Tornadoes in the United States: debris rose to an altitude of 9,000 m!

This weekend, the images of literally blown up houses, collapsed warehouses, collapsed buildings … devastated cities … wiped off the map have looped in the media. The south-east and the Midwest of the United States experienced, last Friday, a tornado episode particularly violent. Services weather report recorded between 20 and 30 tornadoes across seven states in just 24 hours. To give you an idea in terms of numbers, that’s about half of what happens in France … in a whole year!

The power of one of these tornadoes, in particular, was extreme. On thefujita scale which classifies the force of these events according to the damage caused into six categories, those that the Americans already call the “Quad-State Tornado” ranks at least EF4 – since 1950 there have only been 19 such tornadoes over the United States. Even EF5 – only two such powerful tornadoes have been recorded over the country in a month of December since 1950 – the highest category. Even though the month of December is traditionally the month of the year when fewer tornadoes develop over the United States.

A particularly devastating tornado

This incredible “Quad-State Tornado” would have traveled a distance of some 365 kilometers over four states – thethunderstorm who accompanied him, he even traveled more than 900 kilometers. This is roughly the distance that separates Paris from Lyon. What if the data were to be confirmed by the National Weather Service, that would represent a world record. This was previously held by the “Tri-State Tornado” from 1925. She had traveled some 352 kilometers.

In just four hours, the “Quad-State Tornado” passed through Arkansas, Missouri, Tennessee and Kentucky.

The damage caused are considerable. A little more because the “Quad-State Tornado” traveled at night. Radar data show that debris rose to an altitude of 9,000 meters before falling to the ground. More than 100 deaths have already been confirmed.

It seems that the occurrence of such tornado clusters is increasing in the region. Perhaps an effect of temperature and humidity conditions made more and more favorable to the development of such storms. But the researchers note that it is still complicated to link such a one-off event to the global warming.

U.S. tornado surge not due to global warming

With 350 dead and 10,000 homes destroyed, the hundreds of tornadoes that swept across several southern states last week seem exceptional. Apparently, their number has been increasing for several decades, but meteorologists believe that the climatic changes observed on a planetary scale are not in question.

Article by Jean-Luc Goudet published on 05/03/2011

• Browse our file on tornadoes

The event is now compared, in magnitude, to the storm katrina, which notably ravaged New Orleans in late August 2005. Some of the tornadoes that swept through the southern United States last week were perhaps the most powerful seen in forty years. This is the opinion of a meteorologist from the NOAA, Harold Brooks (National Severe Storms Laboratory, Norman, Oklahoma). According to him, the largest must have reached 1.5 kilometers on the ground and fifteen at altitude, with winds over 300 km / h. On average, only one percent of tornadoes turn into serial killers but, according to him, this proportion must have been higher last week.

However, these regions are used to tornadoes. Each spring, they appear in the great plains and cause great damage. These tornadoes are localized and extremely powerful phenomena. These are even the most severe weather manifestations in Earth’s atmosphere, and conditions are particularly favorable for them in the southern United States.

Small but very violent whirlpools

A tornado can form when a air mass hot and humid is topped with a mass ofair cold at altitude. The lower layer tends to rise and cool down due to the drop in pressure. The water condenses and forms a cloud. When the volumes in play are sufficient, appears one of the two giants of clouds : the cumulonimbus, often recognizable by its summit anvil. In the center an air column rises, which generates a depression, sucking the surrounding air. The surfaces concerned are such that the coriolis force, due to the rotation of the Earth, and it is a movement turning which forms around the central depression, while rains and thunderstorms arise.

It can happen that this rotating movement is very localized and that the central depression widens disproportionately as the surrounding winds tighten more and more. A tornado then forms, a sort of upward flared tube which can reach 1 to 20 kilometers in diameter. Her duration life is usually only a few minutes but can be up to a few hours. With winds that can approach 400 km / h, the tornado can cause enormous damage, causing houses to fly, cars, trucks, men, animals, trees…

Natural pulsations of the atmosphere

In the southern United States in the spring, the polar air descending from the north, at an altitude of around 10 kilometers, meets the warm and humid air coming, at ground level, from the Gulf of Mexico. This polar air, moreover, encountered on its way the Rocky Mountains which made it gain altitude, which favors the condensation of water in cloudy masses.

” If you look at the data for the last sixty years, explains Grady Dixon, meteorologist at the University of Mississippi, in comments reported by AFP, you notice that the number of tornadoes increases significantly. But the consensus among tornado experts is that this increase is not real. “He adds that it would be” a big mistake »To attribute this observed increase to global warming global.

On Wednesday April 27, high pressure conditions were exceptional over the Gulf of Mexico. These conditions, according to Grady Dixon, would have been favored by the recent episode of La Niňa, already accused of having flooded australia, and characterized, unlikeEl Niňo, by cooling surface waters in the central and eastern Pacific Ocean. ” We knew then that it would be a big year for tornadoes “Says Grady Dixon, concluding that the phenomenon is” related to the fluctuations of the planet “.