A series of deadly tornadoes have left a path of destruction in their wake after ripping across the US.
More than 30 tornadoes were reported across six states on Friday and Saturday, according to the National Oceanic and Atmospheric Administration, and Kentucky’s governor called it “the most severe and deadly tornado event in Kentucky history”.
One of them, if confirmed, could be the longest ever to touch down upon US soil.
The longest on record occurred in March 1925 and was tracked for about 220 miles (355km) through Missouri, Illinois and Indiana.
One US extreme weather expert, Northern Illinois University’s Victor Genzini, said this latest twister may have touched down for nearly 250 miles (400km).
All the more surprising was that the storm came in December, when normally colder weather limits tornadoes in the US.
Professor Gensini said: “One word: remarkable; unbelievable would be another. It was really a late spring-type of setup in the middle of December.”
So, what causes tornadoes?
Sky weather president Kirsty McCabe says exactly how they form is still subject to much study and debate.
She says: “What we do know is that these rapidly rotating columns of air develop in the massive updraughts beneath severe thunderstorms.
“You need a specific combination of factors to produce these severe thunderstorms: winds that change speed and direction with height; a source of lift, such as a cold front; atmosphere instability; and plenty of moisture in the air.
“Most tornadoes are found in the Great Plains of the United States, aka Tornado Alley. It has earned its name as this is where warm, moisture laden air from the Gulf of Mexico meets cold air from Canada along with dry air from the Rockies.
“Most occur in the spring and summer months especially May and June. They also tend to happen during the late afternoon and early evening, only rarely happening overnight.”
She says the majority of significant or violent tornadoes come from supercells – thunderstorms with a deep rotating updraught.
“Vertical wind shear creates a rolling tube of air parallel to the ground,” she adds. “Heating near the ground then helps lift the horizontal tube of rotating air into a vertical position, forming a funnel cloud.
“If the funnel cloud touches down over land it becomes classed as a tornado, or a water spout over water.”
What made this one so bad?
Kirsty says tornadoes in December are unusual, especially at night.
“There are thoughts that climate change and a La Nina weather pattern may have played a role in these deadly tornadoes, as the temperatures were more like spring than winter,” she says.
It is the La Nina event in the Pacific that is said to have caused a series of bad floods in Australia in the past month or so.
In fact, the temperatures across the eastern seaboard of the US were reported to be between 10C-20C above the seasonal average in the days before the tornadoes struck.
Records for the highest temperature recorded on 11 December were broken in numerous places in the states closer to the coast.
Experts say much higher than average temperatures across much of the Midwest and South in December helped bring the warm, moist air that helped form thunderstorms.
John Gordon, a National Weather Service meteorologist in Louisville, Kentucky, said: “The worst-case scenario happened. Warm air in the cold season, middle of the night.”
Was it caused by climate change?
Scientists expect unusually warm weather to become more common in the winter as the planet warms.
But a direct relationship is not easy to determine.
Experts are still trying to work out whether human-caused climate change is making tornadoes more common – or even more intense.
Ilan Kelman, professor of disasters and health at University College London, says linking specific tornadoes to human-induced climate change is not straight forward.
Winter tornadoes are not unique, or even rare, with one in Florida in February 1998 killing 42 people.
Prof Kelman said: “US tornado disasters more lethal than this year happened, for instance, in 1925 (nearly 700 confirmed dead) and 1936 (over 200 confirmed dead). The 1890 Kentucky tornado disaster killed approximately the same number of people as the 2021 one so far.
“Since these historical catastrophes, population numbers have increased, putting more people in a tornado’s potential path.
“Meanwhile, structures, warnings, evacuations, and shelters have improved, giving many the opportunity to avoid harm irrespective of the tornado.
“Even as human-caused climate change influences tornadoes more, it is tenuous to link climate change to tornado disasters.”
But Harold Brooks, a tornado scientist at the National Severe Storms Laboratory, said it wasn’t so much of a surprise as it might have been.
The US is likely to see more tornadoes occur in the winter, he said, as temperatures across the nation rise above the long-term average.
He said the result would be that, while more tornadoes happen in the winter, fewer events of the type seen over the weekend will take place in the summer.
Prof Gensini agreed, saying, in the aggregate, extreme storms are “becoming more common because we have a lot warmer air masses in the cool season that can support these types of severe weather outbreaks”.
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