Tornadoes have killed dozens of people in five US states including at least 70 in Kentucky. The National Weather Service (NWS) says while tornadoes can occur almost anywhere, the United States is more prone to the devastating weather phenomenon. The powerful storms generally result from “supercell” thunderstorms that are characterised by extremely powerful updrafts. Yet, according to the National Severe Storm Laboratory, part of the National Oceanic and Atmospheric Administration (NOAA), much about tornadoes “remains a mystery.” The NOAA says tornadoes are “rare, deadly and difficult to predict, and they can deal out millions or even billions of dollars in property damage per year.”

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“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 that tornadoes develop extremely rapidly, and may dissipate just as quickly. “Most tornadoes are on the ground for less than 15 minutes.”

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“Tornadoes are nature’s most violent storms,” according to the NWS, with winds that can reach nearly 300 miles per hour (500 kph). Kentucky Governor Andy Beshear said the twister that wreaked havoc in his state had touched down for more than 200 miles (320 kilometers) in his state.

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On average, tornadoes claim 50 lives in the US each year, NOAA says.

Scientists evaluate a tornado’s 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.)