The TORRO Tornado Intensity Scale
Britain has tornadoes, and a scale of its own for grading them.
Origin
Devised by Terence Meaden of the Tornado and Storm Research Organisation (TORRO) in 1972 and published in the mid-1970s. It extends the Beaufort wind scale upwards, with T0 set at gale force 8, and is used for British and European tornado assessment.
The catch
TORRO describes its scale as a pure wind-speed scale, built mathematically on the Beaufort scale, whereas the American Fujita scale was framed around damage. In practice both are rated from what a tornado leaves behind. The real difference is resolution: TORRO has roughly twice as many steps, so a T rating is about double the Fujita number. T2–T3 is roughly F1, T4–T5 roughly F2, and T10–T11 roughly F5.
Loose light litter raised from the ground in spirals. Tents and marquees disturbed, a few twigs snapped. Often passes unnoticed.
Deckchairs and small plants displaced. Awnings ripped, slight damage to hedges and trees.
Heavy mobile homes displaced, light caravans blown over. Garden sheds destroyed, garage roofs torn away, tiles stripped.
Mobile homes overturned, light caravans destroyed. Garages and outbuildings wrecked, house roofs badly damaged.
Motor cars lifted clear of the ground, whole hedgerows uprooted, entire roofs removed from some houses. Roughly the bottom of the American F2.
Heavy motor vehicles lifted. More serious roof damage, and weaker buildings collapse completely. The Birmingham tornado of 2005 was rated at T5 to T6.
Strongly built houses lose entire roofs and perhaps a wall. Older, weaker buildings may collapse. Rare in Britain.
Wooden frame houses wholly demolished. Some walls of stone or brick houses beaten down or collapsed. Steel structures damaged. The Birmingham tornado of 1931 was rated T7.
Motor cars hurled great distances. Wooden frame houses and their contents scattered over long distances. The highest rating TORRO has given any British tornado, the London tornado of 1091.
Many steel structures badly damaged. Locomotives thrown over. Complete debarking of standing tree trunks.
Entire frame houses lifted bodily from their foundations and carried some distance. Steel-reinforced concrete buildings may be severely damaged.
The top of the defined scale, reserved for the most violent tornadoes on earth. Nothing close to this has been recorded in Britain.
The costliest tornado ever recorded in Britain struck Birmingham on 28 July 2005, injuring 39 people and causing around £40 million of damage across Small Heath, Sparkbrook and Moseley. TORRO rated it T5 to T6. Birmingham had been hit before: a T7 tornado in June 1931 killed one person along a ten-kilometre track. The earliest recorded British tornado, in London in October 1091, destroyed the wooden London Bridge and damaged the church of St Mary-le-Bow, and is estimated at T8. The scale is open-ended and can in principle be extended beyond T11, though nothing anywhere has required it.
Common questions
How does the TORRO scale compare with the Fujita and EF scales?
A TORRO rating is roughly double the Fujita number: T0–T1 is about F0, T2–T3 about F1, T4–T5 about F2, T6–T7 about F3, T8–T9 about F4 and T10–T11 about F5. The Enhanced Fujita (EF) scale was designed so that EF ratings match the old F ratings.
What is the strongest tornado ever recorded in Britain?
The London tornado of 1091, estimated at T8. In modern times the strongest have reached around T7, such as Birmingham in 1931.
Does Britain really get many tornadoes?
Yes. Britain records dozens of tornadoes in a typical year, one of the highest rates per unit area in the world, though almost all are weak (T0–T3).
Was there ever a T12 or an F6 tornado?
No. Both scales can be extended in theory, but no tornado has been rated above F5 or T11, and the highest TORRO rating ever assigned is T8.