TORRO scale
Encyclopedia
The TORRO tornado intensity scale (or T-Scale) is a scale measuring tornado
intensity between T0 and T11. It was developed 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
.
in 1975. The scale sets T0 as the equivalent of 8 on the Beaufort scale and is related to the Beaufort scale (B) by the formula:
and conversely:
The Beaufort scale
was first introduced in 1805, and in 1921 quantified. It expresses the wind speed (v) by the formula:
) would be T11 using the following formulae:
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:
or
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 is the primary scale used in North America, 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. The EF Scale is biased to U.S. construction practices however, and as of 2007, no other countries have adapted it for their use, retaining the F scale or T scale.
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 always 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 windspeed 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 anemometer
s 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.
Tornado
A tornado is a violent, dangerous, rotating column of air that is in contact with both the surface of the earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. They are often referred to as a twister or a cyclone, although the word cyclone is used in meteorology in a wider...
intensity between T0 and T11. It was developed by Terence Meaden of the Tornado and Storm Research Organisation (TORRO), a meteorological
Meteorology
Meteorology is the interdisciplinary scientific study of the atmosphere. Studies in the field stretch back millennia, though significant progress in meteorology did not occur until the 18th century. The 19th century saw breakthroughs occur after observing networks developed across several countries...
organisation in the United Kingdom
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
, as an extension of the Beaufort scale
Beaufort scale
The Beaufort Scale is an empirical measure that relates wind speed to observed conditions at sea or on land. Its full name is the Beaufort Wind Force Scale.-History:...
.
History and derivation from Beaufort scale
The scale was tested from 1972-1975 and was made public at a meeting of the Royal Meteorological SocietyRoyal Meteorological Society
The Royal Meteorological Society traces its origins back to 3 April 1850 when the British Meteorological Society was formed as a society the objects of which should be the advancement and extension of meteorological science by determining the laws of climate and of meteorological phenomena in general...
in 1975. The scale sets T0 as the equivalent of 8 on the Beaufort scale and is related to the Beaufort scale (B) by the formula:
- B = 2 (T + 4)
and conversely:
- T = (B/2 - 4)
| Beaufort scale | B | 8 | 10 | 12 | 14 | 16 | 18 | 20 | 22 | 24 | 26 | 28 | 30 |
| TORRO scale | T | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 |
The Beaufort scale
Beaufort scale
The Beaufort Scale is an empirical measure that relates wind speed to observed conditions at sea or on land. Its full name is the Beaufort Wind Force Scale.-History:...
was first introduced in 1805, and in 1921 quantified. It expresses the wind speed (v) by the formula:
- vSpeedIn kinematics, the speed of an object is the magnitude of its velocity ; it is thus a scalar quantity. The average speed of an object in an interval of time is the distance traveled by the object divided by the duration of the interval; the instantaneous speed is the limit of the average speed as...
= 0.837 B3/2 m/s
TORRO scale formula
Most UK tornadoes are T6 or below with the strongest known UK tornado being a T8. For comparison, the strongest detected winds in a United States tornado (during the 1999 Oklahoma tornado outbreak in Moore, OklahomaMoore, Oklahoma
Moore is a city in Cleveland County, Oklahoma and is part of the Oklahoma City Metropolitan Area. The population was 55,081 at the 2010 census, making it the seventh largest city in the state of Oklahoma....
) would be T11 using the following formulae:
- vSpeedIn kinematics, the speed of an object is the magnitude of its velocity ; it is thus a scalar quantity. The average speed of an object in an interval of time is the distance traveled by the object divided by the duration of the interval; the instantaneous speed is the limit of the average speed as...
= 2.365 (T+4)3/2 m/s - vSpeedIn kinematics, the speed of an object is the magnitude of its velocity ; it is thus a scalar quantity. The average speed of an object in an interval of time is the distance traveled by the object divided by the duration of the interval; the instantaneous speed is the limit of the average speed as...
= 8.511 (T+4)3/2 km/h - vSpeedIn kinematics, the speed of an object is the magnitude of its velocity ; it is thus a scalar quantity. The average speed of an object in an interval of time is the distance traveled by the object divided by the duration of the interval; the instantaneous speed is the limit of the average speed as...
= 5.289 (T+4)3/2 mph - vSpeedIn kinematics, the speed of an object is the magnitude of its velocity ; it is thus a scalar quantity. The average speed of an object in an interval of time is the distance traveled by the object divided by the duration of the interval; the instantaneous speed is the limit of the average speed as...
= 4.596 (T+4)3/2 kn
where v is wind speed
Wind speed
Wind speed, or wind velocity, is a fundamental atmospheric rate.Wind speed affects weather forecasting, aircraft and maritime operations, construction projects, growth and metabolism rate of many plant species, and countless other implications....
and T is TORRO intensity number. Wind speed is defined as a 3 second gust at 10 m
Metre
The metre , symbol m, is the base unit of length in the International System of Units . Originally intended to be one ten-millionth of the distance from the Earth's equator to the North Pole , its definition has been periodically refined to reflect growing knowledge of metrology...
AGL
Above ground level
In aviation and atmospheric sciences, an altitude is said to be above ground level when it is measured with respect to the underlying ground surface. This is as opposed to above mean sea level , or in broadcast engineering, height above average terrain...
.
Alternatively, the T-Scale formula may be expressed as:
- vSpeedIn kinematics, the speed of an object is the magnitude of its velocity ; it is thus a scalar quantity. The average speed of an object in an interval of time is the distance traveled by the object divided by the duration of the interval; the instantaneous speed is the limit of the average speed as...
= 0.837 (2T+8)3/2 m/s
or
- vSpeedIn kinematics, the speed of an object is the magnitude of its velocity ; it is thus a scalar quantity. The average speed of an object in an interval of time is the distance traveled by the object divided by the duration of the interval; the instantaneous speed is the limit of the average speed as...
= 0.837(23/2) (T+4)3/2 m/s
Rating process and comparisons to Fujita scale
TORRO claims it differs from the Fujita scaleFujita scale
The Fujita scale , or Fujita-Pearson scale, is a scale for rating tornado intensity, based primarily on the damage tornadoes inflict on human-built structures and vegetation...
in that it is "purely" a wind speed
Wind speed
Wind speed, or wind velocity, is a fundamental atmospheric rate.Wind speed affects weather forecasting, aircraft and maritime operations, construction projects, growth and metabolism rate of many plant species, and countless other implications....
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 is the primary scale used in North America, 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
Enhanced Fujita Scale
The Enhanced Fujita Scale rates the strength of tornadoes in the United States based on the damage they cause.Implemented in place of the Fujita scale introduced in 1971 by Ted Fujita, it began operational use on February 1, 2007. The scale has the same basic design as the original 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. The EF Scale is biased to U.S. construction practices however, and as of 2007, no other countries have adapted it for their use, retaining the F scale or T scale.
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 always 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 windspeed 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 anemometer
Anemometer
An anemometer is a device for measuring wind speed, and is a common weather station instrument. The term is derived from the Greek word anemos, meaning wind, and is used to describe any airspeed measurement instrument used in meteorology or aerodynamics...
s 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
Remote sensing
Remote sensing is the acquisition of information about an object or phenomenon, without making physical contact with the object. In modern usage, the term generally refers to the use of aerial sensor technologies to detect and classify objects on Earth by means of propagated signals Remote sensing...
techniques or as proxies
Proxy (climate)
In the study of past climates is known as paleoclimatology, climate proxies are preserved physical characteristics of the past that stand in for direct measurements , to enable scientists to reconstruct the climatic conditions that prevailed during much of the Earth's history...
from damage surveys. Weather radar
Weather radar
Weather radar, also called weather surveillance radar and Doppler weather radar, is a type of radar used to locate precipitation, calculate its motion, estimate its type . Modern weather radars are mostly pulse-Doppler radars, capable of detecting the motion of rain droplets in addition to the...
is used when available, and sometimes photogrammetry
Photogrammetry
Photogrammetry is the practice of determining the geometric properties of objects from photographic images. Photogrammetry is as old as modern photography and can be dated to the mid-nineteenth century....
or videogrammetry
Videogrammetry
Videogrammetry is a measurement technology in which the three-dimensional coordinates of points on an object are determined by measurements made in two or more video images taken from different angles. Images can be obtained from two cameras which simultaneously view the object or from successive...
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
| TORRO intensity | Wind speeds | Tornado description |
Damage description |
|---|---|---|---|
| FC | - | Funnel cloud aloft (Not a tornado) | No damage to structures, unless on tops of tallest towers, or to radiosondes, balloons, and aircraft. No damage in the country, except possibly agitation to highest tree-tops and effect on birds and smoke. Record FC when not known to have reached ground level. A whistling or rushing sound aloft may be noticed. |
| T0 | 17 - 24 m/s 61 - 86 km/h 39 - 54 mph |
Light | Loose light litter raised from ground-level in spirals. Tents, marquees seriously disturbed; most exposed tiles, slates on roofs dislodged. Twigs snapped; trail visible through crops. |
| T1 | 25 - 32 m/s 87 - 115 km/h 55 - 72 mph |
Mild | Deckchairs, small plants, heavy litter becomes airborne; minor damage to sheds. More serious dislodging of tiles, slates, chimney pots. Wooden fences flattened. Slight damage to hedges and trees. |
| T2 | 33 - 41 m/s 116 - 147 km/h 73 - 92 mph |
Moderate | Heavy mobile homes displaced, light caravans blown over, garden sheds destroyed, garage roofs torn away, much damage to tiled roofs and chimney stacks. General damage to trees, some big branches twisted or snapped off, small trees uprooted. |
| T3 | 42 - 51 m/s 148 - 184 km/h 93 - 114 mph |
Strong | Mobile homes overturned / badly damaged; light caravans destroyed; garages and weak outbuildings destroyed; house roof timbers considerably exposed. Some of the bigger trees snapped or uprooted. |
| T4 | 52 - 61 m/s 185 - 220 km/h 115 - 136 mph |
Severe | Motor cars levitated. Mobile homes airborne / destroyed; sheds airborne for considerable distances; entire roofs removed from some houses; roof timbers of stronger brick or stone houses completely exposed; gable ends torn away. Numerous trees uprooted or snapped. |
| T5 | 62 - 72 m/s 221 - 259 km/h 137 - 160 mph |
Intense | Heavy motor vehicles levitated; more serious building damage than for T4, yet house walls usually remaining; the oldest, weakest buildings may collapse completely. |
| T6 | 73 - 83 m/s 260 - 299 km/h 161 - 186 mph |
Moderately- devastating |
Strongly-built houses lose entire roofs and perhaps also a wall; windows broken on skyscrapers, more of the less-strong buildings collapse. |
| T7 | 84 - 95 m/s 300 - 342 km/h 187 - 212 mph |
Strongly- devastating |
Wooden-frame houses wholly demolished; some walls of stone or brick houses beaten down or collapse; skyscrapers twisted; steel-framed warehouse-type constructions may buckle slightly. Locomotives thrown over. Noticeable de-barking of trees by flying debris. |
| T8 | 96 - 107 m/s 343 - 385 km/h 213 - 240 mph |
Severely- devastating |
Motor cars hurled great distances. Wooden-framed houses and their contents dispersed over long distances; stone or brick houses irreparably damaged; skyscrapers badly twisted and may show a visible lean to one side; shallowly anchored highrises may be toppled; other steel-framed buildings buckled. |
| T9 | 108 - 120 m/s 386 - 432 km/h 241 - 269 mph |
Intensely- devastating |
Many steel-framed buildings badly damaged; skyscrapers toppled; locomotives or trains hurled some distances. Complete debarking of any standing tree-trunks. |
| T10 | 121 - 134 m/s 433 - 482 km/h 270 - 299 mph |
Super | Entire frame houses and similar buildings lifted bodily from foundations and carried some distances. Steel-reinforced concrete buildings may be severely damaged. |
| T0 | T1 | T2 | T3 | T4 | T5 | T6 | T7 | T8 | T9 | T10 | T11 |
| Weak | Strong | Violent | |||||||||
See also
- Enhanced Fujita ScaleEnhanced Fujita ScaleThe Enhanced Fujita Scale rates the strength of tornadoes in the United States based on the damage they cause.Implemented in place of the Fujita scale introduced in 1971 by Ted Fujita, it began operational use on February 1, 2007. The scale has the same basic design as the original Fujita scale:...
- Fujita scaleFujita scaleThe Fujita scale , or Fujita-Pearson scale, is a scale for rating tornado intensity, based primarily on the damage tornadoes inflict on human-built structures and vegetation...
- Saffir-Simpson Hurricane ScaleSaffir-Simpson Hurricane ScaleThe Saffir–Simpson Hurricane Scale , or the Saffir–Simpson Hurricane Wind Scale , classifies hurricanes — Western Hemisphere tropical cyclones that exceed the intensities of tropical depressions and tropical storms — into five categories distinguished by the intensities of their sustained winds...
- Tornado intensity and damageTornado intensity and damageThe Fujita scale and the Enhanced Fujita Scale rate tornadoes by damage caused. The Enhanced Fujita Scale was an upgrade to the older Fujita scale, with engineered wind estimates and better damage descriptions, but was designed so that a tornado rated on the Fujita scale would receive the same...
- List of tornadoes and tornado outbreaks