TORRO scale

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The TORRO tornado intensity scale (or T-Scale) is a scale measuring tornado intensity between T0 and T10. 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.

The scale was tested from 1972-1975 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 by the formula:

B = 2 (T + 4)

and conversely:

T = (B/2 - 4)

Beaufort scale	8	10	12	14	16	18	20	22	24	26	28	30
TORRO scale	0	1	2	3	4	5	6	7	8	9	10	11

The Beaufort scale is expressed by the formula:

v = 0.837 B^3/2 m/s

TORRO claims it also differs from the Fujita scale in the fact that it relies solely on wind speed for classification, where the Fujita scale relies on damage for classification, but in practice, damage is utilised in both systems to infer intensity. This scale is primarily used in the United Kingdom whereas the Fujita scale is the primary scale used in the United States, Europe (to a lesser extent), and the rest of the world.

The TORRO scale has more graduations 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, and the damage descriptors are unclear on the upper end of the scale. It should be noted that in use, Fujita ratings often also have extra qualifications added ("minimal F2" or "upper-end F3 damage").

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. Weather radar is used when available. Sometimes photogrammetry or videogrammetry estimates wind speed by measuring tracers in the vortex. Aerial and ground damage surveys of structures and vegetation are utilised, sometimes with engineering analysis. Also sometimes available are ground swirl patterns left in the wake of a tornado.

Most UK tornadoes are T6 or below with the strongest UK tornado being a T8. For comparison, the strongest detected winds in a United States tornado (during the 1999 Oklahoma Tornado Outbreak in Moore, Oklahoma) would be T11 using the formula:

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 kts

where v is wind speed and T is TORRO intensity number.

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

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

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

At the 2004 European Conference on Severe Storms, Dr. Meaden propsed a unification of the TORRO and Fujita scales as the Tornado Force or TF Scale.

[edit] TORRO scale

TORRO intensity	Wind speeds	Tornado description	Damage description
FC	-	Funnel cloud (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 that 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; 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; 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; steel-framed buildings buckled.
T9	108 - 120 m/s 386 - 432 km/h 241 - 269 mph	Intensely- devastating	Many steel-framed buildings badly damaged; 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

[edit] See also

[edit] References

Grazulis, Thomas P. (1993). Significant Tornadoes 1680-1991, A Chronology and Analysis of Events. The Tornado Project of Environmental Films: St. Johnsbury, VT. ISBN 1-879362-03-1
Meaden, G. T. (1976). "Tornadoes in Britain: Their intensities and distribution in space and time". Journal of Meteorology, UK, 1 (8), pp 242-251.
(1985). Journal of Meteorology, UK, 8 (79), pp 151-153.