radar data, witness testimonies, media reports and damage imagery, as well as photogrammetry or videogrammetry if motion picture recording is available. The Fujita scale, named for the meteorologist Ted Fujita, was replaced with the Enhanced Fujita scale (EF-Scale) in the United States in February 2007. In April 2013, Canada adopted the EF-Scale over the Fujita scale along with 31 "Specific Damage Indicators" used by Environment Canada (EC) in their ratings.[1][2]

Background

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The scale was introduced in 1971 by Ted Fujita of the University of Chicago, in collaboration with Allen Pearson, head of the National Severe Storms Forecast Center/NSSFC (currently the Storm Prediction Center/SPC). The scale was updated in 1973, taking into account path length and width. In the United States, starting in the late 1970s,[3] tornadoes were rated soon after occurrence. The Fujita scale was applied retroactively to tornadoes reported between 1950 and the adoption of the scale in the National Oceanic and Atmospheric Administration (NOAA) National Tornado Database. Fujita rated tornadoes from 1916 to 1992[4][5] 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.[6] The Fujita scale was adopted in most areas outside of the United Kingdom.[citation needed]

On February 1, 2007, the Fujita scale was decommissioned, and the Enhanced Fujita Scale was introduced in the United States.[7] The new scale more accurately matches wind speeds to the severity of damage caused by the tornado.[8]

Though each damage level is associated with a wind speed, the Fujita scale is effectively a damage scale, and the wind speeds associated with the damage listed are not rigorously verified. The Enhanced Fujita Scale was formulated due to research that suggested that the wind speeds required to inflict damage by intense tornadoes on the Fujita scale are greatly overestimated. A process of expert elicitation with top engineers and meteorologists resulted in the EF scale wind speeds, but these are biased to United States construction practices. The EF scale also improved damage parameter descriptions.[citation needed]

Derivation

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A diagram illustrating the relationship between the Beaufort, Fujita, and Mach number scales.

The original scale as derived by Fujita was a theoretical 13-level scale (F0–F12) designed to smoothly connect the Beaufort scale and the Mach number scale. F1 corresponds to the twelfth level of the Beaufort scale, and F12 corresponds to Mach number 1.0. F0 was placed at a position specifying no damage (approximately the eighth level of the Beaufort scale), in analogy to how Beaufort's zeroth level specifies little to no wind. From these wind speed numbers, qualitative descriptions of damage were made for each category of the Fujita scale, and then these descriptions were used to classify tornadoes.[9]

F6 rating

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At the time Fujita derived the scale, little information was available on damage caused by wind, so the original scale presented little more than educated guesses at wind speed ranges for specific tiers of damage. Fujita intended that only F0–F5 be used in practice, as this covered all possible levels of damage to frame homes as well as the expected estimated bounds of wind speeds. He did, however, add a description for F6, which he called an "inconceivable tornado", to allow for wind speeds exceeding F5 and possible advancements in damage analysis that might show it.[10]

Based on aerial photographs of the damage it caused, Fujita assigned the strongest tornado of the 1974 Super Outbreak, which affected Xenia, Ohio, a preliminary rating of F6 intensity ± 1 scale.[11] The 1977 Birmingham–Smithfield F5 tornado's damage was surveyed by Ted Fujita and he "toyed with the idea of rating the Smithfield tornado an F6".[12] In 2001, tornado expert Thomas P. Grazulis stated in his book F5–F6 Tornadoes; "In my opinion, if there ever was an F6 tornado caught on video, it was the Pampa, Texas tornado of 1995".[13] In 2023, it was announced by the Storm Prediction Center and National Weather Service Norman, Oklahoma that the 1970 Lubbock tornado was originally rated F6, which was later downgraded to its official rating of F5.[14]

Furthermore, the original wind speed numbers have since been found to be higher than the actual speeds required to incur the damage described at each category. The error manifests itself to an increasing degree as the category increases, especially in the range of F3 through F5. NOAA notes that "precise wind speed numbers are actually guesses and have never been scientifically verified. Different wind speeds may cause similar-looking damage from place to place—even from building to building. Without a thorough engineering analysis of tornado damage in any event, the actual wind speeds needed to cause that damage are unknown."[10]

The Tornado Project, headed by tornado expert Thomas P. Grazulis, states in reference to the F6 rating, [i]f this level is ever achieved, evidence for it might only be found in some manner of ground swirl pattern, for it may never be identifiable through engineering studies.[15]

Parameters

[edit]

The six categories are listed here, in order of increasing intensity.

  • The rating of any given tornado is of the most severe damage to any well-built frame home or comparable level of damage from engineering analysis of other damage.
  • Since the Fujita scale is based on the severity of damage resulting from high winds, a tornado exceeding F5 is an immeasurable theoretical construct. Frame-home structural damage cannot exceed total destruction and debris dispersal, which constitutes F5 damage. Tornadoes with wind speeds over 319 mph (513 km/h) are possible and such extreme gusts have been determined using mobile radar observation, but no tornado has received an official damage-based F6 rating.
Scale Wind speed estimate[7] Frequency[16] Potential damage[7]
mph km/h
F0 40–72 64–116 44.14% 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. 		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 73–112 117–180 34.24% 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. 		There is 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 113–157 181–253 16.17% 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. 		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 158–206 254–332 4.35% 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. 		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 207–260 333–418 1% 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. 		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 261–318 419–512 0.1% 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". 		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".

Pearson scales

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In 1973, Allen Pearson added additional path length and path width parameters to the Fujita scale. Under this version, each tornado would be assigned one Fujita scale rating and two Pearson scale ratings. For example, a tornado rated F4 based on damage with a path length of 63 miles (101 km) and a path width of 800 yards (730 m) would be rated F,P,P 4,4,4. Use of the Pearson scales was not widespread, however, and it remained more common to simply list a tornado's path length and path width directly.[6]

Pearson scales
Scale Path length Path width
– <0.3 mi (0.48 km) <6 yd (5.5 m)
P0 0.3–0.9 mi (0.48–1.45 km) 6–17 yd (5.5–15.5 m)
P1 1.0–3.1 mi (1.6–5.0 km) 18–55 yd (16–50 m)
P2 3.2–9.9 mi (5.1–15.9 km) 56–175 yd (51–160 m)
P3 10–31 mi (16–50 km) 176–566 yd (161–518 m)
P4 32–99 mi (51–159 km) 0.3–0.9 mi (0.48–1.45 km)
P5 100–315 mi (161–507 km) 1.0–3.1 mi (1.6–5.0 km)

Rating classifications

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Tornado rating classifications
F0 F1 F2 F3 F4 F5
Weak Strong Violent
Significant
Intense

For purposes such as tornado climatology studies, Fujita scale ratings may be grouped into classes.[6][17][18]

Decommissioning in the U.S.

[edit]
Main article: Enhanced Fujita scale

The Fujita scale, introduced in 1971 as a means to differentiate tornado intensity and path area, assigned wind speeds to damage that were, at best, educated guesses.[19] Fujita and others recognized this immediately and intensive engineering analysis was conducted through the rest of the 1970s. This research, as well as subsequent research, showed that tornado wind speeds required to inflict the described damage were actually much lower than the F-scale indicated, particularly for the upper categories. Also, although the scale gave general descriptions of damage a tornado could cause, it gave little leeway for strength of construction and other factors that might cause a building to sustain more damage at lower wind speeds. Fujita tried to address these problems somewhat in 1992 with the Modified Fujita Scale,[20] but by then he was semi-retired and the National Weather Service was not in a position to update to an entirely new scale, so it went largely unenacted.[21]

In the United States, on February 1, 2007,[1] the Fujita scale was decommissioned in favor of what scientists believe is a more accurate Enhanced Fujita Scale. The meteorologists and engineers who designed the EF Scale believe it improves on the F-scale on many counts. It accounts for different degrees of damage that occur with different types of structures, both manmade and natural. The expanded and refined damage indicators and degrees of damage standardize what was somewhat ambiguous. It also is thought to provide much better estimates of wind speeds and sets no upper limit on the wind speeds for the highest level, EF5. Environment Canada began using the Enhanced Fujita scale in Canada on April 1, 2013.[22]

See also

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References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Fujita scale

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The Fujita scale, also known as the F-scale, is a standardized system for estimating the intensity of tornadoes based on the observed damage to human-made structures, vegetation, and other environmental features, correlating such damage to three-second gust wind speeds ranging from 40 mph (64 km/h) for the weakest (F0) to over 261 mph (420 km/h) for the strongest (F5) events. Developed by meteorologist Dr. Tetsuya Theodore Fujita, the scale was first proposed in his 1971 research paper titled "Proposed Characterization of Tornadoes and Hurricanes by Area and Intensity," published as part of the Satellite and Mesometeorology Research Project (SMRP) at the university. It provided a pioneering framework for retrospectively rating tornado strength post-event, as direct wind measurements in tornadoes were rare and challenging prior to modern instrumentation. The scale categorizes tornadoes into six levels, each defined by specific wind speed estimates and typical damage indicators, emphasizing the relationship between structural integrity and aerodynamic forces. For instance, an F0 tornado (40–72 mph) causes minimal damage, such as breaking tree branches, damaging chimneys, or peeling shingles from roofs, while an F1 (73–112 mph) results in moderate effects like overturning mobile homes or pushing vehicles off roads. Higher ratings include F2 (113–157 mph) with considerable damage, such as tearing roofs from well-built homes or uprooting large trees; F3 (158–206 mph) featuring severe destruction like overturning trains or stripping walls from houses; F4 (207–260 mph) involving devastating impacts, such as leveling well-constructed buildings or hurling cars; and F5 (261–318 mph) producing incredible devastation, including debarking trees, carrying frame houses long distances, or generating automobile-sized projectiles. These descriptions were derived from Fujita's analysis of historical tornado damage surveys, particularly following the , and aimed to standardize reporting for meteorological research and public safety. Adopted by the in the early 1970s, the Fujita scale revolutionized tornado documentation and climatology, enabling consistent comparisons of events worldwide until its refinement into the Enhanced Fujita (EF) scale in 2007, which incorporated more damage indicators and engineering-based wind estimates for greater precision. Despite its retirement for operational use in the United States, the original F-scale remains influential in historical records and international contexts, where some countries continue to apply it or adapted versions. Limitations of the scale, such as overestimation of wind speeds due to unverified assumptions from the and variability in construction quality, were acknowledged by Fujita himself and addressed in later iterations.

History and Development

Background and Creation

The Fujita scale, a system for rating based on damage, was developed by Tetsuya Theodore Fujita, a at the , in 1971. Fujita initially proposed the scale in his Satellite and Mesometeorology Research Project (SMRP) Paper No. 91, titled "Proposed Characterization of Tornadoes and Hurricanes by Area and Intensity," which outlined a framework for classifying storms by their path length, width, and associated wind speeds. This work built directly on his earlier research in the and , where he pioneered analyses of downdrafts and mesoscale convective systems using and detailed damage surveys to uncover the internal structures of severe storms. Fujita's motivation stemmed from the limitations of existing tornado assessment methods, which lacked a consistent way to correlate observed damage with estimated wind speeds, hindering accurate forecasting and post-event analysis. He collaborated with Allen Pearson, head of the National Severe Storms Forecast Center (now the ), to create a practical tool for meteorologists to standardize intensity ratings and improve predictions. The scale was formally detailed in Fujita's 1973 SMRP Research Paper No. 98, "Experimental Classification of Tornadoes in FPP Scale," which refined the initial concepts through empirical observations. The , a historic event spawning over 140 tornadoes across the and causing widespread devastation, further highlighted the scale's value by enabling Fujita to conduct comprehensive aerial and ground surveys that validated its application in real-world scenarios. These surveys revealed patterns in damage that reinforced the need for a unified intensity measure, accelerating the scale's adoption within the for operational use.

Derivation and Methodology

Fujita conducted detailed analyses using aerial and ground photography following the to examine damage from 11 significant tornadoes, enabling precise mapping of destruction paths and intensity variations. These photographs, captured during post-event surveys, provided visual evidence of structural deformation, uprooting, and dispersal, which Fujita used to identify consistent patterns of damage across diverse building types and landscapes. To link these damage patterns to wind speeds, Fujita correlated observed destruction with estimated three-second gust velocities, relying on engineering data for the structural integrity of common materials like wood framing, , and roofing under dynamic wind loads. Principles of were applied to model aerodynamic forces, such as drag and lift on objects, informing thresholds where winds would exceed material resistance and cause specific failure modes—for instance, roof peel-off or wall collapse. This approach established ranges for each F-scale category, prioritizing the fastest quarter-mile gust averaged over three seconds as the metric for consistency with standards. A key component of wind speed estimation involved analyzing debris trajectories captured in the photography. Fujita applied photogrammetric techniques to decompose debris velocities into translational and rotational components, thereby estimating maximum rotational winds for rating purposes. Such calculations, repeated across multiple debris paths, refined estimates of maximum rotational speeds. Validation of these estimates came from rare direct measurements in instrumented tornadoes, where anemometer data aligned closely with photogrammetric predictions—for instance, gusts exceeding in documented cases corroborated F4/F5 thresholds derived from damage correlations.

Scale Structure

Parameters and Damage Indicators

The Fujita scale assesses tornado intensity primarily through observed damage to built structures, , and other objects. Primary damage indicators encompass a range of structure types and natural features, such as well-built frame houses, single-family residences, schools, barns, power poles, trees, and vehicles, with ratings assigned based on the highest degree of destruction observed among them. The degree of damage (DOD) refers to the extent of destruction to a given indicator, ranging from minor impacts like broken windows or peeled to total obliteration, such as a structure being swept from its foundation; surveyors select the indicator exhibiting the most severe DOD to determine the overall F rating, ensuring the estimate reflects the peak wind speeds. For instance, in an F1 tornado (73–112 mph estimated winds), damage might include chimneys toppled or roofs partially peeled on frame houses, while shows uprooted trees and scoured surfaces. Non-structural indicators play a crucial role when built structures are absent or insufficient, including vegetation scouring (e.g., bark stripped from trees in F3 or higher), ground debris patterns, and object displacement like vehicles hurled significant distances. These help refine ratings in rural or undeveloped areas, where, for example, an F2 (113–157 mph) might snap or uproot large trees, correlating to considerable but not total destruction. Overall, the scale emphasizes conceptual thresholds of destruction rather than exhaustive lists, prioritizing verifiable evidence from surveys to avoid overestimation.

Rating Classifications

The Fujita scale classifies tornadoes into six intensity levels, designated F0 through F5, based on estimated maximum 3-second gust wind speeds derived from observed damage to engineered structures (such as buildings and power poles) and natural features (like trees and crops). Developed by T. Theodore Fujita, these ratings provide a standardized framework for assessing tornado strength post-event, emphasizing the correlation between wind velocity and destructive effects rather than direct measurements, which are rare due to the dangers involved. The assignment of a rating involves a detailed ground survey of the tornado's damage path by meteorologists, where the highest degree of destruction observed—regardless of its location along the path—determines the overall classification. This process relies on various damage indicators, such as frame houses and forests, to infer wind speeds from the extent of structural or uprooting. F0 (40–72 mph): This weakest category, often termed a "gale tornado," produces light damage, such as broken chimneys, shattered windows, peeled , and snapped branches from trees. Shallow-rooted trees may be uprooted, and sign boards can be damaged or toppled, but overall impacts are minor and rarely threaten human life directly. F1 (73–112 mph): Moderate damage characterizes F1 tornadoes, including the peeling back of roof surfaces on frame houses, overturning or shifting of mobile homes from their foundations, and pushing of moving automobiles off roadways. Garage doors may fail, and weak outbuildings like sheds can be destroyed, though well-built structures typically sustain only superficial harm. F2 (113–157 mph): Tornadoes rated F2 cause considerable damage, such as complete removal of roofs from well-constructed frame houses, of mobile homes, and overturning of boxcars or semis. Large trees are commonly snapped or uprooted, lightweight objects become missiles capable of causing additional harm, and automobiles may be lifted briefly off the ground. F3 (158–206 mph): Severe damage defines F3 intensity, with roofs and some exterior walls torn from well-built homes, leaving interiors exposed; trains may be overturned or derailed; and most trees along the path are uprooted or debarked. Heavy vehicles like automobiles are lifted and thrown significant distances, and poorly anchored homes can be shifted off foundations. F4 (207–260 mph): Devastating effects mark F4 tornadoes, including the leveling of well-constructed houses to their foundations, sweeping away of structures with weak anchors, and generation of small missiles like or vehicles hurled over long distances. Brick homes may have only stub walls remaining, and entire rows of trees can be flattened with significant debarking. F5 (261–318 mph): The most extreme category, F5 or "incredible" tornadoes, result in complete destruction where strong frame houses are lifted cleanly off foundations and disintegrated mid-air, with carried hundreds of yards; automobile-sized objects become airborne missiles traveling over 100 yards; and hardwoods are debarked entirely, leaving only bare trunks. Such damage often exceeds typical expectations, rendering precise estimation challenging.

F6 and Inconclusive Ratings

The F6 category, designated as the "inconceivable tornado" on the original Fujita scale, was conceptualized for estimated wind speeds greater than 318 mph (512 km/h), with anticipated including the total pulverization of even the most robust structures into small and extensive scouring of paved surfaces or beyond typical F5 levels. This rating represented an upper theoretical limit, as Fujita deemed such extreme highly improbable in nature and the localized patterns potentially indistinguishable from severe F5 events amid a broader path. Despite its inclusion in the scale's framework, no has ever been officially assigned an F6 rating, reflecting the practical challenges in verifying consistent with such velocities. In rare instances, the F6 label was preliminarily considered during post-event surveys but ultimately rejected due to insufficient corroborating evidence. For example, following the , Tetsuya Fujita initially rated the tornado as F6 based on revealing near-total debarking of trees, fragmentation of multi-story buildings, and asphalt scouring, effects he interpreted as exceeding F5 criteria; however, the later downgraded it to F5 after ground surveys confirmed the damage aligned with the operational upper limit of the scale. Similarly, the 1999 Bridge Creek–Moore, Oklahoma tornado prompted discussions of F6 potential when Doppler radar recorded 301 mph (484 km/h) winds aloft, implying possible ground-level speeds surpassing 318 mph, yet it was officially classified as F5 because structural damage, while catastrophic, did not provide unambiguous indicators for the higher category. Inconclusive ratings arise when damage surveys yield ambiguous or conflicting results, such as when indicators suggest wind speeds beyond measurable F-scale limits without definitive proof or when the tornado's path features inadequate damage objects for reliable assessment. Criteria for deeming a rating inconclusive typically involve sparse structural remnants, variable construction quality across the path, or environmental factors obscuring damage patterns, preventing assignment of a specific intensity. Guidelines from the recommend withholding a formal rating in these scenarios—labeling it as undetermined—to avoid overestimation, particularly if data alone cannot validate intensity; instead, emphasis is placed on confirming the tornado's occurrence while noting the assessment limitations. This approach ensures ratings remain grounded in verifiable physical evidence rather than speculation.

Pearson Scales

In 1971, Allen D. Pearson, director of the National Severe Storms Forecast Center (now the ), collaborated with T. Theodore Fujita to introduce modifications to the original Fujita scale, creating the Fujita-Pearson scale (FPP scale). These changes addressed limitations in describing tornado characteristics beyond damage intensity by adding logarithmic parameters for path length (PL) and path width (PW), each rated from 0 to 5, to the existing F-scale intensity rating. The updated scale rated each tornado with a three-part notation, such as F2/3/2, where the first number indicated the damage-based intensity (e.g., F2 corresponding to estimated wind speeds of 113–157 mph), the second denoted path length (e.g., PL 3 for paths roughly 1–3 miles long), and the third represented path width (e.g., PW 2 for widths of about 100–200 yards). This refinement improved correlations with observed meteorological data and engineering assessments of tornado impacts by accounting for spatial variability, which could influence overall event severity and forecasting applications. Pearson's advocacy was instrumental in the national implementation of the F-scale starting in 1973, promoting its use for post-event surveys to standardize tornado documentation across the . Despite these enhancements, the full three-component FPP notation saw limited adoption in routine operations, with surveys often focusing primarily on the intensity rating; it remained in use for some detailed analyses until the transition to the in 2007.

International Fujita Scale

The International Fujita Scale (IF-Scale) was developed by the European Severe Storms Laboratory (ESSL) starting in 2018, in collaboration with researchers from various international institutions, to provide a standardized method for assessing tornado intensity and other severe wind events outside . Initial drafts were presented at an ESSL workshop on tornado and wind damage from September 4–7, 2018, addressing limitations in the original Fujita scale's applicability to non-U.S. environments, such as differing building materials and . Version 1.0 of the scale was released on August 1, 2023, with Version 1.0e issued on September 7, 2025, incorporating corrections to treefall ratings; this followed extensive testing and refinement to ensure consistency with historical rating practices while enabling global use. A core feature of the IF-Scale is its use of 21 damage indicators (DIs), adapted from the original Fujita scale's parameters but tailored for international contexts, including and structures common in , as well as s and rural landscapes prevalent worldwide. These indicators categorize objects like residential buildings, power poles, and trees into degrees of (DoD), with each level corresponding to estimated instantaneous three-dimensional wind speeds that align closely with the original F-scale ranges—expressed in both imperial and metric units for broader accessibility (e.g., IF0 at approximately 20–30 m/s or 72–108 km/h). The scale emphasizes conceptual patterns over region-specific assumptions, allowing for assessments in diverse settings like urban areas in or savannas in . For instance, indicators incorporate uprooting and snapping thresholds to differentiate effects from straight-line winds. Ratings on the IF-Scale range from IF0 (light damage) to IF5 (incredible damage), with optional half-step increments (IF0.5, IF1.5, IF2.5) for greater precision in the lower intensities, reflecting wind speeds up to over 90 m/s () for IF5 events. An IF6 rating is reserved for undocumented extreme cases exceeding IF5 thresholds, though it is rarely applied due to the scale's focus on verifiable damage. Unlike tornado-specific scales, the IF-Scale also evaluates non-tornado wind events, such as downbursts and straight-line winds, by analyzing directional patterns in and treefall to distinguish event types. This inclusion broadens its utility for comprehensive documentation. Since its release, the IF-Scale has seen adoption primarily in for both retrospective reanalysis of historical tornadoes and real-time assessments, integrated into the ESSL's European Severe Weather Database (ESWD). For example, in 2024, it was used to rate numerous events, including an IF3 tornado in and the reclassification of the 1930 Montello, Italy, tornado as the first official IF5. Its global framework has facilitated limited application in parts of and , supporting cross-continental comparisons and research on wind damage patterns, with ongoing refinements to indicators like treefall analysis—updated in the September 2025 Version 1.0e—enhancing accuracy in vegetated regions.

Transition and Legacy

Decommissioning in the United States

In 2006, the (NWS), part of the (NOAA), announced plans to phase out the original Fujita (F) scale in favor of an improved system for rating based on damage. The announcement was made public on February 2, 2006, during a conference of the in , marking the formal decision to decommission the F-scale after more than three decades of use. The primary reasons for decommissioning centered on significant limitations in the original scale's methodology. The F-scale overestimated wind speeds associated with higher ratings, such as F5 tornadoes estimated at up to 318 mph (513 km/h), which analyses deemed unrealistic given observed structural failures at much lower velocities. Additionally, the scale suffered from imprecise correlations between damage types and wind speeds, relying on a limited set of vaguely defined damage indicators that hindered accurate assessments. Advances in wind and structural since the provided better on how buildings and other structures respond to tornado , necessitating a revision to align ratings more closely with . The transition took effect on February 1, 2007, when the NWS fully implemented the replacement scale for all tornado damage surveys in the United States, making 2006 the final year of exclusive F-scale use. The decommissioning led to a notable shift in rating distributions, with stricter criteria resulting in fewer assignments of the highest intensities. Post-2007, the proportion of violent tornadoes (ratings equivalent to F4 or F5) declined, reflecting more conservative wind speed estimates and refined damage assessments rather than a true decrease in tornado occurrence. This adjustment improved the overall reliability of tornado climatology data for research and forecasting purposes.

Adoption of Enhanced Fujita Scale

The Enhanced Fujita (EF) Scale was developed over a period from 2000 to 2006 by a committee comprising engineers, meteorologists, and other experts, convened under the leadership of the (NWS) in collaboration with the Wind Science and Engineering Research Center at . This effort involved forums and panels to address limitations in the original Fujita Scale, resulting in a refined system for estimating based on damage assessment. Key enhancements included lowering the estimated wind speed thresholds to better align with engineering analyses of structural failures, such as reducing the EF5 category to winds exceeding compared to the original F5 range of 261-318 mph. The scale expanded to 28 specific damage indicators (DIs), such as one- or two-family residences and small barns, each associated with multiple degrees of damage (DODs) ranging from minor to complete destruction, allowing for more nuanced evaluations. The EF Scale became operational across the on February 1, 2007, replacing the Fujita Scale for all new ratings. Wind speeds are now derived from a function of the selected DI and corresponding DOD, incorporating probabilistic engineering models to provide expected, upper-bound, and lower-bound estimates rather than single values. These changes improved rating accuracy by reducing overestimation of extreme wind speeds and broadening applicability through additional DIs, leading to more reliable correlations between observed damage and inferred intensities. For instance, the first EF5 rating under this scale in 2025 was retroactively assigned to the June 20 Enderlin, tornado, based on severe damage including derailed train cars indicating winds over 200 mph.

Ongoing Use and Retrospective Application

The original Fujita scale (F-scale) has maintained relevance in regions beyond the , particularly where transitions to enhanced variants have been gradual or incomplete. In , tornadoes were rated using the F-scale by Environment Canada until April 1, 2013, when the Enhanced Fujita (EF) scale was adopted with localized damage indicators tailored to Canadian structures and vegetation. Similarly, in parts of and , the F-scale or analogous systems persisted for damage assessment until the introduction of adapted scales, such as Japan's modified EF-scale in the early and Europe's shift toward the International Fujita (IF) scale by 2023 to address regional building differences and documentation gaps. These ongoing applications highlight the F-scale's adaptability in diverse environments before more precise variants took hold. Retrospective application of the EF-scale to pre-2007 U.S. tornadoes has allowed for standardized comparisons across the historical record, with the (NWS) and (SPC) re-evaluating thousands of events based on available damage documentation. For instance, the in , originally classified as F5, was re-rated as EF5 due to evidence of winds exceeding 300 mph from and structural devastation. By 2022, over 78,000 tornadoes from 1953 onward had been assigned (E)F ratings in the SPC database, reflecting comprehensive updates to integrate legacy F-scale data with EF criteria, though not all early records received full re-analysis due to data limitations. The F-scale's legacy endures in tornado research and climatology, where its foundational dataset informs long-term trends in intensity, frequency, and geographic distribution despite the shift to the EF-scale. Studies analyzing multi-decadal patterns, such as those examining changes in violent tornado occurrences, rely heavily on F-scale ratings for pre-2007 events to establish baselines for climate impacts and detection biases. However, re-rating historical events poses challenges, including incomplete or low-quality damage photographs that hinder accurate wind speed estimates and lead to potential under- or over-classification compared to modern surveys with high-resolution imagery. As of 2025, no new are assigned F-scale ratings, with all post-2007 assessments using the EF-scale exclusively. Meanwhile, the IF-scale continues to expand globally, adopted by organizations like the European Severe Storms Laboratory since 2023 and applied in non-U.S. regions to standardize documentation where EF adaptations are not yet implemented, thereby bridging gaps in international tornado records.

References

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