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Savage tornado-spawning storms ripped across the southern and southeastern United States last week, killing at least 340 people and leaving thousands hurt or homeless. Meteorologists have graded two of the around 225 confirmed tornadoes as EF-5, the strongest category on the Enhanced Fujita scale. Winds whirl in these monsters at more than 200 miles per hour.

But it doesn't take such rare, finger-of-God twisters to tear a structure apart. Tornadoes in the EF-2 and EF-3 range packing 111- to 165-mile-per-hour winds can destroy single-family homes, according to experts from the Insurance Institute for Business & Home Safety (IBHS). Four seconds is all a moderately powerful tornado needs to wipe a foundation clean. "A whole house can come apart pretty fast," says Tim Reinhold, IBHS senior vice president of research and chief engineer.

The way a tornado attacks a home from one instant to the next depends upon variables such as the twister's internal wind speeds, size, path and pace along the ground. A residential building's orientation to a tornado and design particulars also greatly influence the outcome. (In short, no two tornadoes or homes are ever exactly alike.)

Nevertheless, years of field studies and simulations have revealed the general science of how tornadoes reduce homes to scrap heaps. This timeline tracks the destruction of a typical single-family, wood frame house built on a slab-on-grade foundation over just four seconds. These type of homes are common in the well-known "Tornado Alley" found in the Plains states, as well as in "Dixie Alley," the portion of the southern U.S. where killer tornadoes have struck once again.

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As a twister barrels toward a home, it brings flying debris that shatters windows and pounds away at the exterior walls. Because they're going so fast, the winds blowing over the roof exert uplift, the same aerodynamic force that allows airplanes to fly. Roof shingles and possibly even pieces of the roof decking tear away, becoming part of the maelstrom as the twister's funnel begins its sweep over the unlucky home.

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Air rushes into the home through the busted windows, filling the structure with pressure like a balloon being inflated, Reinhold says. Internal pressure pushes up against the ceiling, joining the uplift on the roof from the gales outside in putting pressure on the roof. The relatively weak connections between the roof and the walls give way and the roof blows off.

"Homes are not designed to withstand tornadoes," says Timothy Marshall, principal engineer at Haag Engineering Co. and an expert on tornado damage. Building codes in non-hurricane designated areas—that is, everywhere except for southeastern Florida and off the coasts of Louisiana and Alabama—call for two 16-penny (3 1/2 inch) nails connecting roof trusses to exterior wall top plates. These connections are intended to gird homes against gusts up to 90 mph for 3 seconds at a height of 33 feet. But even an EF-1 tornado is capable of doling out more punishment.

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With the roof gone, the walls are next. "Unless there are a lot of interior walls bracing and going into them, the [exterior walls] are flimsy and not well attached to each other at the corners," Reinhold says. Without a roof, an ordinary home becomes a house of cards in the face of a tornado.

Though tornadoes spin in a cyclical motion, the fact that they're so big—with a typical footprint measuring 500 feet wide—means that a house is effectively hammered by straight-line winds. The side walls parallel to the direction of these winds will typically go first, Reinhold says, because they feel the most suction. The front, windward wall then gets pushed in by the tornado, and finally the back wall blows out, all within about a second.

About 40 years ago, documentation of this typical destruction pattern—one wall in, three out—overturned the notion that homes "exploded" due to low pressure inside a tornado's vortex. Actually, people were once taught to open their windows ahead of a tornado. Now the opposite is now known to be true: It's those homes whose envelopes do not rupture that tend to survive, Reinhold says.

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The last bits of a house that a tornado strike sweeps away are the interior walls. "A progressive failure begins top-down, then outside-in," Marshall says.

Small central rooms, therefore, such as a bathroom or closet, are probably the best bet for survival if you don't have a basement or tornado shelter, according to Randy Shackelford, a research engineer with Simpson Strong-Tie, the world's leading makers of structural connectors to shore up homes against high winds. "An interior room gives you a couple extra layers of protection that debris has to go through before it hits you," he says.

Without exterior walls or a roof to brace them, however, these inner walls can easily collapse or get blown over in the twister's fury. To avoid being crushed by the tumbling walls, occupants should try to hide under sturdy objects, such as stairwells or tables. Because tornadoes usually move through a home-sized area so quickly, the surging winds might just pass before reducing the entire edifice to rubble, sparing the lives of those cowering inside.