Huffing, Puffing, Blowing Your House Down
When I asked a weather expert if Alaska has tornadoes, I got a quick answer: No tornado has been officially recorded here. Unofficially, tornadoes---or at least funnel clouds---are reported most years. Because Alaska has few trained observers but many pilots, boaters, campers, and other wanderers watching the state's skies every summer, casual observations are easier to come by than official ones.
Yet it isn't only that Alaska's wide open spaces make tornadoes seem rare here. They are rare. Our weather and terrain, like those in the neighboring chunks of Canada, just don't make for ideal tornado habitat.
That's something for which we can be truly glad. A tornado can do an astounding amount of damage. The June 1991 issue of Weatherwise magazine contains an article by C. Hugh Snyder, a retired National Weather Service meteorologist, that offers clear explanations of the behavior of these peculiar and dangerous storms as well as some examples of their destructiveness.
Snyder's article also debunks a common but wrong explanation of one tornado effect. After a twister has passed through a residential area, what look like exploded houses are likely to be among the ruins. Walls will be fanned outward; roofs will lie atop foundations, covering furniture and floors. As it was explained to me, the walls seem to have been blasted outward while the roof did not blow away because the house exploded. The air pressure at the core of the revolving funnel clouds is so low that the air of normal pressure trapped inside a house blows it apart. Thus windows should be left open when a tornado threatens, so the air inside can escape without damaging the house.
That sounds good, but it isn't what happens. According to Snyder, recent research shows that lift is the real culprit---lift exactly analogous to the force that allows a big jetliner to take off. With the airplane, wind striking a wing's leading edge produces reduced air pressure above the wing airs increased pressure below it. A loaded DC-10 becomes airborne when its speed down the runway reaches about 220 miles an hour. The lift then on each of its 3647 square feet of wing area reaches 159 pounds, and all 290 tons of it can take off.
This airfoil effect seems to be what destroys buildings. Envision what happens as a mass of racing air hits a house. The front wall stops the wind's forward progress (in itself quite a test of a wall's strength). The obstacle forces the air mass up and over the roof and around the other sides. That reduces the air pressure outside the building, generating airfoil-type lift on the roof and outward thrust (sideways lift, so to speak) on the side and lee walls.
High-speed winds can generate considerable lift, as the DC-10 example above illustrates. Structural engineers have calculated that tornado-speed winds of 300 miles an hour could put lifting forces of over 200 pounds per square foot on a roof. Even winds of 160 miles an hour, moderate ones 8 for a tornado, would exert over 31 tons of lift on the roof of a 1000-square-foot house. Leaving a window open wouldn't help that.
Thus, when the tornado has passed and the photographers come out, among the things they record are the unsuccessful attempts of houses to fly. What happens to a house in the turmoil of the storm is that its roof lifts, breaking the reinforcing ties holding the walls in position. The windward wall then blows in while the lee and side walls blow out. The roof falls down atop what looks like an exploded house...but it's actually one that crash-landed.
