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Tornado in the Canadian province of Manitoba , 2007.
Video of a tornado in southern Indiana (March 2012)

A tornado (from Spanish tornar , to German “to turn, turn, turn”, from Latin tornare , with the same word meaning), also large currents , tornados or waterspouts , is a small-scale air vortex in the earth's atmosphere with an approximately vertical axis of rotation . It is related to convective clouds ( cumulus and cumulonimbus ) and thus differs from small drums (dust devils). The vortex extends continuously from the ground to the cloud base, but does not have to be condensed throughout. This definition goes back to Alfred Wegener (1917) and is still generally accepted today.

The terms wind and waterspout ( Engl. : Waterspout ) designate in the German language a tornado over land or over larger areas of water (sea, large inland lakes).

The term `` wind trousers '' - still well-defined in the older literature (Wegener) - has been used in the recent past increasingly undifferentiated for various phenomena in connection with suddenly occurring strong winds ( e.g. downburst ) or incorrectly referred to small bombs. It also gave the impression of a difference between large tornadoes in North America and small tornadoes in Europe. However, there is no difference between tornadoes and tornadoes in terms of their physical nature or their strength.


A young tornado. The visible part of the trunk has not yet reached the ground, but the (poorly recognizable) dust cloud on the ground indicates that the air vortex is already reaching down.

The formation of tornadoes is very complex and is still a topical research topic today. Despite unanswered questions regarding details, the prerequisites and the basic mechanisms of tornadogenesis are quite well known. Under the right conditions, tornadoes can form anywhere throughout the year. Nevertheless, there are both spatial and seasonal and time of day priorities, which are described in more detail below under climatology .


For a tornado to develop, the conditions for deep moisture convection must first be given. These are conditional instability , i.e. a sufficiently strong vertical decrease in temperature, sufficient moisture supply ( latent heat ) in the lower 1–2 km of the atmosphere and an increase in the air mass in order to trigger moisture convection. Elevation mechanisms can be thermal (solar radiation) or dynamic (fronts) in nature. The main source of energy for such storms and thunderstorms in general is the latent heat stored in the water vapor of the moist air mass, which is released during condensation. Only this additional amount of heat enables the air to rise freely (moisture convection), since the atmosphere is stable to dry convection , apart from overheating close to the ground. In the latter case, only small currents can form. At the gust front of a shower or thunderstorm , small currents, the so-called gust front vortices or gustnados , can arise. These can develop into tornadoes if they come into contact with the moist convective updraft and are thus intensified.

Tornado types

With regard to the method of formation, two classes of tornadoes can be distinguished:

Mesocyclonic tornadoes

Schematic representation of a super cell with a tornado

In mesocyclonic tornadoes, in addition to the basic ingredients for shower or thunderclouds described above, there is a strong vertical wind shear , i.e. an increase in wind speed and a change in wind direction with altitude . This wind profile enables the formation of thunderstorm cells with a rotating updraft ( mesocyclones ), so-called super cells , which are characterized by a durability of up to several hours and violent side effects such as large hail , torrential rain and thunderstorm gusts of over 200 km / h. Tornadoes form in 10–20% of all supercells. In many cases, a reduction of the rotating cloud base, a so-called is before the tornadogenesis Wallcloud (German: wall cloud ) observed. Due to the upward movement in the center, air flows in the lower area towards the axis of rotation, which, due to the pirouette effect, leads to an enormous increase in wind speed towards the axis. Soil friction seems to play an essential role here ; however, the details of the intensification of the rotation up to ground contact are not yet fully understood. The direction of rotation of mesozyklonalen tornadoes is on the northern hemisphere mainly cyclonic , i.e. counter clockwise . But this is not a direct effect of the Coriolis force , because tornadoes are too small for that. Rather, this, together with the ground friction, which is strongly orographically influenced, determines the large-scale wind profile of low pressure areas in whose area tornadoes can occur. In most cases, in the northern hemisphere, the wind turns with height to the right, with the air flowing into the mesocyclones from the south, which leads to cyclonic rotation in an anti-clockwise direction. In the southern hemisphere , there is also a corresponding cyclonic rotation, but clockwise there. Occasionally, in addition to the cyclonally rotating tornado, another, anticyclonally rotating tornado forms. In such cases, another area of ​​updraft arises in which convective clouds form. This updraft area is carried away by the rear right main updraft, creating a so-called anti- mesocyclone from which the anticyclonally rotating tornado can develop. Such an anti-mesocyclone is usually much weaker than the main mesocyclone and moves with it.

Non-mesocyclonic tornadoes

This formation mechanism does not require any mesocyclones. Rather, existing ground-level horizontal wind shear disintegrates , z. B. along a line of convergence into individual eddies with a vertical axis, which are stretched and thus intensified by an overlying moist convective updraft of a shower or thundercloud (see figure and literature on the right). This happens in an otherwise rather weak wind environment with a simultaneous strong vertical temperature decrease in the lower layers. In contrast to mesocyclones, the rotation does not extend far beyond the cloud base. The connection to lines with horizontal wind shear (convergence), which often also represents the uplift drive for moisture convection, often creates families of large turbines arranged along the line . Most waterspouts also belong to this rather weaker non-mesocyclonic tornado type, but tornadoes can also develop over land in this way - called land spout in English . The sense of rotation of non-mesocyclonic tornadoes shows a less strong preference for cyclonic rotation.

Cold air currents, on the other hand, occur in connection with convective clouds, which develop within a cold air reservoir in environments with relatively little wind shear at height. They very rarely hit the bottom, but sometimes a touchdown occurs and they turn into weak, short-lived tornadoes.


Size and appearance

Tornado on the sea (waterspout)
Multivortex tornado over Dallas, Texas, 1957

In the early stages, a tornado is almost invisible at first. Only when water vapor condenses or dust, debris, water and the like are whirled up inside the vortex due to the pressure drop and the associated adiabatic cooling does the tornado appear visually. Continuous condensation from the cloud to the ground cannot be observed in every case. Such originating from the mother cloud condensation is as a funnel cloud (English: funnel cloud ), respectively. If the air vortex does not reach the ground, one speaks of a blind vortex. For a tornado, the ground contact of the air vortex is decisive, not its continuous visibility. If, for example, wind effects can be detected under a funnel cloud, i.e. usually damage to the ground, then it is a tornado. The shape of the air vortex is very diverse and ranges from thin tube-like shapes to a more or less wide funnel that widens upwards (see adjacent images and web links ). The diameter can be a few meters up to 500 m and even up to 1 km. It is not uncommon - especially with large diameters - to have several eddies orbiting around a common center, which is known as a multivortex tornado . Dust, debris and condensed water can sometimes prevent a multivortex tornado from being recognized as such, because the individual eddies are not visible.


Destruction of an F3 tornado

The classification is based on the Fujita scale , which is defined by the wind speed. In practice, however, in the absence of direct measurements, this scale is estimated based on the damage caused by the tornado. These range from light storm damage to the complete destruction of massive buildings. So far, tornado strengths F0 to F5 have been observed in reality; For energetic reasons, physical estimates result in the intensity F6 as the upper limit. In Europe there is also z. For example, TorDACH uses the TORRO scale that is twice as fine as the Fujita scale ; in the USA the Fujita scale has been further developed into the so-called Enhanced Fujita Scale , or EF scale for short, which has levels EF0 to EF5 and the Tornadoes classified based on 28 damage indicators.

Effects and damage

The force of a tornado can cause a variety of damage. It can destroy houses and cars and is a danger to animals and people. Stone houses are not safe either. A lot of damage occurs indirectly from flying debris. The main cause of the damage is the dynamic pressure of the wind and, above approximately 300 km / h, there is also increasing indirect damage from flying debris. The earlier assumption that the strong negative pressure inside a tornado, which can be up to 100 hPa, would cause buildings to explode, is no longer tenable. Because of their high wind speeds, which change in a confined space, tornadoes are in principle a danger to air traffic; However, due to the small size of this weather phenomenon, accidents are rare. A spectacular fall occurred on October 6, 1981, when NLM Cityhopper Flight 431 got caught in a tornado and crashed after the right wing was torn off. All 17 people on board died.


Duration and speeds

The duration of a tornado is between a few seconds and more than an hour, on average it is less than ten minutes. The forward movement of a tornado follows the associated mother cloud and is on average at 50 km / h, but can also be significantly lower (practically stationary, not infrequently with waterspouts) or higher (up to over 100 km / h with strong currents). The tornado track is essentially linear with minor deviations caused by the orography and the local wind field in the vicinity of the thunderstorm cell.

However, the internal speed of rotation of the wind is usually much higher than that of the linear movement and is responsible for the severe devastation a tornado can leave. The highest wind speed ever recorded within a tornado was determined with a Doppler radar during the Oklahoma Tornado Outbreak on May 3, 1999 near Bridge Creek , Oklahoma ( USA ) . At 496 ± 33 km / h it was in the upper range of class F5 on the Fujita scale; the upper error limit even extends into the F6 range. This is the highest wind speed ever measured on the earth's surface. Above the earth's surface, only jet streams reached higher wind speeds. In the official statistics, however, this tornado falls below F5, taking into account the most likely value and the uncertainties.

In the US, around 88% of observed tornadoes are weak (F0, F1), 11% strong (F2, F3), and less than 1% are devastating (F4, F5). This distribution function is very similar worldwide and in this form is dominated by mesocyclonic tornadoes, which fill the full intensity spectrum. In contrast, the intensity of non-mesocyclonic tornadoes hardly exceeds F2.

Annual and time of day occurrence

Overland tornadoes occur most frequently in early summer, with the maximum occurring later as the latitude increases . The maximum is reached over water in late summer, because then the water temperature and consequently the instability is highest. The same applies to the daily course. Land tornadoes are most likely to occur in the early evening hours, while waterspouts peak in the morning hours. Furthermore, waterspouts show a climatological difference over the year, depending on whether they go ashore or remain above the water. The seasonal distribution for the first case is similar to that for tornadoes over land, while pure waterspouts show the said late summer maximum.

Distribution and frequency

Tornadoes are observed worldwide wherever there are thunderstorms . The focus is on regions with fertile plains in the subtropics to the temperate latitudes . In the first place is the frequency after the Midwest of the USA , where the climatic conditions for the formation of severe thunderstorms and supercells due to the wide plains ( Great Plains ) east of a high mountain range ( Rocky Mountains ) and north of a tropical sea ( Gulf of Mexico ) are very cheap. For weather conditions with high storm potential, the mountains require relatively dry and cool air masses in the middle to upper area of ​​the troposphere with south-westerly to westerly winds, while in the deeper layers, warm, humid air masses from the Gulf region can be transported to the north without hindrance. As a result, an unstable stratification of the atmosphere with a large supply of latent heat comes together with a directional shear of the wind.

Other important regions are Argentina , Central and Southern and Eastern Europe , South Africa , Bengal , Japan and Australia . Numerous, though on average weaker, mostly non-mesocyclonic tornadoes occur in the Front Range (eastern edge of the Rocky Mountains ), in Florida and over the British Isles .

About 1200 tornadoes are registered annually in the USA, most of them occur in Texas , Oklahoma , Kansas and Nebraska along Tornado Alley with about 500 to 600 cases per year. This is given by the special climatic conditions mentioned above, which offer the prerequisites for the development of mesocyclonal tornadoes in particular far more frequently than in other regions. In addition, there are several regional clusters in the U.S. B. New England and Central Florida.

In Europe, the annual number of tornado observations is 330, of which 160 are above water, including the number of unreported cases, an estimated 590 tornadoes, of which an estimated 290 water pants. As in the US, most European tornadoes are weak. Devastating tornadoes are rare, but eight F4 and two F5 events from Germany have been documented so far. The latter were already described by Alfred Wegener in 1917 in a work on the tornado climatology of Europe. Other devastating cases are known from northern France, the Benelux countries, Austria, northern Italy and Switzerland (an F4 and an F5 event documented here).

Waterspout in front of Usedom

In Germany, the number of tornadoes observed annually is several dozen with a fairly high number of unreported cases, especially weaker events. Exact figures are not available because there are not yet sufficient statistics on this. According to the figures currently available, around five or more F2s, one F3 every two to three and one F4 every 20 to 30 years must be expected annually. According to current knowledge, an F5 is an event of the century or even rarer.

An overview of the spatial and temporal distribution of tornadoes in Germany and their intensity can be found in the web links . In general, it can be stated that the tornado risk is highest in the west of the North German Plain .

In Austria, an average of three tornadoes have been observed annually for the past 30 years. However, since 2002 the increased spotter and statistical activity v. a. volunteers to observe an average number of around five tornadoes / year. Including a possibly quite high number of unreported cases and the still very underrepresented F0 cases, the actual, averaged, annual number could be up to ten tornadoes.

Several F0 and F1 cases occur each year. On average, an F2 can also be expected annually, or once every two years, every five to ten years with an F3. So far an F4 tornado has also been documented in Austria.

The highest tornado density can be observed in southeast Styria (around three tornadoes / 10,000 km² / year), followed by the area around the Hausruck in Upper Austria, the Vienna basin , the region around Linz , the western Weinviertel , the Klagenfurt basin , Lake Constance - Region as well as the Inn valley in the area of Innsbruck .

Tornado near Cala Ratjada ( Mallorca )

In general, the incidence is subject of tornadoes strong fluctuations, (in clusters outbreak called English: Outbreak ) within very short periods of time - often in a single day - expresses followed by fairly long sections of relative calm. The outbreaks are justified by the close connection with certain weather conditions , where several factors for the formation of the tornado come together (see above under formation ). Larger events of this kind with devastating tornadoes are mainly known from the USA (see following section). For Western and Central Europe, the years 1925, 1927 and 1967 should be mentioned with the focus on Northern France / Benelux / Northwest Germany. This region can also be seen as the European tornado alley . The numerically most significant outbreak in Europe with a total of 105 but mostly weaker tornadoes (max. F2) hit the British Isles on November 23, 1981.

At present, the database for Central Europe does not allow a statement to be made as to whether tornadoes occur more frequently due to global warming , as the increase in the number of cases observed is primarily due to better recording in recent years. Thanks to systematic tornado research and due to the high number of cases, reliable statistics have existed in the USA since the 1950s. However, this shows neither a tendency towards increased occurrence nor to greater violence of tornadoes, as shown in the IPCC report from 2001.

Major tornado events

Tornado research

Historical context

Illustration of waterspouts in The Philosophy of Storms by James Pollard Espy , 1841.

American meteorologists like James Pollard Espy were already researching waterspouts and storms in the 19th century .

Radar technology made great strides in the 1930s and 1940s. It was discovered that z. B. Precipitation affected radar readings. The weather radar was created. Although tornadoes have long been a natural phenomenon in the USA , tornado research there is still quite young. The first successful tornado forecast was made in 1948 at Tinker Air Force Base . Only since the 1950s has the US been systematically devoted to recording and forecasting.

Interestingly, tornado research in Europe is older than in the US. Alfred Wegener already did pioneering work here in the first half of the 20th century. In the 1930s, the now almost forgotten meteorologist Johannes Peter Letzmann undertook systematic tornado research in Germany, which was severely restricted by the events of the Second World War and was not continued afterwards. On the contrary, interest in tornadoes subsequently sank practically to insignificance and was limited to a few spectacular cases such as the tornado over Pforzheim in 1968. It was not until the TorDACH network was founded in 1997 that tornado research took off again in German-speaking countries. In 2003, Skywarn was founded in Germany, Austria and Switzerland as an association of voluntary spotters to improve short-term severe weather warnings in German-speaking countries. At the European level there is a pilot project to set up a European Severe Storms Laboratory, ESSL (see web link).

Two technical advances greatly advanced tornado research:

  • The Doppler weather radar made it possible to determine the radial velocity of the precipitation in addition to the spatial distribution of the precipitation intensity by measuring the Doppler effect . Small-scale changes in the radial speed can be signs of strong air circulation (→ tornado, tornado)
  • The polarimetric weather radar is a Doppler weather radar that can send and receive pulses with different polarizations . By sending multiple polarizations of electromagnetic waves , information can be obtained about the shape and type of precipitation.

Many warring parties used weather planes to gain knowledge of weather phenomena at high altitudes. In 1931 the first pressurized cabin was used.


Typical radar echo of a tornadic super cell, here using the example of the strongest tornado (F5) from the Oklahoma tornado outbreak of 1999

Research encompasses aspects of psychology, meteorology and disaster research. The goal in meteorology is to improve the advance warning time. The time between the warning and the occurrence of the event is known as the lead time. It currently averages 13 minutes. An exact / exact prediction of a tornado, its strength and its path is not possible with the current means. To do this, the researchers would need better knowledge of the factors wind speed, temperature and air pressure. Because of the short-term occurrence of tornadoes, science focuses on early detection, with Doppler radar being an essential tool. This enables suspicious rotation in thunderclouds to be detected at an early stage. A clear indication are hook-shaped echoes on the radar image. Today's tornado research focuses on climatology and the creation of case studies on the mechanisms of tornadogenesis (see above). For this purpose, complex numerical simulation calculations are carried out in order to gain a better understanding of how tornadoes develop. The method is to work out similarities and differences by comparing tornado phenomena in order to better represent the development and thus the conditions of tornadoes. In this way, beneficial factors can be identified.

In addition, there is a dense network of voluntary observers, so-called spotters , who bring current warning messages about sighted tornadoes and other weather hazards, such as thunderstorms , hail and flash floods , into the short-term warning system. The spotters are organized in the Skywarn network . In addition, there is a growing number of storm chasers (private storm chasers ), who pursue thunderstorms and tornadoes primarily out of a fascination with the forces of nature, but also provide valuable information for storm and tornado research. Eyewitnesses are indispensable for good research, since even the best radar devices are prone to errors and verified feedback can only be provided by on-site observers. The headquarters of severe weather research in the USA is the National Severe Storms Laboratory (NSSL) founded in 1964 and based in Norman , Oklahoma. Thanks to the warning system, the number of tornado victims in the USA has been significantly reduced.

Howard Bluestein is an important researcher. He further developed the Doppler radar so that a mobile unit that can be installed on a truck is able to scan the sky every 2 seconds. His thesis is that the raindrops have an influence on the development and size of a tornado. In addition, his research showed that there is a rain-free zone below the cloud line within the air. This could provide another way of better predicting tornadoes.

The German Meteorological Service is also planning to set up a tornado early warning center, mainly because of the tornado reports that have accumulated at the beginning of the 21st century, which are primarily due to increased awareness among the population.

Psychology deals with the phenomenon of tornado warning. One question is how predictions have to be designed in order to make people aware of the event that is dangerous for them.

In disaster research, the aim is to use the damage caused to find out how the building fabric can be improved inexpensively in order to reduce the damage to natural phenomena.

Protection of the population

The population is protected in a variety of ways. There is a network of 159 ground-level radar systems in the USA. If a tornado is detected, a report is made on national TV and local radio stations. The messages prompt the user to visit the basement or shelters. In the meantime these have been further developed and can be structurally reinforced.


  • Gottlob Burchard Genzmer (1765): Description of the hurricane, which on June 29, 1764 devastated a line of several miles in the Stargardian district of the Duchy of Mecklenburg. Friedrich Nicolai, Berlin and Stettin 1765. Copy (PDF; 2.0 MB)
  • Alfred Wegener (1917): Wind and water pants in Europe. Vieweg, Braunschweig, On digital copies by Nadine Reinhard at (9 PDFs)
  • Johannes Peter Letzmann (1937): Guidelines for researching trombi, tornadoes, waterspouts and small trumpets. International Meteorological Organization, Climatological Commission, Publ. 38, Salzburg, pp. 91–110. Transcript (PDF)
  • Thomas P. Grazulis (1993): Significant Tornadoes: 1860-1991. Environmental Films, ISBN 1-879362-00-7
  • Nikolai Dotzek (2003): An updated estimate of tornado occurrence in Europe. Atmos. Res. 67–68, 153–161 articles (PDF; 41 kB)
  • James M. Caruso and Jonathan M. Davies (2005) Tornadoes in Non-mesocyclone Environments with Pre-existing Vertical Vorticity along Convergence Boundaries. NWA Electronic Journal of Operational Meteorology June 1, 2005 article


Individual evidence

  1. ^ Friedrich Kluge, Elmar Seebold: Etymological dictionary of the German language , 24th volume. Walter de Gruyter, Berlin / New York 2001, ISBN 978-3-11-017473-1 , page 921.
  2. Amstler, Katharina: Diploma thesis climatological-statistical elaboration of tornado events in Europe , p. 33
  3. ^ Family von Wasserposen over the Adriatic Sea ( Memento of September 27, 2007 in the Internet Archive )
  4. ^ Charles A. Doswell III: Doswell: What is a tornado? 9. Cold air currents., August 14, 2012, accessed on June 2, 2015 .
  5. Natascia Lypny: 'Cold core funnels' give Ottawa commuters a twister fright. , July 30, 2013, archived from the original on August 28, 2013 ; accessed on June 2, 2015 .
  6. Lee Sandlin, Storm Kings: The Untold History of America's First Tornado Chasers . Pantheon Books, New York 2013, ISBN 978-0-307-37852-1 , pp. 46-62 .

Web links

Commons : Tornado  - collection of images, videos and audio files
Commons : Waterspout  - Collection of images, videos and audio files
Wiktionary: Tornado  - explanations of meanings, word origins, synonyms, translations

German-speaking area:

Europe as a whole:

USA / North America: