As another major tornado hits the United States – this time killing dozens and flattening buildings and neighbourhoods in Oklahoma City – Channel 4 News looks at the science behind the super storms.
At least 91 people, including 20 children, are feared dead after a devastating tornado tore through Oklahoma City in the United States.
While the storm is a tragedy, it is not a one-off. Tornados hit the south central states of the United States on a fairly regular basis, causing widespread destruction when they hit populated areas. In 2011, hundreds died when a twister hit Alabama. In just the last few weeks, tornados have hit Texas and Mississippi.
The geography of the area is a key factor in the tornado risk, says Tom Tobler, a forecaster at Meteogroup.
“In the plains of America where tornados form so frequently, you have the Gulf of Mexico to the south, so there is a lot of warm, moist air coming up from the south in the lower part of the atmosphere,” he told Channel 4 News.
It’s a very large tornado. It’s just very, very unlucky that it has struck such a populated area. Tom Tobler, Meteogroup
“Then the Rockies are to the west, so you get cold, dry air, and if that comes across from the west above the warm, moist air to the south, then that warm air wants to rise up through the cool air.”
If there is a “cap” in the atmosphere – a layer of hot air preventing the warm air from rising – this can make the warm air heat up faster and eventually when it breaks through the cap it leads to “intense convection”, a rapidly rising column of air.
Things get more serious, and can develop into tornados, if there is “wind shear” – wind blowing in different directions at different levels in the atmosphere.
“That gives you more powerful thunderstorms which helps to set that column of air rotating,” said Mr Tobler.
However, he warned that the finer details of tornados are still not completely understood, which can mean that predictions are tricky. In Oklahoma, residents only had 16 minutes warning of the storm.
“Usually the warnings are pretty good – but a tornado forming can be a rapid process, and it’s sometimes difficult to give too much warning. And the worst effects are often very localised, so you can’t forecast too far ahead where that is going to happen,” he said.
Another key factor in the devastation in Oklahoma is the sheer power of this tornado – it rates as an EF4 on the Fujita scale, one below the most powerful, EF5.
“It’s a very large tornado. It’s just very, very unlucky that it has struck such a populated area,” said Mr Tobler.