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Tornadoes: The Science Behind the Destruction

How tornadoes form and how they die is not fully understood, yet scientists probing those mysteries’ and aiming to improve warning systems’ have pinpointed key risk factors.

A tornado, or twister, is a violently rotating column of air that extends between the Earth’s surface and a cloud, usually a cumulonimbus cloud.

Most tornadoes last for less than ten minutes. Large tornadoes usually last longer’ around 30 minutes. The most powerful twisters have wind speeds of more than 300 miles (483 kilometres) per hour, which can rip buildings off their foundations. They can be more than two miles (3.2 kilometres) wide, and can spin across the ground for dozens of miles.

The more common tornadoes have wind speeds of less than 110 miles (177 kilometres) per hour, are about 250 feet (76 metres) across, and travel only a few miles before they dissipate.

Tornadoes kill an average of 60 people a year in the US, mostly from flying or falling debris. Half of those deaths are caused by the strongest one per cent of the most violent storms.

How Tornadoes Form

True tornadoes, unlike smaller swirling winds like dust devils and waterspouts, emerge from what are called supercell thunderstorms. For such a storm to form, you first need the ingredients for a regular thunderstorm which include warm moisture near the surface and relatively cold, dry air above. The warm air will be buoyant, and like a hot-air balloon it will rise.

A supercell requires more: winds that increase in strength and change direction with height. The supercell churns high in the air and, in about 30 per cent of cases, it leads to the formation of a tornado below it.

Scientists believe strong changes in winds in the first kilometre of the atmosphere and high relative humidity are important for the formation of tornadoes. There also needs to be a downdraft in just the right part of the storm.

Tornado formation also requires a “Goldilocks” situation, in which air must be cold but not too cold. It should be a few degrees more frigid than surrounding air.

Where and When Twisters Strike
Tornadoes have been observed on every continent except Antarctica. They have been most documented in North America, where an estimated 1,200 strike the United States each year, but they frequently appear in many other countries.The most notoriously affected region in the United States, called “Tornado Alley,” includes the Great Plains states of Kansas, Nebraska, and the Dakotas, as well as parts of Texas. Large-scale weather patterns tend to converge on that area, making tornadoes more likely.

Still, the state that receives the highest number of tornadoes per square mile is Florida, according to the American Meteorological Society. Indiana, Illinois, Iowa, and Louisiana also have many tornadoes per square mile.

Tornadoes can happen at any hour of the day and any time of the year, though they are most common in the spring, especially during May and June in North America.

In many countries, including the United States, Canada, and continental Europe, the strength of tornadoes is often measured by the Fujita scale or the updated Enhanced Fujita Scale. An F0 or EF0 tornado damages trees but substantial structures are left unharmed; a tornado in the strongest category ‘F5 or EF5 ‘blows away buildings.

Since measuring wind speeds inside a twister is extremely difficult, scientists typically rely on damage to estimate velocities.

The Difficulties of Forecasting

Tornadoes are much harder to forecast than are hurricanes, which are larger storms that last a lot longer. According to NOAA, the average amount of time between a tornado warning and the arrival of a storm is about 13 minutes. A tornado warning means a twister has been sighted, while a tornado watch means one is possible.

Predicting the path of a tornado across the landscape can also be challenging as tornadoes tend to follow the general movement of the thunderstorm they are associated with, but the route can be erratic.

How a Tornado Forms
While tornadoes can differ in size, strength, and location, they all share certain characteristics. They are spawned from a type of rotating storm called a supercell thunderstorm.

WIND SHEAR
Fast-moving winds roll air below into a horizontal vortex ‘a spinning tube’ above opposing surface winds.

Updraft
Warmed by the sun, buoyant air near the ground begins to lift a section of the horizontal vortex into a vertical position.

SUPERCELL
Upper-level winds tilt the rotating updraft, called a mesocyclone. This allows the storm to keep growing, as warmed air is sucked into the storm away from the cool downdraft.

Only a fraction of supercells (rotating thunderstorms) produce tornadoes.

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