How are tornadoes measured
In order to make the wind measurable and to be able to compare observations at different locations, but above all to find out where favorable wind conditions are to be found, e.g. for sailing ships, wind tables were drawn up as early as the middle of the 18th century, which were based on the effects in the area around the Observer. In 1759, the British engineer John Smeaton designed a twelve-part scale describing the effect on windmill blades. In 1806 the British navigator Sir Francis Beaufort (1774-1857) transferred the scale to a three-masted full ship and later made it known. The Beaufort scale was named after him.
Tab. 1: Beaufort scale
Wind force 12 was thought to be open at the top. The scale was expanded to 17 levels by an international agreement in 1949 in order to include more severe events. The wind strengths from 13 onwards could hardly be differentiated and it was hardly applicable for extreme events. For example, additional scales were created for recording tropical cyclones and tornadoes. In 1970 the expansion of the Beaufort scale was abolished by the World Meteorological Organization. Since then, the original 13-part scale (wind force 1-12 and calm) has been used again for “normal” wind events.
Saffir-Simpson hurricane scale
Especially after the devastating Hurricane CAMILLE, which devastated entire areas of the southern United States in August 1969, the need increased to classify the hurricanes in different strength classes in order to be able to warn the population more clearly and better of an approaching hurricane. In 1971 the engineer Herbert Saffir and the then director of the National Hurricane Center in Miami / Florida, Bob Simpson, designed the Saffir-Simpson hurricane scale named after them and still valid today.
Tab. 2: Saffir-Simpson hurricane scale
The scale has been used for all tropical cyclones in the Northeast Pacific and the Atlantic since 1973. Initially, the hurricanes were not only classified according to the mean wind speeds, but also according to the lowest air pressure in the center and the expected storm surge. It wasn't until 2009 that the Saffir-Simpson Hurricane Scale became a pure wind scale. The absolute air pressure in the center has little informative value, only the difference in air pressure to the surroundings is important. And hurricane IKE, which razed entire locations on the Texan coast as a category 2 hurricane in September 2008, landed with a storm surge that corresponded to a category 4 hurricane on the scale used up to that point. As a result, such information was deleted from the scale.
Fig. 1: Gilchrist, Texas, destroyed by Hurricane IKE (Source: US Environmental Protection Agency)
Terms such as hurricanes, typhoons and cyclones describe the same phenomenon in different sea areas on earth. The Saffir-Simpson hurricane scale is only valid for the central and eastern North Pacific and for the entire Atlantic. In the Australia area there is a separate scale that is not based on the mean wind but on the peak gusts. There is no equivalent scale for tropical cyclones in the rest of the oceans. Only in the Northwest Pacific (including Japan, China, the Philippines) has it established itself that from a mean wind of 240 km / h is spoken of a "super typhoon".
Hurricanes occur on tropical or subtropical waters when showers and thunderstorms form a tropical low under favorable wind conditions. The earth's rotation ("Coriolis force") sets the low into rotation. A cycle is created: warm and humid air rises, the humidity condenses into clouds and condensation heat is released, which further promotes the rise of the air. The more air rises, the more air has to flow faster from the outside into the deep. From an average wind speed of 62 km / h the system becomes a tropical storm, which is given a name, from 118 km / h it becomes a hurricane (typhoon, cyclone).
Hurricanes are usually several hundred kilometers in diameter and can devastate a large area if they hit land. The predominantly weak wind eye in the center of the tropical cyclone is characteristic, but it can only be clearly seen on satellite images during stronger storms. Hurricanes, typhoons and cyclones can only occur over the oceans. In contrast, tornadoes are small-scale eddies that can travel both on land and at sea.
Fujita tornado scale
At the same time as researching hurricanes, especially from the 1960s onwards, research into and increasingly predicting tornadoes was promoted in the USA. A separate scale was also developed for tornadoes, but it was originally purely a damage scale. This is because the wind speeds that occur in the small-scale eddies are hardly or not at all measurable with conventional measuring methods. So a different approach had to be created. Based on the damage that has occurred, tornadoes can be classified and then assigned to wind speeds. Tornadoes over bodies of water, also known as waterspouts, cannot therefore be classified.
Tab. 3: Fujita tornado scale with damage effects
In 1971, Professor Tetsuya Theodore Fujita (1920-1998) at the University of Chicago developed the scale named after him, which quickly established itself worldwide. It applies not only to tornadoes, but also, for example, to thunderstorm gusts, which in extreme cases can have wind speeds of over 250 km / h. These thunderstorm gusts ("downbursts") were also discovered by Tetsuya Theodore Fujita. The wind speeds assigned to the individual levels follow:
Tab. 4: Fujita tornado scale with wind speeds
In 2007, an "Extended Fujita Scale" was published in the USA, but it is very controversial because of some inconsistencies and reduced wind values and is hardly used outside the USA. The technical discussions about this new scale continue to this day. Another tornado scale is the so-called “Torro scale”, which is used in Europe especially in German-speaking countries and has twice as many levels as the Fujita scale. Since 1972, it has been able to classify damage even more precisely in specialist circles. The different building structures and, above all, the vegetation are recorded much better on the Torro scale. It complements the Fujita scale well and is used in parallel.
Fig. 2: Tornado damage of magnitude F3 in July 2004 in North Rhine-Westphalia (source: Thomas Sävert)
Tornadoes occur in numerous regions of the world. They are mainly known from the USA, but they can also be found on all other continents except Antarctica. In Germany they are also known as tornadoes, but there is no difference to the American tornadoes. On a long-term average, around 1200 tornadoes occur in the USA, several hundred in Europe, 40 to 60 of them in Germany. They have a diameter of a few meters to a few hundred meters, in extreme cases even more than a kilometer, and with wind speeds of over 500 km / h can cause enormous damage. In the railway with a length of a few meters to a few dozen kilometers, they usually only last a few minutes, in exceptional cases even up to an hour.
Tornadoes occur in the area of showers or thunderstorms with strong updrafts and vertical wind shear. Vertical wind shear means an increase and change in direction of the wind with height. Tornadoes are dangerous not only because of the pure wind speeds, but above all because of objects flying around, which can injure or kill people and cause additional damage. In strong tornadoes, vehicles, trucks, combine harvesters and even entire houses can be whirled through the air.
Tornadoes are tied to shower and storm clouds or storm fronts. They are not to be confused with the so-called "dust devils". The latter occur during the day in clear or sunny weather on overheated areas (fields, sand or sports fields, etc.). They often have a lifespan of just a few seconds to minutes, the range of action is limited to just a few meters and they rarely cause any damage worth mentioning. Sand, dust, leaves, straw, etc., but also tents and parasols, for example, can be whirled through the air.
This compilation was created by Thomas Sävert, Andreas Wagner and Stefan Laps, meteorologists at MeteoGroup Severe Weather Center, in October 2011.
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