A death toll that could reach a hundred dead, buildings destroyed as far as the eye can see, entanglements of rubble: six states in the central and southern United States (Kentucky, Illinois, Arkansas, Missouri, Tennessee and Mississippi) were crossed by one of the worst series of tornadoes – about thirty – in the history of the country on the night of Friday 10 to Saturday 11 December. Back to an exceptional phenomenon.
How do tornadoes form?
Tornadoes are produced by supercell storms, very violent, larger and more durable than conventional ones. These supercells form under very unstable conditions. First, when warm, humid air near the surface is under a layer of cooler, drier air, in which the temperature decreases rapidly with altitude. Then, in the presence of a significant variation of direction and speed of the wind with the altitude. This wind shear as well as the thermal shock between hot and cold air causes a rapid rotation of the air column, from the base of the clouds to the ground.
“During the night from Friday to Saturday, we recorded a difference in wind speed of more than 160 km / h over the height of the storm”, explains Paul Markowski, professor of meteorology at the University of Pennsylvania and specialist in tornadoes. The temperatures were also particularly mild. The year 2021 was also marked by a La Niña episode (cooling of the waters of the equatorial Pacific), a phenomenon that tends to increase the intensity of tornadoes in the United States in winter and early spring.
Was Friday’s series of tornadoes atypical?
The United States is hit by 1,200 tornadoes per year on average – a world record – with high variability from year to year, according to the United States Agency for Ocean and Atmospheric Observation (NOAA). The center and the south of the country are particularly affected because of the meeting between the warm air of the Gulf of Mexico and the cold masses coming from Canada, downwind of the Rocky Mountains. The majority of these tornadoes occur in May and June. About 500 of these are rated EF1 or higher on the Fujita Enhanced Scale (0-5) – we start to see damage to homes from the EF1 category.
“Friday’s event was exceptional, assures Harold Brooks, researcher at NOAA, while specifying that the ranking on the Fujita scale has not yet been established. The death toll, although not definitive, seems to be established in the first ten events of the last seventy years. “ One of the tornadoes that ravaged Kentucky may have broken a length record set in 1925 (352 kilometers). Events combining intense tornadoes are unusual during the winter months, as the air is more rarely hot and humid. When they occur in December, “They are a challenge from a warning point of view, as the public is less aware of the risk of dangerous thunderstorms at this time of the year and nocturnal tornadoes, which are therefore difficult to see, are more frequent due to the the brevity of the days ”, says Paul Markowski.
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