Terrifying and mesmerizing at the same time, tornadoes have earned every inch of their fearsome reputation. They produce the strongest winds on the planet – close to 480kph/300mph. If that weren't enough, tornadoes can be astoundingly flaky. A tornado – often informally referred to as a twister – might last only seconds or churn for upwards of an hour. It can be as narrow as a house or as wide as a town. Its path might be a straight line, an arc, or even, on rare occasions, a circle.
The US is pockmarked by about 1000 twisters a year, far more than the number reported through-out the rest of the world combined. Each year a dozen or so American tornadoes are classified as “violent”, meaning their winds have topped 320kph/200mph. Yet the twister risk in many other places is underrated. The United Kingdom averages 33 tornadoes a year, though the majority of them are really quite weak.
Per unit land area, that's not too different from the rate in parts of the north-east United States. A few other places see enough tornadoes to take them seriously, including southern Canada, western Russia, France and the Low Countries, Australia, South Africa, the river valleys of eastern China and the pampas of Argentina. Many other countries will get a stray twister on occasion. The only spot that rivals the US for deadly tornadoes is the populous Ganges Valley of northeast India and Bangladesh, where hundreds of residents have been killed in the last couple of decades.
Tornadoes are masters at taking the spin in the air around them and rendering it lethal. Like a pirouetting ballerina who circles ever faster as she brings in her arms, tornadoes gain speed by tightening and focusing rotation in and near a thunderstorm.
Unlike other atmospheric swirls, such as dust devils, a tornado can only develop out of a parent thunderstorm. That's why the US is so twister-prone. The Great Plains, Midwest and South are prime breeding grounds for the violently rotating thunderstorms called supercells. They form as air arrives from several different directions at different heights. Sultry breezes flowing north from the Gulf of Mexico are overlaid by cooler, stronger west winds imported from the Pacific Ocean and wrung dry by the Rocky Mountains. Sometimes a warm, dry layer off the desert Southwest flows in between. These contrasts of temperature and moisture make for highly unstable conditions, and the change in winds with height helps induce in the storm a slow cyclonic turning (counter-clock-wise in the Northern Hemisphere). No other part of the world has quite the geographic blend to produce intense, rotating storms as frequently as North America does. Since tornadoes rely on thunderstorms, they're most common during the afternoon and evening in the spring and early summer – May is the peak US month – but they have been observed at all times of the day and throughout the year. European twisters have little seasonal preference.
In the 1970s and 1980s, dozens of storm chasers began prowling the Great Plains, many of them working for universities and federal labs. They quickly learned to recognize the visual cues that preceded tornadoes. Often a storms rotation is visible in the billowing walls of cumulus on its south end, where tornadoes are most likely. As the updraught intensifies, a broad, flat, rotating mass called the wall cloud may descend from the main storm. Hailstones bigger than walnuts or even grapefruit can fall, products of the strong updraught. Tapered lines of cloud extend outward, spoke-like, from the wall cloud.
With the help of visual reports like these, along with radar returns high-lighting the areas at risk, specific warnings are now issued in the US twenty minutes or more before many twisters develop, although some tornadoes still aren't caught by radar. More general areas of alert (called tornado watches), usually about the size of a Midwestern state, go into effect hours in advance. Yet the final ingredient that causes a tornado to emerge from a rotating storm still eludes researchers. Several large studies have canvassed the southern Great Plains since 1994. Using wind-probing Doppler radars mounted on trucks and tiny weather stations on top of sedans, scientists have probed tornadic storms in detail. They're now sifting through the results. Rain-driven downdraughts that descend next to a thunderstorm's main updraught appear to be a key piece of the puzzle.
Many tornadoes, especially outside the US, live and die in ways that depart from the standard picture. Conditions in a storm may only come together briefly to produce a short-lived, weak twister, and even in the States these far outnumber their more violent cousins. Although a weak tornado may develop winds of 160kph/100mph or more, its damage is typically limited by a width that averages less than 60m/200ft and a path that usually extends less than 16km/10 miles. By comparison, the most damaging US twister of recent years – passing through the south suburbs of Oklahoma City on May 3,1999 – spanned 1.2km/0.75 miles at its peak and traversed 61 km/38 miles. Its peak winds topped 320kph/200mph most of the way.
Tornadoes that emerge from supercells tend to rotate cyclonically, like their parent storms. This appears to be true in the Southern as well as Northern Hemisphere, although statistics are hard to come by. Tornadoes themselves are too small and fleeting to be ruled by the Coriolis force that makes large low-pressure centres rotate cyclonically, so a small percentage of twisters are anti-cyclonic.
What a tornado does
The brute force of the wind within a tornado can cause a great deal of damage, some of it weird and unexpected. Trains have been thrown off track, houses stripped to their foundations, and entire forests levelled. Cancelled checks and other bits of paper have travelled over 160km/100 miles after being lifted into the air by twisters. Even a humble piece of straw can penetrate a plank of wood when driven by tornadic winds. Although most creatures don't survive intact, some people have taken wild rides in tornadoes – via bathtubs, mattresses or other impromptu vehicles – and lived to tell their tales.
Tornadoes can seem capricious, destroying one house while leaving the one next door almost untouched. Careful analysis has shown that large tornadoes often have multiple vortices – mini-tornadoes that spin around the parent funnel.
These tiny whirls may leave narrow streaks of intensified damage. Building construction can vary as well: a brick house may be able to withstand the same force that levels its wooden neighbour. In the US, mobile homes are notorious for flying apart in twisters. Their occupants account for more than 40 percent of all US tornado fatalities between 1985 and 2005, far out of proportion to their share of the total populace. Cars and trucks are also quite vulnerable. The best shelter is a small interior room on the lowest floor of a sound building.
Officials once urged people to open windows if a tornado approached. The idea was to equalize the air pressure and keep the house from exploding. Engineers in the 1970s put this notion to rest. The air pressure inside even the strongest tornado is only about 10 percent less than normal. It's believed that tornadic winds lift up a building's roof by tucking under its eaves, after which the walls collapse (which can give the appearance of an explosion). Opening windows may actually increase the pressure on the roof and hasten destruction – not to mention putting the occupant in danger from windblown debris.