Tuesday, 7th of June 2016. It was a cloudy but warm day for Dublin. Normally during the summer months, I get jealous of my family and friends posting pictures of summer fun in nearly 30 degree sunshine. Even in North Germany! But that night I was glad to enjoy the good Irish weather whilst in Hamburg hailstones of the size of tennis balls pummelled down and thunderstorms darkened the skies accompanied with heavy rain and… a tornado!?
Yes, a tornado in Hamburg (Fig. 1) that kept roughly 1000 fireguards busy. Only two days earlier twin tornadoes were captured dancing across Schleswig-Holstein. Now, someone could ask if the tornado season has started in Germany. But is this actually a justified question?
It turns out, yes!
Everyone’s heard of the season of extreme weather along the “Tornado Alley” across the middle states of America where on average 1170 observed tornadoes can reach disastrous dimensions (Nikolai Dotzek, 2001). A less known fact is that Europe also is haunted regularly by twisters. Alfred Wegener was one of the first to estimate the number of twisters almost a century ago with “at least 100 tornadoes per year in Europe” (Alfred Wegener, 1917). In 2001, Nikolai Dotzek, who was a pioneer of tornado research in German, investigated the tornado occurrence in Europe and discovered that on average over 700 tornadoes are estimated to strike land or sea. Most of those estimates are located within Germany, the Netherlands, Spain and the UK. The actually observed numbers are much lower with only 330 per year due to the general lack of tornado reports across Europe. Still, Europe is hit by tornadoes four times fewer than the USA (330/1170) and to understand why, we have to understand how tornadoes form.
A tornado is a small-scale hurricane that forms on hot days in connection with thunderstorms, usually during spring or summer (Fig. 2). The twisting air columns form when warm, moist air rises from the ground and meets cold, dry air in the higher atmosphere. Furthermore, strong crosswinds in the upper atmosphere are needed to cause the rising air to spin. Then, out of the thunderstorm cloud a funnel builds, and when the funnel reaches the surface a tornado is born.
Generally, tornados can form wherever these climatic conditions occur. For example, disastrous tornadoes have occurred in Bangladesh with the Saturia–Manikganj Sadar tornado believed to be the deadliest tornado recorded in history. The frequency of destructive tornadoes that occur in the USA is due to the favourable geographical setting. While in Central Europe the Alps acts as a barrier to prevent the exchange between cold and warm air masses, in North America the different temperate air masses often collide. The Rocky Mountains running from north to south offer no protection from the north- or southward moving air masses. Furthermore, hot and dry air coming from the Rocky Mountains as well as the jet stream can strengthen the thunderstorm generation.
Even though the tornado record is getting better and better, meaningful predictions are very difficult. So far it is believed that an F5 tornado, the highest rating on the Fujita scale, will occur once in a century in Germany. Generally, such strong twisters are very rare in Europe. Yet, this estimate lacks historical background. A geological record could improve the understanding of twister appearance. However, such a record is controversial. While past hurricanes can be evaluated based on remobilized and redeposited sediments, similar sedimentary attributes are unknown for twisters. Even waterspouts (tornadoes over water) are not able to mobilize enough sediment to provide enough evidence of its occurrence. Scientists’ debate if shredded plant material could be an indicator of twister appearances as it could be also easily mistaken with rapid flooding events. The only record considered to be caused by tornado has been described by Carozzi and Gerber (1978) on a rock outcrop of a Missouri limestone.
Nevertheless, tornadoes are not a rare phenomenon in Europe with three observed twisters in Germany so far and the summer having just started. We should keep our eyes open in Ireland (FIG. 3)! Following the “tornado year” in 2013 (several warnings and spotted tornadoes), tornados are well known to the Irish with an average of 10 tornadoes spotted each year (Nikolai Dotzek, 2001), but none so far this year…
By Sabrina Renken, postgraduate student, Trinity College Dublin.
http://www.tornadoliste.de/nikolai (German only) or http://www.flame.org/~cdoswell/Dotzek_tribute/Dotzek_tribute.html (English)