How do tornadoes damage homes?

Pressure pries up roofs, sends them sailing

November 17, 2010|By Julie Scharper, The Baltimore Sun

The tornado-like storm that whipped through Northeast Baltimore Wednesday morning stripped away roofs, windows and even roofs, leaving apartments as open as a child's dollhouse.

Residents awoke to find cars jumbled in a heap, furniture scattered across lawns and wooden roofs that had sailed hundreds of feet away — in some cases, across Northern Parkway.

How does a tornado, essentially a whirling column of air, create enough force to tug a roof off a building and send it hurtling like a hockey puck? The explanation is related to the difference between the air pressure inside the structure and the air outside, as well as the force exerted by air molecules themselves.

As a tornado or other storm system passes over a building, low pressure can tug a roof upward, scientists say.

And the winds to the side of it exert a high pressure that pushes roofs up from the sides, said Jewel Barlow, a professor of aerospace engineering at the University of Maryland College Park, who directs the Glenn L. Martin Wind Tunnel at the university.

When those forces surpass the force exerted by the weight of the roof, the structure flies up and is swept away by wind currents, he said.

"If you watch the leaves fly by, it's the same thing at a lower speed," Barlow said.

Most roofs are held down by nails and the force of their own weight, making it relatively easy for a tornado to flip them into the air, he said. In Florida, laws were passed following Hurricane Andrew in 1995 that require roofs to be strapped to the walls and foundations of new homes, he said.

The shape of roofs can make some easier to pry up than others, Barlow said. "Because most roofs are peaked, there is a tendency to create that suction where the flow goes over the top of it," he said.

And a tornado does not need to pass directly over a building to damage it, said Bruce Barnett, a physics professor at the Johns Hopkins University.

The difference in pressure between a tornado and the interior of a house can make windows pop, he said.

"A house tends to explode when a tornado goes by," said Barnett.

A similar difference in pressure is created when a car travels down a highway with the windows closed, he said. The pressure outside is lower than inside the car. When the windows are opened, the pressure is equalized, occasionally causing passengers' ears to pop, he said.

Tornadoes can lift up a building, and, occasionally pick a home off its foundation and set it down a few feet away. But — unlike in "The Wizard of Oz" — structures that are swept high in the air by a storm are generally destroyed by strong winds, the scientists said.

julie.scharper@baltsun.com

twitter.com/juliemore

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