Tornadoes in United States: an unusual phenomenon during winter months

The United States is struck by 1,200 tornado per year on average, especially in the center and south of the country.

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A balance sheet that could reach a hundred deaths, constructions destroyed as far as the eyewear, entanglements of rubble: six states of the center and southern United States (Kentucky, Illinois, Arkansas, Missouri, Tennessee and Mississippi) Have been crossed by one of the worst series of tornadoes – about thirty – from the history of the country on the night of Friday 10 to Saturday 11 December. Back on an exceptional phenomenon.

  • How do the tornadoes are formed?

Tornadoes are produced by supercell thunderstorms, very violent, larger and durable than those classic. These supercells are formed under very unstable conditions. First, when hot and wet air near the surface is under a cool and dry air layer, in which the temperature decreases rapidly with the altitude. Then, in the presence of a large variation of direction and wind speed with altitude. This wind shear as well as the thermal shock between hot and cold air cause rapid rotation of the air column, from the base of the clouds to the ground.

“On the night Friday to Saturday, there was a difference in wind speed of more than 160 km / h on the height of the storm,” explains Paul Markowski, Professor of Meteorology at the University of Pennsylvania and tornado specialists. The temperatures were particularly soft. The year 2021 was also marked by an episode La Niña (Equatorial Pacific Waters Cooling), a phenomenon that tends to increase the intensity of tornadoes in the United States in winter and early spring.

  • was the Friday tornado series atypical?

  • The United States is struck by 1,200 tornado per year on average – a world record -, with a high variability from one year on the other, According to the American Ocean and Atmospheric Opening Agency (NOAA) . The center and south of the country are particularly affected because of the meeting between the warm air of the Gulf of Mexico and the cold masses from Canada, under the wind of the Rocky Mountains. The majority of these tornadoes occur in May and June. About 500 of them are classified EF1 or more on the improved Fujita scale (0-5) – we begin to see dwellings from the EF1 category.

    “The Friday event was exceptional, assures Harold Brooks, researcher at NOAA, while stating that the Fujita scale ranking has not yet been established. The number of deaths, although not definitive , seems to be established in the first ten events on the last seventy years. “One of the tornadoes that ravaged Kentucky could have beaten a record of length established in 1925 (352 kilometers). The events combining intense tornadoes are unusual during the winter months, because the air is more rarely hot and humid. When they occur in December, “they constitute a challenge from the point of view of the alert, because the public is less aware of the risk of dangerous storms at that time of the year and the nocturnal tornadoes, therefore difficult to See, are more frequent because of the brevity of the days, “says Paul Markowski.

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    /Media reports.