RESEARCH//TORNADOES//THE T SCALE

The International Tornado Intensity Scale

Background

Dr. G. Terence Meaden devised The International Tornado Intensity Scale in 1972 to categorize wind speeds of tornadoes. The scale is directly related to the Beaufort Scale and is the only true tornado intensity scale with a sound scientific base.

For information on the origins of the Tscale and how it was devised visit the T Scale Origin page.

The scale allows a tornado's wind speed to be determined by various means:

• visiting the tornado damage site to make non-engineering assessments
• obtaining an engineering assessment of the damage
• using Doppler radar
• applying photogrammetric analysis
• directly measuring observed tornadoes.

Few anemometers have survived even the weakest tornado to record a peak gust and few engineering studies of tornado damage have been made. Furthermore not many observers have been close enough for Doppler measurement. Consequently most ratings are derived from the least accurate method of a non-engineering based study of damage. Nevertheless, it is important to note that the International Tornado Intensity Scale based on sound scientific formulae allows tornadic winds to be rated even if a tornado has no opportunity to cause damage (as when crossing open countryside for instance). Thus the TORRO scale is a true tornado wind speed intensity scale rather than purely a damage scale.

The T Scale is precise. It is well-suited for the more accurate methods of tornadic wind speed determination and also for rating weak tornadoes which account for the vast majority of global events. It is relatively easy for a basic non-engineering study of damage to produce a reasonably accurate rating of damage caused by a weak tornado because the wind speeds are relatively low. Conversely, rating strong tornadoes becomes harder and the precise nature of the scale makes the accurate rating of violent tornadoes by viewing the damage caused particularly difficult. To reduce this, adjacent points on the scale can be grouped together – thus T5-6 is acceptable as a rating.

A tornado is only rated T0 if it passes over vulnerable objects, which are not damaged (leaving aside the more accurate methods of determining wind speeds in this instance). If a tornado is known to have occurred but details of damage or other rating information are not forthcoming, then the tornado is not given an intensity rating. Whenever information is available to provide an intensity rating, the qualifying value is the one corresponding to the tornado at its most intense.

Care is given to the rating of slow-moving tornadoes, because they will create more intense damage than a faster-moving tornado of the same wind speed. In such cases, if there is no other information available to give an intensity rating, the scale number to be used will be lower than what usually corresponds to the observed damage.

As a true wind speed scale, the T Scale can be applied to determine any wind speed, whether the wind is tornadic or not.

The T Scale

The table below highlights the International Tornado Intensity Scale, where tornadoes can be rated on separate wind speed, track length, track width and track area categories. For instance the 7 December 2006 Kensal Rise, London, tornado was found by Kirk (2014) following a site investigation to be rated T5, L4, W6 and A5...

Scale	Wind speed (T)	Description of damage (for guidance only)	Track length (L)	Track width (W)	Track area (A)
0	Light tornado 17-24 m/s (39-54 mph)	Loose light litter raised from ground level in spirals. Tents, marquees, awnings seriously disturbed. Some exposed tiles, slates on roofs dislodged. Twigs snapped; trail visible through crops. Wheelie bins tipped and rolled. Garden furniture & pots disturbed.	≤215 m	≤2.1 m	≤464 m2
1	Mild Tornado 25-32 m/s (55-72 mph)	Deck chairs, small plants, heavy litter becomes airborne. Minor damage to sheds. More serious dislodging of tiles, slates. Chimney pots dislodged. Wooden fences flattened. Slight damage to hedges and trees. Some windows already ajar blown open breaking latches.	216-464 m	2.2-4.6 m	465-2150 m2
2	Moderate Tornado 33-41 m/s (73-92 mph)	Heavy mobile homes displaced. Light caravans blown over. Garden sheds destroyed. Garage roofs torn away and doors imploded. Much damage to tiled roofs and chimneys. Ridge tiles missing. General damage to trees, some big branches twisted or snapped off, small trees uprooted. Bonnets blown open on cars. Weak or old brick walls toppled. Windows blown open or glazing sucked out of frames.	465-999 m	4.7-9.9 m	2160-9990 m2
3	Strong Tornado 42-51 m/s (93-114 mph)	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. Some heavier debris becomes airborne causing secondary damage breaking windows and impaling softer objects. Debris carried considerable distances. Garden walls blown over. Eyewitness reports of buildings physically shaking. Mud sprayed up the side of buildings	1.0-2.1 km	10-21 m	0.01-0.046 km2
4	Severe Tornado 52-61 m/s (115-136 mph)	Motorcars 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. Traffic Signs folded or twisted. Some large trees uprooted and carried several yards. Debris carried up to 2km leaving an obvious trail.	2.2-4.6 km	22-46 m	0.047-0.21 km2
5	Intense Tornado 62-72 m/s (137-160 mph)	Heavier motor vehicles (4x4, 4 Tonne Trucks) levitated. Wall plates, entire roofs and several rows of bricks on top floors removed. Items sucked out from inside house including partition walls and furniture. Older, weaker buildings collapse completely. Utility poles snapped.	4.7-9.9 km	47-99 m	0.22-0.99 km2
6	Moderately-Devastating Tornado 73-83 m/s (161-186 mph)	Strongly built houses suffer major damage or are demolished completely. Bricks and blocks etc. become dangerous airborne debris. National grid pylons are damaged or twisted. Exceptional or unusual damage found, e.g. objects embedded in walls or small structures elevated and landed with no obvious damage.	10-21 km	100-215 m	1.0-4.6 km2
7	Strongly-Devastating Tornado 84-95 m/s (187-212 mph)	Brick and Wooden-frame houses wholly demolished. Steel-framed warehouse-type constructions destroyed or seriously damaged. Locomotives thrown over. Noticeable de-barking of trees by flying debris.	22-46 km	216-464 m	4.7-21 km2
8	Severely-Devastating Tornado 96-107 m/s (213-240 mph)	Motorcars carried great distances. Some steel framed factory units severely damaged or destroyed. Steel and other heavy debris strewn over a great distances. A high level of damage within the periphery of the damage path.	47-99 km	465-999 m	22-99 km2
9	Intensely-Devastating Tornado 108-120 m/s (241-269 mph)	Many steel-framed buildings demolished Locomotives or trains hurled some distances. Complete debarking of any standing tree-trunks. Inhabitants survival reliant on shelter below ground level.	100-215 km	1.0-2.1 km	100-464 km2
10	Super Tornado 121-134 m/s (270-299 mph)	Entire frame houses and similar buildings lifted bodily from foundations and carried some distances. Destruction of a severe nature, rendering a broad linear track largely devoid of vegetation, trees and man made structures.	216-464 km	2.2-4.6 km	≥465 km2

Tornadoes of strength T0, T1, T2, T3 are termed weak tornadoes.
Those reaching T4, T5, T6, T7 are strong tornadoes.
T8, T9, T10, T11 are violent tornadoes.

Because the Tornado Scale is open-ended, it can be extended beyond T10 using the formulae below where v = wind velocity, T = Tornado Intensity number, and B = Beaufort Force number.

v = 2.365 x (T+4)^3/₂ metres per second
v = 8.511 x (T+4)^3/₂ kilometres per hour
v = 5.289 x (T+4)^3/₂ miles per hour
v = 4.596 x (T+4)^3/₂ knots

Thus, B = 2 x (T + 4) and T = (^B/₂) – 4.