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Weather isn't always predictable. If it were, daily forecasts would be spot-on, hurricanes anticipated, and picnics safe from abrupt summer downpours. Instead, climate systems are complex, and tornadoes are no exception. So, whereas guessing the direction of a tornado's torque is possible, like any weather prediction the forecast will only be correct most of the time.
It's true that tornadoes tend to revolve counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. However, according to research meteorologist Richard Rotunno of the U.S. National Center for Atmospheric Research in Boulder, Colo., the opposite has also occurred. There has even been the occasional appearance of both counterclockwise and clockwise rotating tornadoes under the same thunderstorm. These deviations undercut the common misperception that the direction of a tornadoes' spin results from the Coriolis force.
To set the record straight, Rotunno explains that the Coriolis force only has a significant influence on the spin direction of Earth's largest atmospheric and oceanographic circulation systems, such as the Gulf Stream, jet stream, trade winds and hurricanes. Earth's rotation around its axis causes this effect, making Northern Hemisphere winds deflect to the right and those in the Southern Hemisphere deflect to the left. It is also why an airplane flying from Anchorage to Miami must consider the Earth's counterclockwise rotation (as seen from the North Pole) to land at its destination, instead of splashing into the Gulf of Mexico.
The Coriolis force isn't, however, omnipotent, compelling all currents great and small to spin counterclockwise when north of the equator and clockwise to its south. Though many people have seen videos of toilets flushing in Australia and the U.S. that swirl in opposite directions, these experiments are based on luck and, perhaps not surprisingly, the toilets' varying designs. Pranksters have even gone so far as to blame the Coriolis effect for hair curling in a certain direction.
Despite the large amount of misinformation, toilets—and even tornadoes—are too small to be affected by the Coriolis, whose force would only begin to directly influence a storm's swirling mass if it were approximately three times larger than the supercell storm systems that typically generate tornadoes.
"Tornadoes are only indirectly influenced by the Coriolis force," says meteorologist Harold Brooks of the National Oceanic & Atmospheric Administration's National Severe Storms Laboratory in Norman, Okla. The majority of tornadoes happen in "tornado alley," in the Great Plains of the U.S., but they can happen anywhere in the world, including southern Brazil, northeastern Argentina and Bangladesh. These violently roiling columns of air originate from parent thunderstorms called supercells. In the U.S., supercells form when dry polar air from Canada meets moist tropical air from the Gulf of Mexico, causing the warm air to rapidly rise.
The upwelling current of air within a thunderstorm is referred to as an updraft. "If sufficient vertical wind shear (the increase of wind speed with height) exists, this updraft will rotate," Brooks says. "Tornadoes usually rotate in the same direction as the thunderstorm they're associated with." Therefore, if the warm winds blowing north from the equator meet cool upper-level winds out of the west, the tornado will rotate counterclockwise. And if the warm equatorial winds blow to the south and clash with aloft winds, a tornado will rotate clockwise.
This is because in both hemispheres, upper-level winds blow out of the west due to planetary rotation. These winds are Coriolis's subtle claim to a tornado's torque.
Although understanding Coriolis's weak influence over the direction of a tornado's spin seems feasible, fully grasping how tornadoes function may not be. And predicting exactly when and where tornadoes will occur-and which way they will spin-seems even less so. Uncertainty may be the only certainty of weather.