Fujita scale

Scale for rating tornado intensity

Fujita scale
Scale	Speed		Damage
Scale	mph	km/h	Damage
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), also known as the Fujita–Pearson scale (FPP scale), is a retired scale used to rate tornado intensity based primarily on damage to structures and vegetation. Tornado ratings were assigned following damage surveys, with the most severe reliable damage generally determining the final rating.

The scale was developed by meteorologist Ted Fujita and was widely used for tornado ratings in the United States until it was replaced by the Enhanced Fujita scale on February 1, 2007. Canada continued using the Fujita scale until adopting the Enhanced Fujita scale in 2013.

Background

Frequency of tornadoes by rating on the Fujita scale

F0 (44.1%)
F1 (34.2%)
F2 (16.2%)
F3 (4.35%)
F4 (1.00%)
F5 (0.10%)

The Fujita scale was introduced in 1971 by Ted Fujita of the University of Chicago, with contributions from Allen Pearson of the National Severe Storms Forecast Center. In 1973, path length and width classifications developed by Pearson were added to create the Fujita–Pearson scale.

Tornadoes in the United States began being routinely rated using the scale during the 1970s. The scale was also applied retroactively to many historical tornadoes.

Although wind-speed ranges were assigned to each category, the Fujita scale was primarily a damage scale. The estimated wind speeds were not directly measured and were later found to substantially overestimate the winds necessary to produce certain types of damage, particularly at F3 through F5 intensity.

These limitations eventually contributed to the development of the Enhanced Fujita scale, which introduced more detailed damage indicators and revised wind-speed estimates.

Derivation

Fujita originally developed a theoretical 13-level scale ranging from F0 to F12. It was designed to connect the Beaufort scale with the Mach number scale, with F1 corresponding approximately to the highest level of the Beaufort scale and F12 corresponding to Mach 1.

Only F0 through F5 were intended for practical tornado ratings. These categories represented increasing levels of structural and vegetation damage, ranging from minor damage at F0 to incredible destruction at F5.

F6 rating

Fujita included F6 as a theoretical category above F5, describing it as an "inconceivable tornado." However, F6 was never intended to be routinely assigned because damage to conventional structures cannot meaningfully exceed the complete destruction represented by F5.

Several exceptionally violent tornadoes have historically been discussed as possible F6 candidates. The 1974 Xenia tornado was initially considered to have damage near F6 intensity, while Fujita also considered whether the 1977 Birmingham–Smithfield tornado warranted an F6 rating. The 1970 Lubbock tornado was also originally listed as F6 before receiving its official F5 rating.

No tornado has ever received an official final F6 rating.

Parameters

The Fujita scale contains six operational categories, ranging from F0 to F5. A tornado's rating is generally based on the most severe damage that can be reliably attributed to it.

Because F5 represents the complete destruction and removal of well-built structures, damage-based ratings above F5 are considered impractical. A tornado may theoretically contain winds exceeding the original F5 wind range without receiving a rating higher than F5.

Scale	Wind speed estimate		Frequency	Potential damage
Scale	mph	km/h	Frequency	Potential damage
F0	40–72	64–116	44.14%	Minor 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	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.
F2	113–157	181–253	16.17%	Significant damage. Mobile homes are heavily damaged or even destroyed. Well-built structures can suffer severe damage, including roof loss, and the collapse of some exterior walls may occur in poorly built structures. 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.
F4	207–260	333–418	1%	Extreme 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	261–318	419–512	0.1%	Catastrophic 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".

Rating classifications

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

For climatological purposes, Fujita ratings are sometimes grouped into broader intensity classifications. F0–F1 tornadoes are generally considered weak, F2–F3 tornadoes strong, and F4–F5 tornadoes violent. F2–F5 tornadoes may also be classified as significant, while F3–F5 tornadoes may be classified as intense.

Decommissioning in the U.S.

Although the Fujita scale became the standard method of rating tornadoes in the United States, later engineering research identified several limitations. Most importantly, the original wind-speed estimates were found to be too high, particularly for violent tornadoes.

The scale also provided relatively broad damage descriptions and did not adequately account for differences in construction quality. A poorly constructed building could be destroyed by substantially weaker winds than a well-built structure, complicating attempts to estimate tornado intensity from damage alone.

To address these problems, meteorologists and engineers developed the Enhanced Fujita scale. The new scale retained the familiar 0–5 rating system while introducing revised wind estimates, numerous damage indicators and more detailed degrees of damage.

The Fujita scale was officially replaced by the Enhanced Fujita scale in the United States on February 1, 2007. Canada adopted the Enhanced Fujita scale in 2013.