TORRO scale

The TORRO tornado intensity scale (or T-Scale) is a scale measuring tornado intensity between T0 and T11. It was proposed by Terence Meaden of the Tornado and Storm Research Organisation (TORRO), a meteorological organisation in the United Kingdom, as an extension of the Beaufort scale.

History and derivation from Beaufort scale

The scale was tested from 1972 to 1975 and was made public at a meeting of the Royal Meteorological Society in 1975. The scale sets T0 as the equivalent of 8 on the Beaufort scale and is related to the Beaufort scale (B), up to 12 on the Beaufort scale, by the formula:

B = 2 (T 4)

and conversely:

T = B/2 - 4

Beaufort scale	B	8	10	12	12	12	12	12	12	12	12	12	12
TORRO scale	T	0	1	2	3	4	5	6	7	8	9	10	11

The Beaufort scale was first introduced in 1805, and in 1921 quantified. It expresses the wind speed as faster than v in the formula:

v = 0.837 B^3/2 m/s

TORRO scale formula

Most UK tornadoes are T6 or below with the strongest known UK tornado estimated as a T8 (the London tornado of 1091). For comparison, the strongest detected winds in a United States tornado (during the 1999 Oklahoma tornado outbreak) would be T11 using the following formulas:

v = 2.365 (T 4)^3/2 m/s

v = 8.511 (T 4)^3/2 km/h

v = 5.289 (T 4)^3/2 mph

v = 4.596 (T 4)^3/2 kn

where v is wind speed and T is TORRO intensity number. Wind speed is defined as a 3-second gust at 10 m AGL.

Alternatively, the T-Scale formula may be expressed as:

v = 0.837 (2T 8)^3/2 m/s

or

v = 0.837(2^3/2) (T 4)^3/2 m/s

or

$T=(v/2.365)^{2/3}-4$

Rating process and comparisons to Fujita scale

TORRO claims it differs from the Fujita scale in that it is "purely" a wind speed scale, whereas the Fujita scale relies on damage for classification, but in practice, damage is utilised almost exclusively in both systems to infer intensity. That is because such a proxy for intensity is usually all that is available, although users of both scales would prefer direct, objective, quantitative measurements. The scale is primarily used in the United Kingdom whereas the Fujita scale has been the primary scale used in North America, continental Europe, and the rest of the world.

At the 2004 European Conference on Severe Storms, Dr. Meaden proposed a unification of the TORRO and Fujita scales as the Tornado Force or TF Scale. In 2007 in the United States, the Enhanced Fujita Scale replaced the original Fujita Scale from 1971. It made substantial improvements in standardizing damage descriptors through expanding and refining damage indicators and associated degrees of damage, as well as calibrated tornado wind speeds to better match the associated damage. However, the EF Scale, having been designed based on construction practices in the United States, is not necessarily applicable across all regions. The EF-scale and variants thereof are officially used by the United States, Canada, France, and Japan, as well as unofficially in other countries, such as China.

Unlike with the F scale, no analyses have been undertaken at all to establish the veracity and accuracy of the T scale damage descriptors. The scale was written in the early 1970s, and does not take into account changes such as the growth in weight of vehicles or the great reduction in numbers and change of type of railway locomotives, and was written in an environment where tornadoes of F2 or stronger are extremely rare, so little or no first-hand investigation of actual damage at the upper end of the scale was possible. The TORRO scale has more graduations than the F scale which makes it arguably more useful for tornadoes on the lower end of the scale; however, such accuracy and precision are not typically attainable in practice. Brooks and Doswell stated that "the problems associated with damage surveys and uncertainties associated with estimating wind speed from observed damage make highly precise assignments dubious". In survey reports, Fujita ratings sometimes also have extra qualifications added ("minimal F2" or "upper-end F3 damage"), made by investigators who have experience of many similar tornadoes and relating to the fact that the F scale is a damage scale, not a wind speed scale.

Tornadoes are rated after they have passed and have been examined, not whilst in progress. In rating the intensity of a tornado, both direct measurements and inferences from empirical observations of the effects of a tornado are used. Few anemometers are struck by a tornado, and even fewer survive, so there are very few in-situ measurements. Therefore, almost all ratings are obtained from remote sensing techniques or as proxies from damage surveys. Weather radar is used when available, and sometimes photogrammetry or videogrammetry estimates wind speed by measuring tracers in the vortex. In most cases, aerial and ground damage surveys of structures and vegetation are utilised, sometimes with engineering analysis. Also sometimes available are ground swirl patterns (cycloidal marks) left in the wake of a tornado. If an on site analysis is not possible, either for retrospective ratings or when personnel cannot reach a site, photographs, videos, or descriptions of damage may be utilised.

TORRO scale parameters

The 12 categories for the TORRO scale are listed below, in order of increasing intensity. Although the wind speeds and photographic damage examples are updated, which are more or less still accurate. However, for the actual TORRO scale in practice, damage indicators (the type of structure which has been damaged) are predominantly used in determining the tornado intensity.

Scale	Wind speed (Estimated)			Potential damage	Example of damage
	mph	km/h	m/s
T0	39 - 54	61 - 86	17 - 24	Light damage. Loose light litter such as paper, leaves and twigs raised from ground level in spirals. Secured tents and marquees seriously disturbed; a few exposed tiles/slates on roofs dislodged. Twigs and perhaps weak small branches that are in leaf snapped from some trees; minimal or no damage to trees with no leaves, trail visible through crops.
T1	55 - 72	87 - 115	25 - 32	Mild damage. Deckchairs, small plants/plants in small pots, heavy litter becomes airborne; minor damage to sheds. More serious/numerous dislodging of tiles, slates and chimney pots with some tiles/slates blown off typical/average strength roofs. Low quality wooden fences damaged or flattened. Slight damage possible to low lying shrubs/bushes, particularly of the evergreen variety. Moderate damage to trees, with a few medium sized branches in leaf snapping on the upper bound of T1, trees without leaves on them likely remaining mostly unscathed except for significant twig breakage, although for some a few small branches could break. Very weak/unhealthy trees, particularly those in leaf and of softwood variety such as conifers are likely to be nearly or completely uprooted.
T2	73 - 92	116 - 147	33 - 41	Moderate damage. Heavy mobile homes displaced with some damage to exterior, light caravans lose majority of roof and/or are blown over, particularly from upper bound winds of T2, bonnets blown open on some vehicles, average strength sturdy garden sheds destroyed, greenhouses of weak/average construction lose entire plastic/glass roofing cover with a total collapse of some weak/average greenhouse structures likely. Garage roofs torn away, some to significant damage to tiled roofs and chimney stacks with many tiles missing, particularly to weak wooden framed homes, though typically thatched roofs with small eaves/smooth surface suffer only minor damage, outbuildings lose entire roofs and suffer some degree of damage to actual structure. Guttering pulled from some houses with some siding damage possible, older single glazed windows blown in or out of frames or smashed. Significant damage to most tree types, some big branches twisted or snapped off, most small and shallow rooted trees whether in leaf or not are uprooted or snapped.
T3	93 - 114	148 - 184	42 - 51	Strong damage. Mobile homes overturned / badly damaged; light caravans severely damaged or destroyed; garages and weak outbuildings severely damaged or destroyed; house roof timbers considerably exposed with more strongly built brick masonry houses suffering major roof damage with chimneys at risk of collapse, though structure/walls of the building below roof itself mostly intact except for windows breaking especially from any small flying objects. Most large healthy trees lose many big branches and many are snapped or uprooted, lighter cars flipped.
T4	115 - 136	185 - 220	52 - 61	Severe damage. Cars levitated. Mobile homes/lighter caravans airborne / destroyed; garden sheds obliterated and airborne for considerable distances; entire roofs removed from some houses; roof timbers of stronger brick or stone houses completely exposed; gable ends torn away. "Weak" framed wooden houses will receive some damage to structure though most of structure still standing. Numerous strong trees uprooted or snapped with all trees within damage path receiving some debranching.
T5	137 - 160	221 - 259	62 - 72	Intense damage. Heavy vehicles such as buses/lorries (trucks) overturned or overturned and displaced some distance in excess of 10 metres though with minimal levitation, lighter vehicles such as passenger cars thrown large distances. Wind turbines built from strong material suffer significant blade damage with blades ending up shredded or broken/ possibly suffering permanent deformation of tower/blades with winds on the upper bounds of T5. Strong framed wooden buildings/weak brick masonry buildings receive more significant damage than T4 though walls on ground floor will probably remain, some wall damage on second/upper floor connected to roof is likely though with one or two walls blowing down/collapsing, some/significant damage likely inside of these buildings. Stronger brick masonry homes may lose a few rows of bricks on second floor, though overall structure below roof itself largely standing with bottom floor relatively intact except for doors and windows, the roof mostly or entirely blown/torn off. The oldest, weakest buildings may collapse completely.
T6	161 - 186	260 - 299	73 - 83	Moderately-devastating damage. Strong framed wooden buildings largely or completely destroyed, Strongly built brick masonry houses lose entire roofs just like T5 though exterior walls on second floor now likely blown down or collapsed with significant interior damage, windows broken on skyscrapers, more of the less-strong buildings collapse, national grid pylons severely damaged or blown down/bent and deformed, Strong trees that aren't uprooted /snapped will suffer major debranching with most leaves torn off, other trees excluding the widest and strongest ones are snapped/uprooted, very large heavy branches thrown large distances. Lighter vehicles thrown upto a mile in some cases, heavy vehicles such as buses lofted and tossed tens of metres away, trains derailed/blown over while in motion.
T7	187 - 212	300 - 342	84 - 95	Strongly - devastating damage. Strongly built wooden-framed/weak brick masonry buildings/houses wholly demolished; some walls of more strongly built stone / brick masonry houses beaten down or collapse with significant damage to overall structure, with some shifting on foundations likely; skyscrapers twisted; steel-framed warehouse-type constructions may buckle slightly. Well built steel reinforced concrete buildings/houses suffer total roof loss with some damage to overall structure though most walls remain standing, particularly the lower floors. Trains whether stationary or not are blown over. All large branches torn/stripped from trees down to the trunk, some small-medium sized trees are thrown. Noticeable debarking of any standing tree trunks from flying debris.
T8	213 - 240	343 - 385	96 - 107	Severely - devastating damage Cars and other larger/heavier vehicles such as trucks hurled great distances. Strong wooden-framed houses and their contents dispersed over long distances; strong stone or brick masonry buildings severely damaged or largely destroyed with one or two sections of walls blown away; steel reinforced concrete homes/large buildings suffer significant to major structural damage. Skyscrapers badly twisted and may show a visible lean to one side; shallowly anchored high rises may be toppled; other steel-framed buildings buckled.
T9	241 - 269	386 - 432	108 - 120	Intensely -devastating damage. Many steel-framed/concrete buildings badly damaged though some of structure may remain standing albeit shifted in position on foundation; skyscrapers toppled; locomotives or trains likely blown over and rolled a short distance from tracks with damage to its exterior, empty train cars however are likely to be flipped and rolled repeatedly some distance away from tracks with some levitation likely along the way. Strong brick masonry buildings/houses almost or completely destroyed with large sections of houses/building blown away from foundation. Concrete pathways slightly above soil level could be shifted in position by several inches. Complete debarking of any standing tree-trunks.
T10	270 - 299	433 - 482	121 - 134	Super damage. Entire very well built houses/buildings lifted bodily or completely from foundations and carried a large distance to disintegrate. Steel-reinforced concrete buildings severely damaged or almost obliterated. (e.g. 1930 Montello tornado)
T11	>300	>483	>135	Phenomenal damage. Exceptionally well built very thick walled (40-80cm) brick masonry buildings are completely destroyed and swept off foundations entirely with only flooring or foundations remaining with even these potentially damaged or with sections pulled off entirely; Well built steel-reinforced concrete structures/homes are completely destroyed. Tall buildings collapse. Cars, trucks and train cars thrown in excess of 1-3 miles. In terms of man made objects, only the very heaviest ones for example locomotives/trains weighing hundreds of tons and the strongest of buildings made low to the ground with specific very aerodynamic designs and incredibly thick load bearing steel concrete walls with no windows/discernible roof will "survive" a tornado of this strength, survival would be reliant on these specialised structures or out of path of the tornado itself. But the precise design needed and possibility of it actually successfully providing adequate safety during such a tornado is very speculative for now.

T0	T1	T2	T3	T4	T5	T6	T7	T8	T9	T10	T11
Weak				Strong				Violent

See also

• Saffir-Simpson Hurricane Scale
• Tornado intensity and damage
• Wind engineering
• List of tornadoes and tornado outbreaks

References

• Grazulis, Thomas P. (1993). Significant Tornadoes 1680-1991, A Chronology and Analysis of Events. St. Johnsbury, VT: The Tornado Project of Environmental Films. ISBN 1-879362-03-1.
• Meaden, G. T. (1976). "Tornadoes in Britain: Their intensities and distribution in space and time". Journal of Meteorology. 1 (8). UK: 242–51.
• Meaden, G. T. (1985). "A study of tornadoes in Britain, with assessments of the general tornado risk potential and the specific risk potential at particular regional sites". Journal of Meteorology. 8 (79). UK: 151–3.

External links

• TORRO Tornado Intensity Scale (TORRO)
• T-Scale Origins and Scientific Basis (TORRO)
• Wind speed scales: Beaufort, T Scale, and Fujita's Scale (TORRO)
• Determination of tornado or downburst intensity (TorDACH)