Discussion of particular topics
This page gives a discussion of the customary German terminology for atmospheric vortex phenomena and their distinction from straight-line thunderstorm winds. Unfortunately, in this context there are frequent confusions, mostly being caused by the word Windhose, which is only imprecisely used nowadays.
Experience of the last decades shows that the media, public, and sometimes also meteorologists named nearly all small-scale, damaging wind phenomena "Windhosen", even if they had been downbursts in reality or just dust devils (Kleintromben) occurring with nice and sunny weather.
However, the US-coined word "tornado" is unambiguous and should therefore also be applied in German instead of "Windhose". For a most exact representation of the climatology of thundertstorm phenomena described here, existence and application of a unique terminology is very important. Especially, a clear distinction between tornadoes and downbursts is a main task for the future in order to gain a more precise knowledge of downbursts in Europe. Up to now, downbursts are still being confused with tornadoes or unspecifically termed as "wind damage with thunderstorms". Therefore, the current nnumber of downbursts in the TorDACH data archive is still significantly lower than the number of tornado reports - strongly contradicting expectations based on physical arguments.
First, the different kinds of small-scale atmospheric vortices are discussed. Afterwards, a description of straight-line wind phenomena with thunderstorms is given.
The various small-scale atmospheric vortices are called Tromben in German. Thermally induced dust devils (Kleintromben) occur without vigorous cumulus convection (e.g. thunderstorms). An overview of the terminology is given by the following Table ( click to enlarge ).
The table shows that the word "tornado" and the somewhat archaic term "Großtrombe" are synonyms. Just as unambiguous from the definition (but not from its contemporary usage) are "Windhosen" = tornadoes over land, "Wasserhosen" = tornadoes over water surfaces. Note that for physical reasons, it is not meaningful to classify a tornado as a waterspout, if it just traverses a river or a small lake.
Alfred Wegener has earned a lot of merit for his research on tornadoes in Europe. In his book "Wind- und Wasserhosen in Europa" (Wegener, 1917), he has unified the tornado terminology in German. And most notably, he has given an excellent definition of tornadoes, also pointing out that tornadoes over land (Windhosen) and tornadoes over water (Wasserhosen) present the identical physical phenomenon - see the following two quotations:
"Windhosen und Wasserhosen sind gleichartige Erscheinungen, nur
ist erstere Bezeichnung auf dem Lande, letztere auf See gebräuchlich.
Sollte es einmal gelingen, geringe grundsätzliche Unterschiede
zwischen beiden zu entdecken, was bisher mit Sicherheit nicht möglich
ist, so werden sich solche vermutlich ohne weiteres auf die verschiedene
Reibung am Untergrunde zurückführen lassen."
(Wegener, 1917, p. 3)
("Tornadoes and waterspouts are identical phenomena, only that the
former term is used over land, while the latter is used over the sea.
Should it happen that small underlying differences between the two are
discovered, something that has certainly not happened until now, these
will presumably be attributable to the differences in surface
friction").
"Wind- und Wasserhosen sind große Luftwirbel mit vertikaler Achse,
die vom Rande einer Cumulo-Nimbus-Wolke meist bis zum Erdboden herabreichen,
in ihrem Inneren durch Kondensation in Form eines herabhängenden
Zapfens, Trichters, Schlauches oder Säule, im unteren Teil auch durch
Staub, ganz oder teilweise sichtbar sind und in einer meist nach Hektometern
zählenden Spurbreite durch stürmisches Hinzuströmen der Luft
zu dem stark luftverdünnten Raum um die Wirbelachse gewöhnlich
derartige Verwüstungen verursachen, wie sie auch bei den schwersten
Stürmen größerer Ausdehnung nicht beobachtet
werden."
(Wegener, 1917, S. 5).
("Tornadoes and waterpouts are large vortices with vertical axis extending
from the base of a cumulonimbus cloud to the surface, visible completely or
in part through condensation or, in the lower part through dust, in the
form of a pendant cone, funnel, hose or column. In a track typically on
the order of hundreds of meters wide, with intense convergence towards the
region of strongly reduced air pressure around the vortex axis, they in
general cause damage of a kind not observed in even the strongest larger
scale storms.")
These definitions were well ahead of their time. The often-cited, but not very helpful, tornado definition in the Glossary of Meteorology's first edition (Huschke, 1959) has been a subject of criticism for many years. Only recently, the second edition of the Glossary (Glickman, 2000) provided a definition similar to Alfred Wegener's given 83 years before:
A tornado is "a violently rotating column of air, in contact with the ground, either pendant from a cumuliform cloud or underneath a cumuliform cloud, and often (but not always) visible as a funnel cloud."
In this context, see also Chuck Doswell's readable essay "What is a tornado?"
A meaningful subclassification of tornadoes comes from the fact if the cumuliform parent (thunderstorm-)cloud is a supercell or not. Supercells are thunderstorms with a persistent, rotating updraft. For this reason, they often have a propagation direction different from other thunderstorms. Supercell tornadoes can attain the highest intensities on the Fujita scale. Non-supercell tornadoes (including most of the waterspouts) on the contrary, only attain up to F2-intensity.
Chuck Doswell has also made a contribution to defining supercell storms: "What is a supercell?"
Note: The Fujita scale reaches from F-2 to F6. In most cases, only the range from F0 to F5 is applied. F0 matches Beaufort 8, and here, the first light damage occurs. Tornadoes with negative F scale do not cause damage and are therefore almost never observed. The existence of F6 tornadoes has never been proven, even in the USA. However, that a tornado could someday just attain this extreme intensity, is at least not unthinkable.
Funnel clouds can be precursors or intermediate stages of tornadoes - in those cases when the vortex has not yet reached the ground or if a tornado undergoes a transient weakening, and therefore does no longer cause damage at the ground (what was frequently interpreted in the past as a "jumping" or "skipping" of the tornado). In all these cases there will in most cases already (or still) be a funnel cloud pending from cloud base more or less far down to the ground, indicating the presence of the vortex.
However, there are several possibilities for confusion. The criterion for existence of a tornado is ground contact of the vortex, not ground contact of the funnel. Unfortunately, for this reason many weaker tornadoes have been misclassified as "only" funnel clouds, just because the cloud did not reach to the ground - even if there was damage at the surface or the vortex had been visible there due to dust or small debris. Past cases have led to the following rule of thumb: If the funnel cloud reaches down rather far from the cloud base, and the funnel diameter is relatively large at cloud base, then the vortex is very likely to have ground contact and it is a weak tornado. Funnel clouds of vortices not in contact with the ground, on the other hand, ar often very narrow and short.
In addition, small and short-lived funnels can also form under different circumstances and then have nothing to do with tornadoes. This is the case for shear or cold air funnels. With these, local wind shear at the cloud boundary or small-scale descending air parcels from cloud base can generate vortices in which condensation of cloud droplets occurs.
These vortices which are not tornadoes, but dynamically induced whirlwinds, form at gust fronts or other convergence- or shear lines in the atmospheric boundary layer, where vertical vorticity is present. With vigorous gust fronts, these vortices can attain F1 on the Fujita scale and therefore cause already considerable damage. This is one of the reasons why gust front vortices are frequent reasons for false tornado reports. The vortex itself becomes visible mainly by dust lifted up from the ground, less so by condensation within the funnel.
However, a gust front vortex can develop into a non-supercell tornado. Namely then, when it comes below the updraft region of a convective cloud (cu cong or cb) and intensifies by vortex stretching. This can happen when a gust front propagates below another thunderstorm, or if two such convergence lines collide and interact. In this case, the vertical vorticity is enhanced, and also the strong horizontal convergence is beneficial for the formation of a new cumuliform cloud.
Many waterspouts (e.g. over Lake Constance) are formed from the interaction of gust front vortices and towering cumulus clouds. As there are frequently several vorticity centers along a boundary layer convergence line, it becomes clear why waterspouts or other non-supercell tornadoes over land often occur having several funnels at the same time along the convergence line.
These thermally induced whirlwinds are not tornadoes!
They are small-scale vortices which form at the earth's surface (over land or water) under fair weather conditions and strong solar radiation. They are limited to the atmospheric boundary layer. There is no contact with deep cumulaus clouds.
Usually, dust devils are harmless, although optically and acoustically already impressive vortices which in rare cases can reach several hundreds of meters altitude. Their intensity remains within F-1 or F0 in most cases, and no serious damage occurs. An exception from this rule was the whirlwind forming in Koblenz on 9 August 1997, reaching F1 on the Fujita scale and damging a large number of cars on a parking lot by flying debris.
Gust fronts form with many intense showers or thunderstorms. Depending on the size spectrum of raindrops and the relative humidity of the air through which the rain (or also hail) falls, this air is more or less being cooled by evaporation and melting processes. Together with the drag force exerted by the falling hydrometeors (rain, hail, ...) on the air, a cold downdraft area forms and a cold air pool below the cloud.
This cold air spreads out at the ground, mainly in the direction of storm propagation and laterally. So the turbulent leading edge of the cold air (the gust front) often moves farther and farther away from the parent thunderstorm cell - in many cases visible by a characteristic, arc-shaped stratiform cloud. If the atmospheric conditions are favorable, such gust fronts can propagate for hours and cause sudden, apparently unexplicable strong winds in regions without thunderstorms.
As mentioned above, locally vortices may form along gust fronts, and the merger of two gust fronts propagating in opposite directions may lead to explosive formation of new thunderstorms.
In principle, downbursts form very similar to ordinary gust fronts - only that they are limited to a smaller area, but are also much more intense. When thunderstorms are present in the vicinity of an airport, downbursts present a big hazard to aircraft in their take-off and landing phase. For the formation of a downburst, the same downdraft-producing mechanisms within the thundercloud are neccessary as for gust fronts. If, however, large copious amounts of precipitation rush down from large altitude and encounter relatively dry air already several kilometers above the ground, then the forming downdraft may be of the order of 20 to 30 m/s. Simultaneously, this deep downdraft carries with it the horizontal momentum of the upper air flow downward, such that after the redirection of the downdraft at the ground, windspeeds of far more than 100, in extreme cases far more than 200 km/h may develop. One can imagine this process as a swinging tip-over of a bucket of water - the flow in a downburst is similar to this spreading of the water at the ground.
From the USA, some few downbursts of F3 intensity are known, but in
general F2 on the
Fujita scale
presents an upper limit both in the USA and in Europe.
The thunderstorm near Weilheim on 21 June 2002
(in 
)
presents an example of a T3/F1 downburst in Germany.
Unfortunately, downbursts are often confused with tornadoes. Many reports using the German word Windhose have not been tornadoes, but downbursts. Once in a while, vortices may form at the edge of a downburst, such that the damage may locally indicate damage from a tornado. On the other hand, also some tornadoes may lead to almost straight-line damage. Therefore, an a posteriori decision if a tornado or a downburst had caused the observed damage is not always easy.
Fig. 1: Schematic of the flow fields within downbursts
and tornadoes (Fujita, 1985).
Nevertheless, there is one principal difference between downburst and tornado windfields: The windfield of a downburst is always divergent (by the dispersion of the cold air coming down), whereas the windfield of a tornado is always convergent (by the inflow into the tornado vortex). Note the schematic picture in Fig. 1 above.
Especially with supercell thunderstorms, gust front, downburst and tornado may be generated together or successively from the same thunderstorm. In tis context, the tornado is the most specialized and therefore most unlikely event, while the gust front is the most general and most frequent phenomenon.
If lines of thunderstorms (squall lines) propagate faster in their central parts than at their fringes, the squall line will be curved like a bow and display a so-called bow echo on a radar screen. Such structures indicate that the thunderstorms in the center of the bow are the most intense and that in this region, the gust fronts of individual thunderstorm cells are superposed such that one must expect very strong, straight-line winds in the propagation direction of the bow echo. These can attain F1 intensity and cover a larger area compared to a downburst. If a very elongated squall line forms several bow echoes over its entire length, a wave-like structure of the squall line develops which is called a line echo wave pattern (LEWP).
Tornadoes with bow echoes may arise and are most likely at the (northern) tip of the bow echo. If tornadoes form at the southern tip of the bow echo, they often have an anticyclonic sense of rotation. In addition, gust front vortices may frequently form which then could wrongly be reported as tornadoes - as with the case of 23 June 2003 in Norderstedt-Glashütte, when a small gust front vortex was seen at the edge of a downburst. In this case, Norderstedt was located at the southern edge of the bow echo.
The largest-scale extreme wind event with thunderstorms is the derecho. This term comes from the Spanish word derecho = "straight on", and was coined by Gustavus Hinrichs in the USA in 1888 to make a distinction from the tornadoes (span. tornado = turned or twisted). A derecho consists of an extremely extended zone with damaging, straight-line thunderstorm winds, which could be caused e.g. by a succession of bow echoes within a long-lived squall line. However, even with derechos, some tornadoes may locally form.
Johns and Hirt (1987) revived the term derecho in the meteorological community and defined a derecho by:
In Europe, the term "derecho" is still relatively seldom used. Before, such events had been classified as "extended wind damage with a squall line" or similar terms. The event which hit Berlin on 10 July 2002 was classified as a derecho by Gatzen (2003). Also the CLEOPATRA-squall line of 21 July 1992 in southern Germany might have met the above criteria for a derecho.