THE INTERNATIONAL

TORRO TORNADO INTENSITY SCALE

Dr. G. Terence Meaden devised TORRO’s Tornado Intensity Scale in 1972, in order to categorize wind speeds in tornadoes. The scale is directly related to the Beaufort Scale, and is the worlds 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 Tscale Origin page.

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

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

Few anemometers have survived even weak tornadoes and recorded a peak gust. Few engineering studies of tornado damage have been made, and not many observers have been close enough for Doppler measurement. Consequently, most ratings are derived from the least accurate method, the non-engineering study of damage. Nevertheless, it is important to note that the TORRO Tornado Intensity Scale, which is 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 scale is quite precise. Not only is it well-suited for the more accurate methods of tornadic wind speed determination, but also for rating weak tornadoes which account for the vast majority of global events. It is relatively easy for even a 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. On the other hand, rating strong tornadoes becomes harder and the precise nature of the scale makes the accurate rating of violent tornadoes by viewing the damage caused alone particularly difficult. To reduce this difficulty, adjacent points on the scale can be grouped together – thus T5-6 is quite 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 TORRO SCALE can be applied to determine any wind speed, whether the wind is tornadic or not.

T-SCALE WIND SPEEDS TYPICAL DAMAGE

INTENSITY (descriptive, for guidance only)

TORRO Intensity	Description Of Tornado & Windspeeds	Description Of Damage (for guidance only)
T0	Light Tornado 17 - 24 m s-1 (39 - 54 mi h-1)	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	Mild Tornado 25 - 32 m s-1 (55 - 72 mi h-1)	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	Moderate Tornado 33 - 41 m s-1 (73 - 92 mi h-1)	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	Strong Tornado 42 - 51 m s-1 (93 - 114 mi h-1)	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	Severe Tornado 52 - 61 m s-1 (115 - 136 mi h-1)	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	Intense Tornado 62 - 72 m s-1 (137 - 160 mi h-1)	Heavy motor vehicles levitated; more serious building damage that for T4, yet house walls usually remaining; the oldest, weakest buildings may collapse completely.
T6	Moderately-Devastating Tornado 73 - 83 m s-1 (161 - 186 mi h-1)	Strongly-built houses lose entire roofs and perhaps also a wall; more of the less-strong buildings collapse.
T7	Strongly-Devastating Tornado 84 - 95 m s-1 (187 - 212 mi h-1)	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	Severely-Devastating Tornado 96 - 107 m s-1 (213 - 240 mi h-1)	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	Intensely-Devastating Tornado 108 - 120 m s-1 (241 - 269 mi h-1)	Many steel-framed buildings badly damaged; locomotives or trains hurled some distances. Complete debarking of any standing tree-trunks
T10	Super Tornado 121 - 134 m s-1 (270 - 299 mi h-1)	Entire frame houses and similar buildings lifted bodily from foundations and carried some distances. Steel-reinforced concrete buildings may be severely damaged.

Tornadoes of strength T0, T1, T2, T3 are termed weak tornadoes.

Those reaching T4, T5, T6, T7 are strong tornadoes, and T8, T9, T10, T11 are violent tornadoes

Because the TORRO 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 (T+4)^1.5 metres per second v = 8.511 (T+4)^1.5 kilometres per hour

v = 5.289 (T+4)^1.5 miles per hour v = 4.596 (T+4)^1.5 knots.

Thus, B = 2 (T + 4) and T = (B/2 – 4)