Due to the high wind speeds of up to 500 km/h in the peripheral area and the large depression in their centre, tornadoes exhibit an extraordinarily high damage potential. The diameter of a tornado is between 50 and 1,000 m (average ~ 100 m); the wind speed according to the Enhanced Fujita scale (EF) is more than 105 km/h (EF0, see Table 1). Very severe tornadoes from category EF3 upwards occur only in connection with supercell thunderstorms, while weaker tornadoes can also form at the flanks of a squall line or in a hurricane during landfall. The wind speeds and thus the category are estimated from the damage patterns on the ground, as direct measurements are usually not available. The highest wind speed to date (~ 480 km/h) was observed with a mobile Doppler radar (DOW) in the USA (May 3, 1999, Oklahoma).
The lifetime of a tornado is from a few seconds to more than half an hour; in very rare cases a tornado can also last over one hour. Its rotation is usually cyclonic (anticlockwise) in the northern hemisphere due to the clockwise rotation of the wind in the boundary layer. The formation of a tornado is a very complicated process which is to this day not fully understood. The necessary requirements are identical to those for the formation of deep moist convection – in particular warm and moist air masses (= energy) and high wind shear, especially directional shear (vertical changes of the horizontal wind direction).
USA: 1,100 tornadoes every year
Tornadoes occur on almost every continent and in very many regions in the mid-latitudes. They are most frequently observed in the USA, where 1,100 tornadoes occur on average every year (Brooks, 2013). In Europe there are around 300 – 500 tornadoes every year according to the European Severe Weather Database (ESWD). In Germany 30 - 50 tornadoes are observed on average every year. One of the most severe tornadoes in Germany was that on July 10, 1968 in Baden-Württemberg (Pforzheim) with an intensity of EF4 (or F4) and maximum wind speeds of more than 300 km/h.
The relative distribution among the different intensity classes (EF) in Germany is similar to that in the US (Dotzek et al., 2003). This means that tornadoes in the US are not stronger compared to Germany or Europe, but occur much more frequently. The difference in the relative frequency between the two continents can be plausibly explained by the fact that in Europe the Alps prevent direct meeting of cold (polar) air masses with warm, humid Mediterranean air. In North America, the central mountain range, the Rocky Mountains, has a north-to-south orientation, and warm and moist air masses from the Gulf of Mexico can directly meet cold air masses from Alaska or dry air from the West (Graf et al., 2011). As the temperature gradient is still very pronounced in spring, but the air close to the ground already has a high energy content, intense tornadoes in the US occur most frequently in the months of May and June compared to July and August in Europe.
Studies for the US do not at present indicate an increase in the number of tornadoes or tornado days (except the increase due to the larger number of observations). However, Brooks (2013) reports on an increase in the number of tornadoes on one day and a change in the times of year in which they occur. For Germany or Europe such studies does not exist, mainly due to the incomplete records of tornado observations.
Literatur:
Brooks, H., 2013: Increased variability of tornado occurrence in the United States in recent years. 7th Eur. Conf. on Severe Storms, 3 – 7 Juni 2013, Helsinki, Finnland.
Dotzek, N., 2001: Tornados in Germany, Atmo.Res. 56, 233 – 251.
Dotzek, N., 2003: An updated estimate of tornado occurrence in Europe, Atmo.Res. 67, 153 – 161.
Dotzek, N., Grieser, J. und Brooks, H. E., 2003: Statistical modeling of tornado intensity distributions. Atmo.Res., 67, 163 – 187.
Graf, M., 2008: Synoptical and mesoscale weather situations associated with tornadoes in Europe. Institut für Geographie, Universität Zürich, Zürich, Schweiz.
Sävert, Th., 2008: Tornadoliste Deutschland. Velbert, Deutschland.
