Tornadoes

A tornado is a violently rotating column of air that reaches from a thunderstorm cloud all the way to the ground. They are some of the most destructive natural forces on Earth: wind speeds can exceed 400 km/h inside the strongest tornadoes, easily strong enough to flatten houses and throw cars hundreds of metres. Most tornadoes happen in the central United States, in a region nicknamed Tornado Alley, where roughly 1,200 occur each year. The UK has a few weak tornadoes too (around 30 per year) but nothing remotely on the American scale.

  • Wind speedsUp to 480 km/hIn the most extreme tornadoes
  • USA tornadoes per yearApproximately 1,200Most in the world by far
  • Tornado seasonSpring and early summerApril to June in the US
  • Strongest scaleEF5On the Enhanced Fujita scale
  • Average lifetime10 to 15 minutesBut some last over an hour
  • UK tornadoes per yearApproximately 30Mostly weak

How tornadoes form

Tornadoes form inside special kinds of thunderstorms called supercells. A supercell has a large rotating updraft in the middle, called a mesocyclone. The rotation comes from differences in wind direction at different altitudes: at ground level, wind may be blowing one way, while higher up it is blowing the other way. This wind shear makes the air spin.

When the spinning mesocyclone is tilted upright by the rising warm air, and air below the cloud is sucked rapidly upwards, a narrow rotating funnel can drop from the base of the cloud. If the funnel reaches the ground, it becomes a tornado. Most tornadoes only last 10 to 15 minutes, but the strongest can last over an hour and cut paths over 200 km long.

The Enhanced Fujita scale

Tornado strength is measured on the Enhanced Fujita (EF) scale, from EF0 to EF5, based on observed damage.

  • EF0: 105 to 137 km/h. Light damage. Branches broken, road signs knocked down.
  • EF1: 138 to 177 km/h. Moderate damage. Roofs damaged, mobile homes overturned.
  • EF2: 178 to 217 km/h. Considerable damage. Roofs torn off, frame houses shifted.
  • EF3: 218 to 266 km/h. Severe damage. Trains overturned, most trees uprooted.
  • EF4: 267 to 322 km/h. Devastating damage. Well-built houses levelled, cars thrown.
  • EF5: 322 km/h+. Incredible damage. Even reinforced buildings destroyed. Very rare.

Tornado Alley

The central United States gets more tornadoes than the rest of the world combined. The reason is geography: warm wet air from the Gulf of Mexico meets cold dry air from Canada over a flat landscape with nothing to stop them mixing. The collision zone, stretching from Texas up through Oklahoma, Kansas, Nebraska and the Dakotas, is called Tornado Alley. It produces approximately 700 tornadoes a year, far more than any other region of the world.

A second tornado-prone region called Dixie Alley stretches across the south-eastern United States (Mississippi, Alabama, Georgia). Dixie Alley tornadoes are particularly dangerous because they often happen at night and in heavily wooded areas, making them hard to see.

Fact The fastest wind speed ever measured was inside a tornado that hit the suburbs of Oklahoma City in May 1999. Doppler radar measured wind speeds of approximately 484 km/h inside the funnel: the fastest natural wind ever recorded on Earth. The tornado was an EF5, killed 36 people, and cut a path 60 km long through the city's outskirts.

Famous tornado disasters

  • Tri-State Tornado, 1925: the deadliest in US history. Cut a 350 km path across Missouri, Illinois and Indiana. Killed 695 people.
  • Bangladesh Daulatpur-Saturia Tornado, 1989: the deadliest in world history. Killed approximately 1,300 people and made 80,000 homeless in one of the most densely populated parts of the world.
  • Joplin Missouri Tornado, 2011: an EF5 that tore through Joplin, killing 158 people and destroying approximately one-third of the city.
  • Birmingham Tornado, UK, 2005: a small F2 (the European tornado scale) tornado that damaged hundreds of buildings and injured 19 people in the suburbs of Birmingham. The most damaging UK tornado in modern times.

How to stay safe

If a tornado warning is issued in your area:

  • Get to the lowest level of a sturdy building, ideally a basement or storm cellar.
  • If no basement is available, go to an interior room on the lowest floor (bathroom, hallway, closet) without windows.
  • Cover yourself with a mattress or blankets to protect from flying debris.
  • Stay away from windows.
  • If outdoors and no shelter is available, find a ditch or low area to lie flat in. Avoid hiding under bridges.
  • Never try to outrun a tornado in a car. They are unpredictable and can change direction quickly.
Did you know? The UK actually has the most tornadoes per square kilometre of any country in the world, with approximately 30 per year in an area roughly 1/40th the size of the USA. Most UK tornadoes are very weak (F0 or F1) and rarely cause serious damage, although a 2005 tornado in Birmingham did damage hundreds of buildings. UK tornado season peaks in late summer.
Deeper dive: storm chasers and the science of tornadoes

One of the most unusual scientific communities in the world is the storm chasers: scientists, photographers and enthusiasts who deliberately drive towards severe thunderstorms to study and photograph them. The community is largely based in the United States, where Tornado Alley provides plenty of opportunities every spring.

Scientific storm chasing started in the 1970s and 80s, when meteorologists realised they could learn much more about tornado formation by being on the spot than by relying only on radar from a distance. Famous early scientific chasers like Howard Bluestein at the University of Oklahoma pioneered the use of mobile Doppler radar mounted on trucks, which can drive close to a tornado and measure its internal wind speeds in real time.

The work has paid off. Tornado warnings used to be issued only after a tornado was already on the ground. Today, advance warnings are typically issued 10 to 15 minutes before a tornado forms, giving people time to take shelter. The improved warnings have dramatically reduced tornado deaths in the United States despite the country having many more buildings to be hit.

Storm chasing is also dangerous. Several scientific chasers have been killed in tornadoes, most famously the experienced researcher Tim Samaras and his team in 2013, hit by an unexpectedly fast-moving EF3 tornado in Oklahoma. The work continues, but with greater respect for the violence of the storms.

For more, see thunderstorms, wind and hurricanes.