Understanding tornadoes puts scientists in a whirl

Tornadoes are nature's most spectacular storms: a funnel of wind, spinning at speeds greater than 200 miles per hour, leaps from the bottom of a thunderstorm and cuts a swath of destruction, shredding farmhouses, uprooting trees and picking up automobiles and hurling them a quarter-mile.

Despite their frequency - more than 800 tornadoes occur in the US each year - scientists know relatively little about them and how they do the damage they do. Tornadoes are short-lived, erratic and violent. They can be extremely unkind both to instruments and the researchers who operate them.

"I've been analysing their effects for 20 years," said University of Arkansas civil engineer R. Panneer Selvam. "You can look at the damage or use a wind tunnel, but these are all estimates based on a straight wind, and in a rotating tornado there are so many variables."

A month after 583 tornadoes struck 17 states in 10 days in early May, Selvam attended the International Conference on Wind Engineering in Lubbock, Texas, where he presented a computer model describing the forces a tornado imposes on any structure unfortunate enough to be caught in its path.

He concluded that when the tornado engulfs a building, the air in the funnel has no way to go but up. While scientists have long known that tornadoes create updrafts, Selvam suggested for the first time that the upward pressure on the sides and roof of a building may be as great as 10 times the force of gravity.

Moreover, "the pressures are much higher at different places" on a building's surface, Selvam said, and these pressure points change constantly because of wind turbulence and interaction with the diverse surfaces of a fixed structure.

Selvam said that although he still must add variables to improve the quality of the model, his findings could have important implications for building design because they could one day allow engineers to quantify tornado stresses and set standards for overcoming them.

"The state of knowledge on this subject defines how the building will behave in a straight wind," Selvam said. "But nobody has been able to do this with a storm that is moving forward, rotating and going upward all at once."

Early predictions

Still, although computers may eventually predict the tornado forces that will threaten individual buildings, the research may have a long way to go. "Modelling the effect of a tornado on a building is very far-out science," said civil engineer James R. McDonald of Texas Tech University's Wind Science and Engineering Research Centre, host of the June conference. "We're just beginning to do that, and it has yet to be shown conclusively."

To prove the theory, scientists will have to demonstrate it, either by simulation in a controlled environment, such as a wind tunnel, or by studying it live in the middle of a storm. Neither task has been easy.

"Things change very rapidly, and the tornado's movement is not 100 per cent predictable, so it's really a hard place to collect information," said research meteorologist Harold Brooks of the National Oceanic and Atmospheric Administration's (NOAA) National Severe Storms Laboratory. "If you're using graduate students, you presumably want them to survive."

Tornadoes form in thunderstorms when warm, moist winds in the lower atmosphere give way to cold, dry winds coming from a different direction at higher altitude, creating a rotation in the storm. When rising air within the thunderstorm tilts the funnel, one end drops downward until it touches the ground. Only then does it become a tornado.

NOAA describes "weak" tornadoes as lasting about 10 minutes and generating winds of less than 110 miles per hour, while "strong" tornadoes are longer than 20 minutes with winds of as much as 205 miles per hour. "Violent" tornadoes can last more than an hour and have winds that reach 300 miles per hour.

And although tornadoes happen everywhere, no place on Earth works better than the midwestern Great Plains. "South winds bring the warm, moist air in from the Gulf of Mexico, and the Rocky Mountains give us high, dry, cold winds from the west," Brooks said. "It's perfect."

Brooks said scientists have a fair knowledge of how the storms work as they form in the atmosphere and begin to rotate, but the third stage - when the funnels touch the ground, and air begins to rush in and create an updraft - "is not very well understood".'

Scientists have created computer models to describe complicated forces operating inside the tornado, he added, "but if you introduce a building into the equation", as Selvam did, "it makes life even more complicated."

Vertical force

And even though a tornado exerts vertical forces on a trapped building that could be far more powerful than previously thought, those forces may be the least of the building's problems: "Wind travelling at 150 miles an hour is one thing," Brooks said. "But a 2-by-4 travelling at 150 miles per hour is something entirely different."

In fact, Brooks continued, flying debris not only poses the gravest danger to buildings and people, but also damages sensors, distorts radar images and generally plays havoc with scientists' efforts to gather data from the storm as it is happening.

In this context, added McDonald, the vertical movement is "relatively mild". When a tornado engulfs a building, it is the debris-laden rotational wind that "creates pressures on all the surfaces - tears the building apart and picks it up." But McDonald said measuring these effects is difficult: "We are doing some pioneering work to model the vortex and test scale models of buildings," McDonald said.

McDonald said the idea popularised by Dorothy on her trip to Oz, and shared, to some extent, by Selvam, was "a misconception". People "at the time of the story thought that a tornado was a giant vacuum cleaner," he said. "There is some of that, but Dorothy needed to pay more attention to the debris impact."