#1: "The southwest corner of a basement is the safest location during passage of a tornado."
The truth is that the part of the home towards the approaching tornado (often, but not always, the southwest) is the least safe part of the basement, not the safest. This is also true of the above-ground portion of the house. In most tornadoes, many more homes will be shifted than will be blown completely free of a foundation. Homes that are attacked from the southwest tend to shift to the northeast. The unsupported part of the house may then collapse into the basement or pull over part of the foundation, or both. Historically, the few deaths in basements have been caused by collapsed basement walls, houses, and chimneys, rather than by debris that was thrown into the basement from the outside.
For nearly a century, the published conventional wisdom was that the southwest corner of a building, both above and below ground, afforded the best protection. This misconception probably originated from someone's reasoning, rather than from actual observations. They probably assumed that deadly debris would be propelled over the southwest corner and land in the northeast corner.
The idea that it was safe to seek shelter on the side of a house facing the oncoming tornado dates back to at least the first book on tornadoes, the 1887 comprehensive text "Tornadoes", by John Park Finley. He placed in italic for emphasis the following remark: "Under no circumstances, whether in a building or in a cellar, ever take a position in a northeast room, in a northeast corner, or an east room, or against an east wall." He also recommended removing the furniture from the west-facing room and closing all windows in the house. This is all incorrect, deadly, and time-wasting advice. It is quite possible that someone has died following it. While relatively few people probably read the book when it was available, the advice was quoted in many newspapers. It is possible that in the limited number of damage surveys that Finley conducted personally, he came upon a grisly scene involving the northeast portion of a poorly constructed house that had fallen over, and it strongly influenced his thinking.
These assumptions went essentially unchallenged until 1966, when Professor Joseph Eagleman of the University of Kansas undertook a survey of destroyed produced by after the Topeka tornado of June 8th. Professor Eagleman's objective study showed that the south side and southwest corners, the direction of approach for the Topeka tornado, were the least safe areas, and the north side of homes were the safest... both on the first floor and in the basement. He repeated the study after the Lubbock, Texas tornado of May 11, 1970, and the results were even more striking. The southwest portion of the houses were unsafe in 75% of the damaged homes... double the percentage of unsafe areas in the northeast part of homes. As a general rule, people in basements will escape injury despite the extreme devastation above them. Being under a stairwell, heavy TABLE, or work bench will afford even more protection.
Ignorance of this conventional wisdom, combined with common sense, has saved lives in the past. At the Pacolet Mills near Gainesville, Georgia on June 1, 1903, 550 people ran to the northeast corner of the building as the tornado approached from the southwest. That northeast corner was the only part of the building not destroyed. At least fifty people died in other Gainesville fabric mills on that day, and more than 40 more died in homes near the mills.
#2: "Some towns are 'protected'!"
Various Native American tribes perceived tornadoes in different ways. Some saw them as a cleansing agent, sweeping away the ragged and negative things of life. Others saw them as a form of revenge for dishonoring the Great Spirit. Today, only the myths about the protection of towns by rivers and hills linger in modern American culture.
The Osage Indians, native to Kansas, Oklahoma, and Missouri passed on tornado legends to the early settlers. One such legend has it that tornadoes will not strike between two rivers, near the point where the rivers join. In the past 150 years, this idea may have given a false sense of security to some people who thereby failed to take shelter. They may not have lived to help debunk the myth. One by one, the myths that particular towns are protected have fallen by the wayside.
Emporia, Kansas, for instance, had sat "protected" between the Cottonwood and Neosho Rivers, in native Osage territory, for over a century. Emporia was free of damaging tornadoes until June 8, 1974 when a tornado killed six people and destroyed $20,000,000 worth of property on the northwest side of town. Another tornado did $6,000,000 in damage along the west side of Emporia on June 7, 1990. Part of the path of the 1974 tornado was also the site of a deadly twister on September 29, 1881, but the area was farmland then.
The idea that one's town is "protected" is a combination of wishful thinking, short memory, the rarity of tornadoes, and a distorted sense of "here" and "there." Proof of protection has been offered by a very simple statement of fact. The town has never been hit by a tornado, but 10 tornadoes have touched down "outside" of town in the past 30 years. The occurrence information may be fact, but the conclusion that the town must be "protected" does not logically follow.
That logic disregards some very basic ideas. It ignores the likely possibility that rivers, ridges, and valleys have little or no effect on mature tornadoes. Tornadoes have passed seemingly unaffected over mountain ridges 3,000 feet high. Dozens have crossed the Mississippi River, from Minnesota to Louisiana. Both sides of the river, at the confluence of the Mississippi and Missouri Rivers, near St. Louis, have seen devastating tornadoes.
Topography may have some influence, but protection is not one of them. Weak tornadoes may damage hilltops. But well-formed, mature tornadoes may actually stretch themselves into valleys and intensify. During this vortex stretching, the funnel diameter may shrink in diameter and the tornado will spin even more rapidly. This is hardly what one would call protection for buildings in a valley.
The belief that tornadoes don't hit "here," but always seem to hit "north of town" or "south of the river" ignores some very simple mathematics. "Here" may be a small town with an area of one square mile. Just "outside of town" or "there" or "to the north" may be anywhere within visual sighting from the water tower, perhaps 10 miles in all directions. Therefore, if the town has an area of one square mile, then "outside of town" has an area of over 300 square miles. A tornado touchdown is 300 times more likely "outside" of town than in-town. The "protection" of the town does not come from hills, or a mound, or the joining of two rivers. Tornado protection comes from the same source as our protection from falling comets or other heavenly visitors... that afforded by the laws of probability... the very low probability of rare events such as tornadoes.
#3: "Tornadoes never strike big cities."
This misconception has a small kernel of possible truth at its heart. Before we get to that possible bit of truth, we first have to make a number of things clear. When one thinks of a "big city", the image of sky scrapers and large office or apartment buildings come to mind. In actuality, if you were to compare the downtown where these buildings occur with the rest of the city, it would comprise a rather small percentage of the city's area.
Let's look at a map:
In the past 40 years, the city of St. Louis and the surrounding suburbs of St. Louis County have been hit 22 times, although none of them were in the tiny skyscraper heart of the city. There are three possible reasons for that. First, the central city may produce a "heat island" in which turbulent rising air disrupts the formation of small tornadoes(keep in mind that most tornadoes are small). The second possibility is that the "roughness" created by the skyscrapers causes turbulence that disrupts the formation of small tornadoes. The third, is, of course, the idea that tornadoas are rare, and the central city is very small. So it is a matter of coincidence.
Professor Fujita of the University of Chicago suggested that the "heat island" effect takes hold for small tornadoes when a city reaches a population of about 1,000,000. There seems to be a lack of small tornadoes in the central cities of Chicago, Tokyo, and London. These are the only three cities that have been carefully studied over a long time.
None of this applies to intense tornadoes. They are just too rare to assume that they avoid central cities. There are thousands of small towns all across Tornado Alley that have never been hit by an intense tornado. If you have ever seen video of the Wichita/Andover, Kansas tornado, it should seem ridiculous to you to think that this monster, at the bottom of an 8-mile-high rotating column, would be bothered by the presence of a few 300-foot-tall buildings. Perhaps some time in the next century, a central city will be in the path of a violent tornado, and we will learn what will happen. The probability of a violent tornado in the downtown area of any large city is about once in a thousand years.
It is possible that a tornado could actually intensify even more after it forms outside of town and moves into the central city. One speculation has it that the friction of the buildings will slow down the inflow of air into the funnel. This would deprive the funnel of air. The pressure would drop, causing the funnel to shrink in diameter, and spin even faster. So central city tornadoes that began outside the city could be more damaging than average. As you can see, there is a lot more to this than the simplistic idea that heat and roughness keeps tornadoes away. The rarity of intense tornadoes and the fact that St. Louis has been hit by three of them is an interesting curiosity, but that is another puzzle for another day.
#4: "Opening windows to equalize air pressure will save a roof, or even a home, from destruction by a tornado."The idea that moving one thin pane of glass is going to protect a roof or house from one of the most violent natural forces on the planet has a certain absurdity about it. It is probably born of wishful thinking and faulty logic, stemming from the need to do something... anything. In reality, opening windows is a dangerous and useless waste of time, and could actually be harmful to the house.
To get to the very center of a mature tornado (where the pressure may be low enough to cause some explosive effects), the windows would have to endure 100-200 mph winds in the walls of the vortex. Those winds would be laden with boards, stones, cars, trees, telephone poles, and the neighbor's roof shingles as well as wind pressure of more than 100 pounds per square foot. This barrage would blow more than enough ventilation holes in the building to allow any pressure difference to be equalized.
Even with the windows closed, most houses and commercial buildings have enough openings to vent the pressure difference in the time that it takes for a tornado to pass. The engineering team at Texas Tech's Institute for Disaster Research (Minor et al., 1977) point out that the pressure drop inside a tornado with 260 mph winds is only about 10%, or just 1.4 pounds per square inch. Most buildings can vent this difference through its normal openings in about three seconds. That is sufficient time even if the tornado is moving forward at a very rapid 60 mph. In the real world, the discussion is pointless. That violent a tornado would totally blow apart a house before the central low pressure ever arrived. Venting of air to relieve pressure would not be an issue.
If the home owner opens the wrong window, air can rush in and exert pressure on the structure from the inside--like blowing air into a balloon. It is unlikely that the resident knows where the construction weak points are. In addition, the wind fields in a passing tornado are very complex and constantly changing. It is not possible to predict the strongest direction of attack. The best advice from every engineer with whom the author has ever discussed this is to leave the windows alone and get into the basement or other shelter as fast as possible. One should not think first of the house roof, but of the impact of one's death on one's family, or of one's self unnecessarily crippled or scarred for life.
I don't recall the exact origin of the "window opening" advice, but do recall that the original advice was to open windows in both the front and the back of the house. Theoretically, this would allow air to move through the house, and reduce any buildup of interior pressure. Somehow, the advice was altered to include only the windows on the north side of the house, (away from the tornado). There is no evidence that any opening of windows ever helped to hold a roof in place. The best advice is still to forget the windows and get to a shelter.
#5: "Highway overpasses are a safe place to shelter if you are on the road when you see a tornado coming."Myths continue to kill! In a film entitled "Terrible Tuesday", about the Wichita Falls tornado of 1979, a man was interviewed by a reporter about his close brush with death. He had been on the highway when he realized a tornado was coming. He parked his car and ran up underneath the overpass crossing the highway. In the early 1990s, the television crew covering a story was on the way back from the shoot. They saw a tornado, and when they realized it was gaining on them, they parked the car and ran up under the overpass, where several other people had also tried to take shelter. A small tornado was headed straight for them, but tossed around a van before it reached them. The weak tornado passed south of them, but both the experience and the video were very intense. This video was seen on television programs and newscasts by literally millions of people!
Since that video clip aired, many people have come to assume that this is a safe shelter, perhaps because the news crew survived. But this is a modern-day urban myth. Scientists and meteorologists and the emergency management people have become very frustrated with the increasing number of motorists who are doing this routinely. The truth is, any time you deliberately put yourself above ground level during a tornado, you are putting yourself in harm's way.
During the May 3, 1999 Oklahoma tornadoes, dozens of drivers pulled over on the highway and ran up under highway overpasses. Not only did this put them at risk, but they put many other motorists at risk by blocking the roads in the area of the overpass. An informal survey of storm chasers showed that 9 out of 10 storm chasers felt that overpasses were extremely dangerous places to be during a tornado.
Scientists at the SPC and NSSL knew that it was simply a matter of time until someone was hurt or killed because they chose to climb up underneath an overpass. And on May 4th, that prediction proved to be true. A woman and her child climbed up to shelter beneath the overpass, with several other people, and the intense winds caught her in their grasp. She was carried a half a mile to her death. Her child sustained slight injuries. It is regretable that this myth could not have been dispelled before someone paid the price with their life. An excellent explanation of why this is unsafe is at the NSSL Podcast.