Tornado Facts

The questions:

  1. What is a tornado?
  2. What is a whirlwind?
  3. What is a whirlpool?
  4. What are the main characteristics of a tornado?
  5. What is the Tornado Intensity Scale?
  6. How often do tornadoes occur in the United Kingdom?
  7. What about tornadoes that are not reported?
  8. How does this compare with other nations?
  9. Where do tornadoes occur in the United Kingdom?

1 What is a tornado?
A tornado is a violent rotating column of air that reaches to the ground from a storm cloud in the shape of a condensation funnel created and maintained by strong inflowing winds. The spinning winds can attain extremely high speeds which provide great risk to property and life at the ground and in the air. When the humidity is high enough, the tornado funnel is made visible by the circulation of condensed water vapor in its outer sheath, but although the flow of air is inward and upward, the cloud within the low-pressure funnel actually extends downward from the cloud base.

The origin of tornadoes is often associated with well-developed thunderstorm cells on cold fronts, for example at the gust-front boundary where an advancing mass of cold air overruns and displaces pre-existing warmer humid air. Within a cell a strong persistent updraft of warm moist air is maintained as air enters the forward right flank at low altitude. As the air ascends it is forced to turn due to the variation of wind speed with height (known as vertical wind shear) and due to its proximity to a downdraft of drier cold air. By this means, the buoyant warm updraft acquires rotation in an anticlockwise sense as it undergoes local stretching in the vertical. The spinning, spiralling effect gradually extends along the length of the updraft, and the speed of rotation or ‘twisting’ increases as the effective column diameter diminishes.

Just as water exits from a bath more efficiently by spinning down the plug hole, so the humid rising air is carried more effectively through the cloud cell in the form of a spinning updraft. Another way of expressing it is by recognizing that the tornado’s high wind speeds are the result of convergence because angular momentum has to be conserved. An analogy is the way in which an ice-skater increases his or her rate of spin (thereby providing a convergence of mass) by drawing in the arms. The upward flow of air in the tornado being greater and faster than inflow at the base ensures a reduced pressure for the rotating updraft.

When the column is seen extending visibly beneath the cloud base as a funnel cloud, it is because water vapor is condensing into its outer sheath due to falling pressure and temperature. Eventually, given enough time and a high-enough rate of spin and stretching, the tornado’s funnel lengthens to the ground, and with it come the high-speed potentially-damaging winds. By contrast, winds in the limited area at the middle of a tornado funnel are light, if not approaching calm at the epicentre, as with the eye of a tropical storm or hurricane.

In the most intense storms known as supercells, winds in the lowest three kilometers of the atmosphere veer strongly with height and produce, relative to a horizontal axis, vorticity of the inflow towards the updraft before tilting into the vertical and becoming a fiercely-spinning column of air. At the same time in such meteorological conditions a mesocyclone can develop in which the entire cumulonimbus, often 10 kilometers across, rotates with a strong sustained updraft.

Most continents have regions where conditions for tornado formation are more likely to develop. In the USA, across a broad zone from Nebraska to Texas and Oklahoma, violent tornadoes develop more frequently than elsewhere. This region has been named ‘tornado alley’. In this zone cold dry air moving south and east from across the Rockies meets warm moist air from the Gulf of Mexico, especially in late spring and summer when the general conditions are optimum. On some remarkable occasions more than a hundred tornadoes have been recorded in a day, not only in the USA but also in the British Isles. In the US on 3-4 April 1974 within sixteen hours in thirteen states from Georgia to the Canadian border there were 148 tornadoes, of which 48 caused 315 deaths. In Britain on 23 November 1981 as many as 105 tornadoes broke out in five-and-a-half hours as a cold front crossed a comparatively small part of England from north-west to south-east. The country with the highest number of reported tornadoes per unit area is England whose area corresponds roughly with Oklahoma, but many of the latter’s tornadoes are more violent. Holland is another small country reporting a very high frequency of tornadoes per unit area. The part of the USA with the highest number of tornadoes per unit area is central Oklahoma.

Tornadoes of North America and Europe have been studied the most, especially since 1970. In the USA and Europe two tornado wind-speed scales have come into use. There is the international decimal T-Scale which ranges from 0 to 9 and is a simple development of the 200-year old Beaufort wind speed scale, and there is the American F-Scale with its shorter span from 0 to 5. In the severest tornadoes wind speeds have been known to approach 130 meters per second or 300 miles per hour. The strongest known tornadoes for the USA and for Europe are T10 or F5 on these scales. Tornado tracks commonly range from a few dozen to a few hundred meters in diameter but can be up to 5 or more kilometers wide. Lengths of tornado tracks exceeding a hundred kilometers are known, especially for the USA.

Some tornadoes form out to sea as strong waterspouts (q.v.) which sometimes cross the coast, so a waterspout may become a tornado as the twisting funnel moves from land to sea (and vice-versa). A recent powerful and well-documented example is that of Selsey on the south coast of England on the night of 7 to 8 January 1998. When the waterspout made landfall, it carved a trail of damage a kilometer wide through the town as it damaged hundreds of buildings in less than ten minutes.

SELECT REFERENCES

Abbey, R. F. (1976). Risk probabilities associated with tornado wind speeds (page 187 et seq.) in Proc. Symposium on tornadoes, Lubbock, Texas, June 1976.

Dotzek, N., Berg, G., Rauch, E. and Peterson, R.E. (2000). Adaptation of T- and F-Scales for Central Europe with stronger buildings than in US. Meteorol. Zeitschrift. 9, 165-174.

Elsom, D. M. and Meaden, G. T. (1982). Bull. Amer. Meteorol. Soc. 12th Conf. Severe Local Storms. San Antonio, Texas. American Meteorological Society

Elsom, D.M., Meaden,G.T., Reynolds, D.J., Rowe, M.W. and Webb, J.D.C. (2001): Advances in tornado and storm research in the UK and Europe. Atmos. Research 56, 19-29.

Fujita, T.T. (1971). Proposed characterization of tornadoes and hurricanes by area and intensity. SMRP Internal Research Paper, no. 91.

Fujita (1973). Tornadoes around the world. Weatherwise. 26, 56-62.

Fujita, T.T. and Pearson, A.D. (1973). Results of FPP classification of 1971 and 1972 tornadoes. Preprints 8th Conference on Severe Local Storms, Denver. October 1973. pp. 142-145

Meaden, G.T. (1975-76). Tornadoes in Britain: their intensities and distribution in time and space. J. Meteorology, 1, 242-251.

Meaden, GT (1985). A study of tornadoes in Britain, with assessments of the general tornado risk potential and the specific risk potential at particular regional sites. Prepared for H.M. Nuclear Installations Inspectorate Health and Safety Executive.

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2 What is a whirlwind?
Whirlwinds can be divided into two main classes, the greater and the lesser whirlwinds. The former are represented by tornadoes, waterspouts and funnel clouds. The latter are the momentum-driven eddy whirlwinds and the thermal-column wind-devils and fire-devils.

A whirlwind is a particular kind of vortex spinning in a fluid. When a vortex spins in water, the term is ‘whirlpool’ (q.v.). When the fluid is air, the vortex is a whirlwind which can be of various types, represented by two classes. Most violent are the greater whirlwinds -- tornadoes (q.v.) and waterspouts (q.v.). More common are the lesser whirlwinds which include wind devils, fire devils and eddy whirlwinds.

The greater, or major, whirlwinds are created by parent clouds which for the majority of tornadoes and many of the waterspouts are cumulonimbus severe-weather clouds. They are characterized by a columnar funnel of spinning air made visible by condensation of cloud-water droplets caused by reduced air pressure in the spinning core. The rotation is seen to descend from the cloud-base as the funnel develops, and it persists until retraction into the cloud-base is complete. Moreover, a tornado may become a strong waterspout as the powerful rotation moves from land to sea (and vice-versa). When such vortices do not descend far enough to reach a land or water surface, they are simply called funnel clouds.

The majority of lesser whirlwinds are typified by a more or less vertical, whirling column of rising air that is warmer than its surroundings. More frequently formed on warm, sunny days in clear dry air, these are the wind devils -- also known as land devils and water devils. Depending upon the nature of entrained matter that makes them visible, they are called dust devils, sand devils, snow devils, fire devils, etc. Great heights can be obtained in long-lived cases, as a result of which an energetic, tall, devil-whirlwind can be topped by a small cumulus cloud. Whirlwinds of this class are driven by the motion of warmer air as it displaces cooler air and becomes a small-scale, rapidly rotating column of wind at the lower end of a 'thermal'. The degree of warmth of the thermal column is relative to the temperature of its cooler surroundings. Although wind-devils are more likely to form on clear, dry hot afternoons especially in summer, they can form even in winter given appropriate temperature gradients over, for instance, fields of snow or ice.

Another category of lesser whirlwind is the ordinary eddy whirlwind. This is a helical rotation originating in the general wind flow of the day downwind of an obstruction like a hill or high stand of trees. This is similar to how a whirlpool forms as an eddy in water downstream of an obstruction. Because eddy whirlwinds are driven by momentum, they are more likely to form in breezy conditions. Such whirlwinds are also sometimes visible in humid air over lakes downwind of hills or mountains. A different kind of eddy whirlwind – one which is part of the greater whirlwind class -- forms at the interface of two air masses that are moving at different speeds.

A whirlwind may also develop over a fire or region of hot ashes if there is a suitable convergence and rotation of the air. It is then a fire devil, of the lesser whirlwind kind because it is maintained by a rising column of hot air. Fire devils can occur under many weather conditions, but as with other minor whirlwinds, they form more readily in calm or light winds because this allows heat to build up more rapidly. The cause of fire devils is usually forest fires, wild fires and post-harvest stubble burning, but also bonfires, oil fires, volcanic eruptions and nuclear explosions.

References:

Meaden, G. T. (1985). The classification of whirlwind types and a discussion of their physical origin. J. Meteorology, 10, 194-202.

Reynolds, D. J. and Meaden, G. T. (1999). Major whirlwind nomenclature. Weather, 54, 223-225.

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3 What is a whirlpool?
A whirlpool, whether in water or other liquid, is a rotary current or vortex produced by the interaction of opposing currents in the fluid or surface winds over it. In rivers they form as eddies downstream of obstacles like jetties and bridge piers, the more readily when conditions are turbulent. Whirlpools are less common in seas and oceans but can develop from the interaction of opposing tidal flows especially where coastal and bottom configurations offer narrow, deep passages of considerable depth. Whirlpools are also created in the turbulent flow at the base of cascades or waterfalls as at Niagara.

In the vortex of the whirlpool, the dynamic flow is a spiralling motion downwards. A return of water to the surface takes place in the manner of a quasi-stationary rising current known as a kolk or boil. This is visible at the surface as a heaving disturbance of the fluid.

The severest sea-whirlpools are those of Corryvreckan off Western Scotland, Moskstraumen in the Lofoten Islands, Norway, and the Old Sow between Maine and New Brunswick. Other major whirlpools include Naruto between Tokushima and Awaji Island in Hyogo (Japan), Saltstraumen (Norway), and Garofalo (Italy). When their power is intense, the term maelstrom is often used.

The often-cited sea-whirlpool Charybdis is a fiction, being no more than rough choppy water arising from wind blowing obliquely across fast-moving tidal water flowing steadily in one direction, or in the opposite direction when the tide is ebbing.

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4 What are the main characteristics of a tornado, and how does it differ from a hurricane?
A tornado will typically last for a few minutes, track across the land for 2 to 5 kilometres (roughly 1 to 3 miles) and will have a diameter of 20 to 100 metres (roughly 20 to 100 yards). Windspeeds are in the order of 72 to 113 miles per hour - that is, T2 or T3 on the Tornado Intensity Scale. At the more extreme end, some tornadoes track for over 100 kilometres, are over 1 kilometre wide and have winds in access of 300 miles per hour (T10) - such tornadoes are extremely rare, anywhere in the world.

In contrast, a hurricane is an intense area of low pressure that only forms in the tropics where the sea surface temperature is at least 27 degrees Celsius. A hurricane has a diameter of around 100 miles, has mean windspeeds which must average at least 73 mph (by definition), and can last for several days - but will rapidly decay on making landfall.

For more information on definitions, see the Severe Storm Definitions & Whirlwind Classification Scheme.

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5 What is the Tornado Intensity Scale?
The Tornado Intensity Scale was devised by Dr. Terence Meaden of Bradford-on-Avon, Wiltshire, Great Britain, in 1972. This ranges from T0 to T10, with each point representing a range of windspeed, just like the Beaufort Scale (in fact, the Tornado Intenstiy Scale is based directly on the Beaufort Scale). T0 to T3 are weak tornadoes, T4 to T7 are strong tornadoes and T8 to T10 are violent tornadoes.

For more information on the Tornado Intensity Scale, see the TORRO Tornado Intensity Scale.

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6 How often do tornadoes occur in the United Kingdom?
On average, 33 tornadoes are reported each year in the United Kingdom. This average is based on a 30 year period, though in reality the actual yearly figures may vary dramatically from year to year.

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7 What about tornadoes that are not reported?
Many tornadoes which are reported because they have been associated with structural damage to a lesser or greater degree. It is likely that many tornadoes are being missed altogether because they travel over fields. No doubt some are seen and reported, but unfortunately the report never reaches TORRO.

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8 How does this compare with other nations?
In order to accurately compare tornado frequencies between different countries, land area must be taken into account - after all, a large nation will report many tornadoes simply because of its large land area. When land areas are taken into account, the United Kingdom actually has the highest frequency of reported tornadoes per unit area in the world. This was first recognised by an American meteorologist, Dr. T. Fujita, in 1973.

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9 Where do tornadoes occur in the United Kingdom?
Most tornado reports are from the Western Midlands, Eastern Midlands, Central-Southern England, South-Eastern England and East Anglia. Some occur in South-Western England, North-Western England, North-Eastern England and Wales. Tornadoes are rare in Northern Ireland and Scotland.

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