How Do Tornadoes Form? The Spinning Storms of the Great Plains
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The winds in the strongest tornadoes pass 480 kilometres an hour, the fastest measured on the surface of the Earth, and they do it in a funnel a few hundred metres wide that may last ten minutes. About twelve hundred touch down in the United States each year, more than in the rest of the world together, mostly in a band from Texas to Iowa that has been called Tornado Alley since the 1950s. They come out of a particular kind of thunderstorm, and how that storm begins to spin is understood much better than how the spin reaches the ground.
The supercell
Ordinary thunderstorms rise and collapse within an hour, their own rain falling through the updraught and choking it. The tornado-producing storm is different. A supercell is a thunderstorm whose updraught rotates, and the rotation comes from wind shear: when the wind near the ground blows from one direction and the wind a few kilometres up blows faster from another, the air between them rolls, like a pencil between two hands, into a horizontal tube of spin. A thunderstorm's updraught draws that tube upward and tilts it vertical, and the updraught itself begins to turn. A rotating updraught, called a mesocyclone, can be several kilometres across, and it organises the storm so that rain falls to one side rather than into the rising air, letting the storm live for hours and travel hundreds of kilometres.
From cloud to ground
Most supercells never produce a tornado, and the step from a rotating storm to a funnel touching the ground is the least understood part. The current picture is that cool, rain-chilled air descending at the rear of the storm reaches the surface and is drawn back into the updraught, tightening the rotation near the ground the way a skater spins faster by pulling in her arms, and stretching it upward into a narrow, intense vortex. The condensation funnel that makes a tornado visible is water vapour condensing in the low pressure of the core; the tornado may be on the ground before the funnel appears, marked only by a swirl of debris. Once formed, the vortex is fed by the storm above it, and it dies when the cold outflow wraps around and cuts off the warm inflow.
Tornadoes also form without supercells, weaker ones spun up along the leading edge of a squall line or under a growing cumulus cloud over land or water, where they are called landspouts and waterspouts. The violent, long-lived ones are almost all from supercells.
Why the Plains
The central United States has a geography no other place matches. Warm, wet air flows north from the Gulf of Mexico at low level; dry air flows east over the Rockies above it, and cold air comes down from Canada, with the jet stream overhead to supply the shear. The mountains to the west and the flat land to the east let the layers stack without obstruction, so that in spring the ingredients for a supercell meet over Oklahoma more reliably than anywhere on Earth. The ingredients:
- •Warm, humid air near the ground, supplying the energy
- •Cold, dry air above it, making the atmosphere unstable so that the warm air rises fast
- •A lid of warm, dry air in between that holds the energy in until it breaks all at once
- •Wind shear, the change in wind speed and direction with height, to make the updraught rotate
- •A trigger, such as a cold front or the afternoon sun, to start the air rising
Measuring what cannot be measured
A tornado destroys any instrument placed in its path, so its strength is judged afterwards from the damage. The Fujita scale, devised in 1971, and the Enhanced Fujita scale that replaced it in 2007 run from EF0, winds around 105 to 137 kilometres an hour that break branches and damage roofs, to EF5, over 322 kilometres an hour, which sweeps well-built houses off their foundations and carries cars for hundreds of metres. About four in five tornadoes are EF0 or EF1; fewer than one in a hundred reach EF4 and EF5, and those cause most of the deaths. The deadliest in American history, the Tri-State tornado of March 1925, travelled 350 kilometres across Missouri, Illinois and Indiana in three and a half hours and killed 695 people; the 2011 Joplin tornado killed 158 and is the costliest.
Warning
In 1925 nobody knew the storm was coming. Today Doppler radar can see the rotation of a mesocyclone from a hundred kilometres away, and since the 1990s the national network of such radars has raised the average warning time to about thirteen minutes, enough to reach a cellar or an interior room. Forecasters can also say days ahead that an outbreak is likely, and the storm chasers who follow supercells across the Plains, some for science and some for spectacle, have supplied the close-range measurements and film that filled in the picture of how the funnel forms. What the warnings cannot yet do is say which supercell will produce a tornado and which will not, and the false alarm rate remains above half, which is the problem the research is now aimed at.
Myths that put people in danger
Some long-repeated tornado advice is wrong. Opening windows to let the pressure equalise does nothing useful; the damage to a house comes from wind and flying debris, not from the building exploding, and time spent at windows is time exposed to breaking glass. Sheltering under a motorway overpass is dangerous rather than safe, because the gap beneath the bridge can funnel wind and debris into a faster stream. The safest place is the lowest floor, in a small interior room without windows, with something sturdy over your head to stop falling objects.
Other myths concern geography. Tornadoes do not avoid cities, rivers, lakes or hills; cities are simply small targets compared with the open country around them, and many have been struck directly, including the suburbs of Oklahoma City more than once. Nor is the south-western corner of a basement a special refuge, a piece of folklore that predates careful studies of damage. The advice that has held up is plain and effective: know where your shelter is before the season starts, act on a warning at once, and protect your head and neck.
Which way they spin, and why
Most tornadoes in the Northern Hemisphere spin anticlockwise when seen from above, and most in the Southern Hemisphere clockwise. It is tempting to credit the Coriolis effect of the Earth's rotation directly, but a tornado is far too small and too fast for that effect to steer it. The spin is inherited from the parent storm's mesocyclone, which gets its rotation from wind shear. The Earth's rotation enters further back, by shaping the large weather systems that produce the typical shear, with winds turning clockwise with height ahead of a Northern Hemisphere low, which in turn favours storms that rotate anticlockwise.
Because the link is indirect, exceptions exist. A small minority of tornadoes spin the opposite way, most of them weak, and waterspouts and landspouts, which pick up their spin from local wind boundaries rather than a mesocyclone, can turn either way. The same misunderstanding attaches to sinks and toilets. The Coriolis effect is real at the scale of oceans and weather systems, but in a bathroom it is swamped by the shape of the basin and the faint swirl left from filling it, so water can drain in either direction in either hemisphere.
Tornadoes beyond Tornado Alley
The United States has the most tornadoes, but not the deadliest. That grim record is usually given to a tornado that struck the Manikganj district of Bangladesh in April 1989, estimated to have killed around 1,300 people. Bangladesh and eastern India get relatively few tornadoes, but they arrive with little warning in some of the most densely populated farmland on Earth, where many homes are built of light materials and there are no basements. The death toll from a tornado depends as much on how many people are in its path, and what they can shelter in, as on how strong it is.
Elsewhere the numbers can surprise. The United Kingdom records a few dozen tornadoes in a typical year, and relative to its land area that is one of the highest rates in the world, though almost all are weak, short-lived and spun up along lines of heavy showers rather than by supercells. Europe as a whole sees hundreds a year, and a few are violent, such as one that tore through villages in the Czech Republic in 2021. South America has a second great hotspot over the plains of Argentina, southern Brazil and Uruguay, where moist air from the Amazon meets dry air from the Andes.
The takeaway
Tornadoes form beneath supercell thunderstorms whose updraughts rotate because wind shear rolls the air into a spinning tube that the storm tilts upright; cold outflow near the ground then tightens and stretches the rotation into a narrow vortex that reaches the surface. The central United States gets more than anywhere else because Gulf moisture, Rocky Mountain dry air, Canadian cold and the jet stream meet over flat ground; strength is rated from damage on the EF scale, and Doppler radar now gives about thirteen minutes' warning.