Each year, the US gets around four times as many twisters as the rest of the world combined.
The tornadoes that ripped through six states in the US on December 10 were some of the deadliest on record, with at least 90 people dead.
The worst-hit was Kentucky, which reported at least 80 deaths, and several more were reported in Arkansas, Illinois and Tennessee. It would make the event not just one of the most devastating in Kentucky history, but US history, and potentially the deadliest December outbreak on record.
When it comes to tornadoes, the US is saddled with a meteorological exceptionalism compared to the rest of the world, and by a significant margin.
For perspective, countries outside the US witness around 200-300 tornadoes per year, while in the US that number is well over 1,200 per year.
That’s four times as many twisters as the rest of the world combined.
What makes the US so tornado-prone?
Most tornadoes in the US take place in the Great Plains – notably dubbed “Tornado Alley” – an area which is perfect for twisters. It’s typically considered an area which extends from Northern Texas, Oklahoma, Kansas, Colorado, Nebraska, to South Dakota.
The main reason for that is geography. The central part of the US is unique in that there’s a large warm area of water just to the south (Gulf of Mexico), and a high range of mountains (The Rocky Mountains) that extend from north to south.
A tornado requires a few special ingredients that Tornado Alley is full of: warm moist air at low levels, cold dry air at higher levels, and a mechanism to lift that warm air up.
Lots of warm moist air flows into the plains from the Gulf of Mexico, and cool dry air flows from over the Rocky Mountains in the west. Eventually a change of temperature or pressure will arrive and lift that warm air up into the cool air, forming an updraft (an upward movement of air).
Once they meet, the moisture from the warm air begins to condense, forming clouds and the beginning of a thunderstorm.
Under normal conditions, rain would fall from these clouds and cool the warm air, breaking the storm. But in Tornado Alley, there’s a strong air current flowing from west to east, known as the Jet stream. This paired with the cool mountain air blows the rain away, keeping the air in the updraft warm and wet, which allows the storm to intensify.
For a storm to then rotate, winds need to be moving at different speeds and directions, and Tornado Alley has that in abundance.
The air coming from the Gulf moves slowly into the plains. Meanwhile the Jet stream from the mountains provides a steady stream of high, fast-moving air flowing east. Because the Jet stream is flowing faster and in a different direction, it causes the Gulf, when below, to rotate like a spinning football. When the spinning air gets pulled into the updraft it is tilted but continues to spin – causing the entire updraft to rotate.
Storms like this are known as “supercells” and they create prime conditions for tornadoes. They’re rare, but most commonly occur in Tornado Alley.
As the supercell grows the spiraling updraft begins to stretch towards the ground and forcefully pulls air into the cyclone. Air rushes in from the sides and a spinning dust cloud forms below, which brings us to the final stage – getting the vertically spinning air to the ground.
As more air is pulled in tightly, pressure builds and the faster and longer that tornado gets. It stretches closer to the ground until it eventually meets with that dust cloud. And then, it touches down.
In Oklahoma, known as the tornado capital of the world, winds have previously reached a mind boggling 400 kilometres per hour.
However, many scientists and experts in recent years have warned that people living in southern parts of the country are just as much at risk of tornadoes as those in the Plains are.
Research reveals that as the planet warms, Tornado Alley is starting to shift east into the Mississippi River Valley, as evidenced by last week’s deadly tornado which hit southern states.
Apart from the US, southeastern Brazil and northeastern Argentina have some of the same ingredients Tornado Alley does: cool mountain air coming over the Andes, and warm moist air coming from the Amazon.
Another is Bangladesh, where warm humid air spreads from the Bay of Bengal and travels north, where it overlaps with winds blowing southeast out of the Himalayan Mountains.
Averaging around six tornadoes a year, Bangladesh also experienced the world’s deadliest tornado back in 1989, which killed 1,300 people and injured 12,000.