Tornado watch : Issued by the National Weather Service when weather conditions are ripe for tornadoes. Remind your family where to find shelter. Turn on a radio or television and listen for announcements. Since tornado prediction is an inexact science, you may get little warning of an actual funnel cloud.

Tornado warning: A tornado has been sighted (visually or on radar). The danger is serious! Go to a safe place, turn on a battery-powered radio and wait for instructions or an all-clear.

Light in weight, but with large surfaces to catch the wind, in a tornado a mobile home is a disaster waiting to happen. Courtesy James McDonald, Texas Tech University.

"Go to a safe place" sounds simple. But where's safe?

The watchword is down. If your house has a basement, go there. Otherwise, look for an interior room, like a closet or bathroom. Try to hide under a sturdy piece of furniture and protect your head and neck with your arms, blankets or a mattress.

If in a mobile home, go outside, preferably into a basement, or a ditch or even a depression in the ground. Mobile homes have a disastrous tendency to collapse or explode in tornadoes.

Protecting your home from twisters
It would be nice to protect your home against a direct hit, but that's not possible, says Ron Wolfe, a research engineer at the U.S. Forest Products Laboratory, in Madison, Wis. Wolfe, who has investigated storm damage from hurricanes and tornadoes, says, "The drop in pressure can be quite dramatic, and the house will just blow apart."

For lack of anchors, a house blew off this foundation in Barneveld, Wisconsin. Anchor bolts should be sticking up every four feet or so along this foundation, as required by common sense and building codes. Courtesy Ron Wolfe, U.S. Forest Products Laboratory.

But direct hits are rare. Even in tornado alley, a twister hits a given square mile only once every 700 years, Wolfe adds, "It's not economically feasible to build a house to resist that kind of wind. That's why you get insurance."

Still, many houses close to a twister are damaged or destroyed by wind, rain and flying debris. These homes-and their occupants -- can benefit from simple, relatively cheap measures to drastically reduce damage, Wolfe adds.

Many of these improvements concern the roof, which often fails first in windstorms. Once that happens, Wolfe says, torrential rain can soak the insulation and drywall and the walls, no longer braced from above, can collapse.

Roof shingles are usually the first to go, Wolfe says. Shingles near roof edges, which face the worst winds, should be set in special mastic during reshingling.

Roof sheathing, usually a material like plywood, should be nailed securely to the rafters. Nails should be 6 inches apart at the edges, and 12 inches apart elsewhere.

Rafter fastening is critical. These angled beams, which support the roof sheathing, were traditionally nailed to the walls. This is extremely weak, Wolfe says, partly because the nails tend to split the rafters. Simple, cheap "hurricane clips" make a connection that is 5 to 15 times stronger.

Foundation anchors keep a house from blowing away in one piece. Even though Wisconsin building code required that houses be bolted to foundations, Wolfe says the attachments were missing from many wrecked homes in Barneveld. Although builders thought the attachments were unnecessary, he says, the tornado proved them wrong.

More advice on strengthening your home against ferocious winds.

If you're building a new home in an area prone to tornadoes, you may consider building a tornado room. Select an interior room with no windows and use steel doors and reinforced concrete to create a room that is practically bombproof. Because of the degree of modification needed, including good fastening to the ground, Norbert Delatte, professor of civil engineering at the University of Alabama at Birmingham, says tornado rooms are almost impossible to fit into an existing house. "You pretty much have to do it with new construction, you have to build the walls a new way, using reinforced concrete or concrete block with reinforcing and grout." Here's information on safe rooms.

Mobile home? Ideally, not Air-mobile...
Mobile homes present special problems in tornadoes. The best advice is to leave them for underground shelter, says James McDonald, professor of civil engineering at Texas Tech University in Lubbock, who also directs the Institute for Disaster Research. "There's not a whole lot you can do because of the way they are constructed. It's a very lightweight structure with a large, exposed surface area ... that's built with very little margin for error."

A 1993 federal regulation, passed in response to Hurricane Andrew, raised building standards. New mobile homes must withstand 30 to 40 percent faster winds, McDonald says (previously, homes were supposed to survive winds of only 60 mph). Stronger fastenings must be used inside the home, especially at exterior corners.

But a mobile home that remains intact must also remain in place. The standard anchoring technique, McDonald says, is to drive big steel screws into the ground. "But there's a catch-22. If the ground is soft enough for the anchor to penetrate, the soil is too soft to prevent it from pulling out. If the ground is stronger, the stake won't go in deeply enough."

McDonald (see "Review of Standard Practice. ..." in the bibliography) notes that "95 percent of mobile homes are never moved after they are placed." Thus he advocates anchoring mobile homes to concrete foundations, an expensive measure but one that can be done to existing homes.

Still, given the drawbacks of lightweight construction and lack of a basement, McDonald says he would not feel comfortable riding out a tornado warning in a new-standard mobile home, even if it was bolted to a concrete foundation.

Time for fun. Want to make a twister in a bottle?

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