Tornadoes: devastating but still not well understood

Tornadoes are a frequent and often devastating weather phenomenon most commonly seen in the United States, but meteorologists are still unable to say exactly how they originate.“The US typically has more tornadoes than anywhere else in the world, though they can occur almost anywhere,” according to the National Weather Service (NWS).Hardest hit are Great Plains states like Kansas, Oklahoma and Texas, though they are also common in many other states, all east of the Rocky Mountains.Scientists still struggle to pinpoint the precise way in which these powerful storms form.“Much about tornadoes remains a mystery,” according to the National Severe Storm Laboratory, part of the National Oceanic and Atmospheric Administration (NOAA). “They are rare, deadly and difficult to predict, and they can deal out millions or even billions of dollars in property damage per year.”What is known is that they generally result from so-called “supercell” thunderstorms characterised by extremely powerful updrafts, according to NOAA.“Within the storm, a strong vertical wind shear causes a horizontally rotating cylinder of air. The updraft lifts the rotating cylinder within the supercell. The rotating cylinder of air narrows, becoming stretched, and spins faster and faster, forming a tornado.”The NWS notes: “Tornadoes develop extremely rapidly, and may dissipate just as quickly. Most tornadoes are on the ground for less than 15 minutes.”“Tornadoes are nature’s most violent storms,” according to the NWS, with winds that can reach nearly 300 miles per hour (500 kph). They can wreak devastation on a path more than one mile wide and 50 miles long -- or longer.The devastating tornado that killed dozens in Kentucky on December 11, 2021 stayed on the ground for 227 miles, said Kentucky Governor Andy Beshear. That would be a record, if confirmed.On average, tornadoes claim 50 lives in the US each year, NOAA said. The spring of 2011 brought the deadliest spate of tornadoes in recent history, with more than 580 people losing their lives in April and June. They caused damage estimated at $21bn.After a tornado passes, scientists evaluate its strength based on the damage inflicted and on measurements of wind speed. They then classify it using the Enhanced Fujita (EF) scale, which assigns ratings from EF-0 to EF-5. EF-0 means “light” damage and wind gusts of 65 to 85 mph, while EF-5 signifies gusts of over 200 mph and “incredible” damage. (Before 2007, the original Fujita scale used ratings of F-0 to F-5.)