Rotating thunderstorm cells

Peter Wiesen on the American tri-state tornado and German pioneering research

The Tri-State Tornado is considered the most dangerous tornado to hit the United States on 18 March 1925. What is the reason for this?

Wiesen: "Tornadoes" are actually a meteorological phenomenon. I am a physical chemist specialising in air chemistry. Nevertheless, I naturally follow the effects of climate change. Simply put, the rising average temperature means there is more energy in the atmosphere, which increases the likelihood of extreme weather events. This brings us directly to tornadoes, which can be aptly described as an extreme weather event.

The aforementioned tri-state tornado, which occurred almost exactly 100 years ago in March 1925 in the USA, was not the strongest in history, but it was the one that travelled an unusually large distance across three states. It left a trail of devastation in its wake. With 695 deaths, it was the largest number of victims ever to be claimed by a tornado in the USA.

It swept through the country non-stop for around 352 kilometres. That's an extremely long distance, isn't it?

Meadows: This swath of devastation really was extremely long and unusual. However, this tornado was travelling very fast at 95 km/h and lasted a good 3.5 hours. The speed at which tornadoes usually move on the ground is around 50 km/h, the same speed at which the parent cloud, usually a so-called supercell, moves. The rotation speed in the tornado is of course significantly higher. The highest wind speed ever measured within a tornado was 496 ± 3 km/h. At such a speed, even massive stone houses can no longer withstand it.

Tornadoes often dissipate again after a few minutes. This makes forecasting very difficult and the tornado hunters who observe such extreme weather events - partly as a hobby, partly for scientific purposes - find it extremely difficult to track a tornado.

In a single city, 234 people lost their lives, the highest number in a single city in US history. How does a tornado form in the first place?

Wiesen: The formation of tornadoes has still not been conclusively clarified and is still being researched. There are new measurement methods that have significantly advanced tornado research. The basic mechanisms that can lead to the formation of a tornado are fairly well understood. Here we would have to delve deep into the physics of the atmosphere. Just this much: it requires an unstable air mass with a large vertical temperature drop and high humidity. In the past, this was referred to as latent heat. The conditions for the occurrence of tornadoes are found in so-called supercells, i.e. particularly strong thunderstorm cells that have their own rotation.

Are there actually spatial or temporal conditions under which a tornado develops?

Wiesen: In general, tornadoes over land occur most frequently in early summer and then most likely in the early evening hours.

How do you explain these air vortices that appear to be travelling in a spinning motion?

Wiesen: This is really meteorology or physics and the air vortices don't just appear to be rotating. A so-called wind shear must be present for a rotating vortex to form. This means that the wind speed increases with the height above the ground and the wind direction changes with the height (vertical wind shear).

There are mesocyclonic and non-mesocyclonic tornadoes. What is the difference?

Wiesen: The strong vertical wind shear mentioned above only occurs in so-called mesocyclonic tornadoes. This can be observed in supercells, which are sometimes also found in Germany. One such supercell called Ela hit NRW on Whit Monday 2014 and caused extensive damage. Supercells, also known as mesocyclones, are thunderstorm cells with a rotating updraft.

Strong tornadoes are only observed in such supercells, but not in all of them. It is assumed that around 10-20% of supercells lead to the formation of tornadoes. As far as I know, no tornadoes were observed during Ela.

The tornadoes then rotate anti-clockwise in the northern hemisphere and clockwise in the southern hemisphere.

In the case of non-mesocyclonic tornadoes, a tornado forms without a mesocyclone occurring. A wind shear is also required here. However, such tornadoes are usually short-lived and of low intensity.

What diameter can such a tornado have?

Wiesen: The diameter of a tornado can vary greatly. Diameters of a few metres up to 500 m or a kilometre have already been observed. Sometimes several vortices form, which then circle around a common centre. This is known as a multi-vortex tornado.

Tornadoes are categorised according to the Fujita scale. What is that?

Wiesen: The classification of a tornado goes back to the storm researcher Tetsuya Fujita, who introduced this scale, which ranges from F0 to F6 or F12, in 1971. This scale can be used to classify the damage caused by tornadoes. According to this scale, the tri-state tornado would have been classified as F5. An F6 tornado has not yet been observed. The scale is widely used in the USA. The "enhanced Fujita scale" (EF) has been used in the USA since 2007, but is not recognised outside the USA.

Interestingly, tornado research in Europe is older than that in the USA. And in Germany, the researcher Johannes Peter Letzmann (1885 - 1971) is even considered a pioneer. Why was his work not continued?

Wiesen: That is correct. Even Alfred Wegener, who is better known to many as a polar researcher and to whom plate tectonics, i.e. the shifting of the continental plates, can be traced back, was already working on wind or waterspouts and so-called large trombones in 1917. Letzmann had the misfortune that the Second World War severely restricted his research and it came to a standstill after the Second World War.

In the USA, the development of weather radar gave a major boost to tornado research. In addition, tornadoes occur much more frequently in the USA along the so-called "Tornado Alley" in the states of Texas, Oklahoma, Kansas and Nebraska than in Europe

Have there also been tornadoes in Germany?

Wiesen: Of course there have been. Thankfully, however, they occur much less frequently than in the USA and are usually relatively weak and therefore have a comparatively low destructive potential. According to current knowledge, a tornado with a strength of F5 like the Tri-State tornado would be an event of the century or even rarer.

Will such phenomena increase as a result of climate change?

Wiesen: It's impossible to say for sure. As mentioned at the beginning, the energy content of the atmosphere increases as temperatures rise, and with it the probability of extreme weather events. However, the increase in cases observed in recent years is probably due to better recording.

In the USA, intensive research has resulted in a good time series of tornado events, from which neither an increase in events nor a greater intensity can be deduced.

Uwe Blass

Prof Dr Peter Wiesen is an atmospheric chemist in the Department of Physical and Theoretical Chemistry in the School of Mathematics and Natural Sciences at the University of Wuppertal. Under his leadership, the Institute for Atmospheric and Environmental Research, which was founded in 2021, is participating in the European Commission's ATMO-ACCESS project and in the European research infrastructure ACTRIS, whose German contribution is coordinated by the Leibniz Association's TROPOS Institute in Leipzig.