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.
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.