The tornado left a trail of destruction with considerable damage not only to buildings and infrastructure but also in the forests. According to official statistics, 23 people were killed and more than 230 injured.
How and where do tornadoes form?
Tornadoes are short-lived, small and extremely powerful hurricanes which occur in connection with thunderstorms. They grow downwards out of thunderclouds as a tube-like, rotating "trunk". They rarely last more than a few minutes and, like thunderclouds, move with an average speed of 50 - 60 km/h. However, considerably higher or lower speeds can also occur.
For the formation of supercells, the most severe of all thunderstorm systems responsible for most tornadoes, optimum atmospheric conditions prevail above all in spring in the Great Plains east of the Rocky Mountains. From Texas in the south to South Dakota in the north of the United States there extends an almost 2000 kilometre wide region on both sides of the Mississippi known as "Tornado Alley". A large number of tornadoes occur in this region every year. The total number of tornadoes in the USA is between 1,000 and 1,200 every year.
When cold, dry air masses move from the Canadian Shield east of the Rocky Mountains to the east and southeast and meet warm, humid air masses from the Gulf of Mexico, these are perfect conditions for the formation of severe thunderstorms (supercells), which are mostly the basis of the occurrence of tornado events. Likewise necessary is a large change in wind speed and direction with height (wind shear).
Hardest hit by the tornado on May 20 was Moore, a suburb of the metropolis Oklahoma City. Hardly a single house withstood the wind speeds of more than 323 km/h, two schools and a hospital were destroyed and cars were hurled through the air like toys. Afterwards the small town was like a pile of ruins. The financial damage caused by the storm amounts to several billion US dollars.
Course and development of the tornado in Oklahoma:
Two days before the tornado in Moore, thunderstorms with strong hail and violent gusts of wind had occurred in an area from northern Texas to South Dakota; more powerful tornadoes had already been sighted in Kansas. On May 19, the centre of the thunderstorms slowly moved eastwards and reached its climax one day later. Hailstones eight to eleven centimetres in diameter fell in the towns of Yukon and Bigheart (both in Oklahoma). Wapanucka and Eureka Springs recorded hurricane-force winds up to 129 km/h. One day later the climax of the so-called severe storm situation occurred. From 3.00 pm local time (10.00 pm CEST) onwards a tornado devastated the densely inhabited town of Moore. It had a total of 40 minutes‘ ground contact and moved ENE with an average speed of about 40 km/h. A tornado warning was given 16 minutes before the tornado arrived. The number of 23 lives claimed by the storm in Moore is however considerably lower than the deaths caused by previous tornado disasters in the USA.
Previous tornado disasters in the USA:
The most deadly tornado, the so-called "Tri-State" tornado, occurred in March 1925 and claimed 695 human lives. With 158 victims, the Joplin tornado which was recently observed in May 2011 occupies 7th place among the most deadly tornadoes in the history of the USA.
In May 1999 the "Bridge Creek-Moore" tornado hit the same region in Oklahoma, likewise an F5 tornado, which on the basis of the year 2011 caused a total loss of 1.4 billion US dollars. This is to this day the fourth most expensive tornado since 1950. According to initial estimates, the Moore tornado of May 20, 2013 takes its place at least among the top 5 of the most expensive tornadoes in the USA.
The five tornadoes which have to date caused the most damage in the USA (Storm Prediction Center, NOAA, 2011):
1) Joplin, Missouri, May 22, 2011, 2.8 billion US dollars
2) Topeka, Kansas, June 8, 1966, 1.7 billion US dollars
3) Lubbock, Texas, May 11, 1978, 1.5 billion US dollars
4) Bridge Creek-Moore, Oklahoma, May 3, 1999, 1.4 billion US dollars
5) Xenia, Ohio, April 3, 1974, 1.1 billion US dollars
According to the US agency Eqecat, which is specialized in risk assessment, the series of tornadoes occurring from May 18th to 20th caused damage amounting to two to five billion US dollars.
Classification and protection
The Fujita scale is used to classify tornado damage. It is divided into the categories F0 to F12. The figures of the categories F6 to F12 are however only theoretical and have not been measured to date. As the F scale is only of coarse resolution, the Enhanced Fujita scale (EF) has been used in the USA since 2007. The EF scale has six categories (from EF0 to EF5), but begins at higher speeds (EF0: 105 - 137 km/h) and ends at lower speeds (EF5: > 323 km/h). Accordingly, the tornado in Moore is assigned to EF5, the highest damage category.
Protection from tornadoes of category EF5 is in the first instance afforded by well-secured basement rooms with a firm ceiling, as a tornado of this strength can destroy not only wooden houses but also buildings made of stone down to the foundation walls. Most dangerous are the objects flying around like bullets which the tornado brings in its wake for kilometres. A tornado does not even stop before asphalt ceilings. In nature the rule applies that one should escape from a tornado, as even a distance of one kilometre provides relative safety. If it is too late for this, it helps to lie flat on the ground so as not to be hit by pieces of debris.
Do tornadoes also occur in Germany?
The term tornado has also been in use in Germany for some years; formerly they were referred to as wind socks. The danger of tornadoes is particularly great especially in the Midwest of the USA, but destructive tornadoes occur again and again in Europe and in Germany. For example, over Whitsun 2010 a tornado in Brandenburg and Saxony caused severe damage. Another well-known example is the tornado on July 10, 1968 in Baden-Württemberg (Pforzheim). Here a tornado of category EF4 (or F4) with maximum wind speeds of almost 400 km/h was observed.