Misconceptions about tornadoes

Windows and outer walls of the Bank One Building in downtown Fort Worth, Texas were damaged by the 2000 Fort Worth tornado. It is a commonly held belief that tornadoes can not strike downtown areas, but Fort Worth is just one of many cities which have been struck by significant tornadoes.

Despite the fact that many misconceptions about tornadoes are no longer prevalent, many still remain. This can be attributed to many factors, including stories and news reports told by people unfamiliar with tornadoes, sensationalism by news media and the presentation of incorrect information in popular entertainment. Common myths cover various aspects of the tornado, and include ideas about tornado safety, the minimization of tornado damage, and false assumptions about the size, shape, power, and path of the tornado itself.

It is thought by some people that taking shelter under highway overpasses or in the southwest corner of the building provides extra protection from a tornado, but both of these likely increase the danger of injury or death. Some still believe that opening windows ahead of a tornado will reduce the damage from the storm, but this is not true. Some people also believe that escaping in a vehicle is the safest method of avoiding a tornado, but this can increase the danger in many situations. Other myths are that tornadoes can skip houses, always travel in a predictable direction, always extend visibly from the ground to the cloud, and increase in intensity with increasing width; all of these have some basis in fact, but they are certainly not always true. Finally, some people believe that tornadoes only occur in North America, do not occur in winter, are attracted to mobile home parks, or that some areas are protected from tornadoes by rivers, mountains, valleys, tall buildings or other geographical or man-made features; the truth is that tornadoes can occur almost anywhere at any time if the conditions are right. Some geographic areas are just more prone to these conditions than others.

Source

Tornadoes are subject to a great deal of speculation and misinformation. Some tornado myths are remaining bits of folklore which are still passed down by word of mouth. The idea that the southwest corner of a structure is the safest place in a tornado was first published in the 1800s, and is still quoted today despite being thoroughly debunked in the 1960s and 70s.^[1] Many tornado myths are actively spread by media outlets. News reporters unfamiliar with the science behind tornadoes tend to repeat "common wisdom" which has since been proven incorrect. One notable instance of mass media spreading a tornado myth was after the 1999 Oklahoma tornado outbreak, where TIME magazine ran a caption on a picture suggesting that highway overpasses were safer tornado shelters than houses.^[2][3] The spread of some myths can even be attributed to sensationalism in the media or in popular tornado-themed movies such as The Wizard of Oz and Twister.^[4]

Safety

Safest location in a building

The central room on the lowest floor of a house is by far the safest area during a tornado. In multilevel apartment buildings, this will mean the ground floor units. Often the upper levels are built with lighter, weaker materials. This house near Denton, Texas was destroyed by an F2 tornado, with only a few interior walls still standing.^[5]

In 1887, the first book on tornadoes was written by John Park Finley, a pioneer in the field of tornado research. While it was a revolutionary book containing many breakthrough ideas, it contained a few ideas which have since been proven false.^[1] One of these was the idea that the northeast or east part of a structure was the least safe, and should be avoided when seeking shelter from a tornado. This advice was unrefuted and heeded by many until the 1960s.^[1]

This myth was derived from two misconceptions: First, that tornadoes always travel in a northeasterly direction, and second, that debris from a structure will be carried away in the direction of the tornado's propagation, leaving anyone taking shelter on the side of the structure facing the tornado's approach unharmed.^[1][6] The seriousness of these misconceptions began to be revealed in the 1960s and 1970s, when surveys of major tornado damage in residential areas showed that the section of a house in the direction of the tornado's approach is actually the least safe.^[1] Additionally, many tornadoes have traveled in directions other than northeasterly, including the Jarrell Tornado (F5 on the Fujita scale), which moved south-southwesterly.^[1][7] Because determining a tornado's direction of approach can take time away from seeking shelter, official advice is to seek shelter in an interior room on the lowest floor of a building, under a staircase, I-beam, or sturdy piece of furniture if possible.^[6]

Opening windows to reduce tornado damage

One of the oldest pieces of tornado folklore is the idea that tornadoes do most of their damage due to the lower atmospheric pressure at the center of a tornado, which causes the house to explode outward. As the theory goes, opening windows helps to equalize the pressure.^[8]

The source of this myth is from the appearance of some destroyed structures after violent tornadoes. When one wall receives the extreme pressure of tornado winds, it will likely collapse inward. This then leads to a considerable outward pressure on the three remaining walls, which fall outwards as the roof falls down, creating the impression of a house which has exploded. Indeed, damage surveys of "exploded" houses usually show at least one wall which has blown inward.^[8] Additionally, if the roof is lifted before any walls fail, the walls can fall in any direction. If they fall outward, this structure can also appear to have exploded.^[9]

In even the most violent tornadoes, there is only a pressure drop of about 10%, which is about 1.4 pounds per square inch (9.7 kPa).^[10] Not only can this difference be equalized in most structures in approximately three seconds, but if a significant pressure differential manages to form, the windows will break first, equalizing the pressure.^[1] Additionally, as the windows are the most fragile parts of a house, in a significant tornado flying debris will likely break enough windows to equalize any pressure difference fairly quickly. Regardless of any pressure drop, the direct effects of a tornado's winds are enough to cause damage to a house in all but the weakest tornadoes.^[1][6]

Current advice is that opening windows in advance of a tornado wastes time that could be spent seeking shelter. Also, being near windows is very dangerous during a severe weather event, possibly exposing people to flying glass.^[11]

Using highway overpasses as shelter

The El Dorado Lake tornado overpass, above, had an unusual construction which provided a sheltered area for the camera crew. Most overpasses, like the Shields Boulevard Overpass at bottom, provide little or no shelter to tornado winds and flying debris.^[7]

The first documented instance of a person using a highway overpass for shelter from a tornado occurred in Wichita Falls, Texas on April 10, 1979. A man stuck in a traffic jam as the tornado approached left his vehicle and lay flat on an embankment beneath an overpass, surviving a violent tornado with only minor injuries.^[7] In 1991, a highly publicized event occurred in which a television crew and several others survived the passage of a tornado near El Dorado, Kansas by huddling underneath an overpass, bracing themselves against steel girders. The sensational footage taken by the television crew was broadcast across the United States. This and other media coverage helped to fuel the myth that the underside of bridges or overpasses are good shelters when a tornado strikes.^[7]

However, in the El Dorado tornado, several unique factors came together to protect the film crew and others seeking shelter under the bridge.^[7] The tornado did not pass directly over the filmed bridge, but instead tracked slightly south of the area, exposing the people to winds less damaging than those in the tornado core. The overpass had an unusual design which included a hollow crawlspace at the top of the embankment, which was large enough to allow people to crawl inside and hold the exposed girders against the wind. This design also allowed for added protection against high-speed debris.^[7]

These cases led to a false belief among many that highway overpasses were good shelter from a tornado.^[7] The belief was so strong among some that, in at least one case during the 1999 Oklahoma tornado outbreak, an individual left her well-built home and drove several miles to seek shelter under an overpass, in the mistaken belief that she was safer there than inside her house.^[7] During the outbreak, a violent tornado directly struck three highway overpasses, and at each one there was a fatality. All of the individuals received significant injuries from tornadic debris, and several were swept into the tornado itself.^[7] By contrast, the same tornado struck approximately 2,000 homes in Moore, Oklahoma, completely destroying many, yet resulting in only 3 fatalities.^[7]

From scientific lessons learned, especially in the wake of the 1999 Oklahoma outbreak, meteorologists insist that overpasses are insufficient shelter from tornado winds and debris, and may be the worst place to be during a violent tornado.^[7] The embankment under an overpass is higher than the surrounding terrain, and the wind speed increases with height. Additionally, the overpass design may create a "wind-tunnel" effect under the span, further increasing the wind speed. Many overpasses are completely exposed underneath and most lack hanging girders or a crawlspace-like area to provide sufficient protection from debris, which can travel at high speeds even in weak tornadoes. People stopping underneath overpasses block the flow of traffic, putting others in danger.^[7][12]

Escaping a tornado in a vehicle

A 2008 tornado lifted this school bus and flipped it on top of a damaged elementary school in Caledonia, Mississippi.

Often people try to avoid or outrun a tornado in a vehicle. In theory, cars can travel faster than the average tornado, and so it is better to avoid the tornado altogether than take shelter in its path.^[13] The official directive from the National Weather Service is for house-dwellers in the path of a tornado to take shelter at home rather than risk an escape by vehicle.^[14] This is a result of several factors and statistics. An interior room inside a well-built frame house (especially one with a basement) provides a reasonable degree of protection from all but the most violent tornadoes. Underground or above-ground tornado shelters, as well as extremely strong structures such as bank vaults, offer almost complete protection. Cars, on the other hand, can be heavily damaged by even weak tornadoes, and in violent tornadoes they can be thrown large distances, even into buildings. High-profile vehicles such as buses and tractor trailers are even more vulnerable to high winds.^[15][16]

There are many reasons to avoid cars when a tornado is imminent. Severe thunderstorms which produce tornadoes can produce flooding rains, hail, and strong winds far from the tornado-producing area, all of which can make driving difficult or even impossible. Some tornadoes move faster than cars, even when the road is clear and flat. Any of these situations can leave drivers stranded in the path of the tornado far from substantial shelter.^[16] When coupled with driver panic, they may also result in dangerous but preventable accidents.^[16] This situation would be magnified greatly if all the residents of a warned area left in their vehicles, which would cause traffic jams and accidents as the tornado approached.^[16] Numerous victims of the deadly Wichita Falls, Texas tornado on April 10, 1979 died in their vehicles in such a situation.^[15]

If a person spots a nearby tornado while driving, the official National Weather Service directive has been for the individual to abandon the car and seek shelter in a ditch or culvert, or substantial shelter if nearby.^[14] Far-away, highly-visible tornadoes, however, can be successfully fled from at right angles (90-degrees) from its direction of apparent movement.^[11] Despite dangers inherent with operating a vehicle during a tornado, given sufficient advanced warning, mobile home residents have been instructed by the National Weather Service to drive to the nearest secure shelter during a warning.^[17]

Tornado behavior

Tornadoes "skipping" houses

Several different phenomena have lent credence to the idea that tornadoes "skip".^[11] Tornadoes vary in intensity along their path, sometimes significantly. If a tornado were causing damage, then weakened to the point where it could cause no damage, followed by a re-intensification, it would appear as if it "skipped" a section. Occasionally with violent tornadoes, a small suction vortex will completely destroy a structure next to another building which appears almost unscathed and apparently "skipped".^[11][18]

It is true that a house that is between two destroyed homes can be "untouched", but this is not the result of a tornado "skipping" as was previously thought. After the 1974 tornado Super Outbreak, Dr. Ted Fujita studied many films of tornadoes from that day. Included in his review was damage and tornado film footage of F4 and F5 tornadoes. Fujita concluded that multiple vortices, highly volatile tornadic satellites which orbit the parent tornado at high speeds, are responsible for making tornadoes appear to "skip houses".^[19]

Weaker tornadoes, and at times even stronger tornadoes, can occasionally "lift", meaning their circulation has ceased to affect the ground. The result is an erratic and discontinuous linear damage path, leading to the term skipping tornado. These discontinuities tend to occur over areas larger than the small neighborhoods where the "house-skipping" effect is observed, except possibly at the time of the birth and organization of the tornado.^[20]

Association of size with intensity

The Elie, Manitoba Tornado of June 22, 2007 appeared small and narrow throughout its lifespan, yet was the first tornado in Canadian history to cause F5 damage.

Some people have been led to assume that small, skinny tornadoes are always weaker than large, wedge-shaped tornadoes.^[6] There is an observed trend of wider tornadoes causing worse damage. It is unknown whether this is due to an actual tendency of tornado dynamics or an ability for the tornado to affect a larger area.^[11] However, this is not a reliable indicator of an individual tornado's intensity. Some small, rope-like tornadoes, traditionally thought of as weak, have been among the strongest in history.^[11] Since 1950, more than 100 violent tornadoes (F4/EF4 or higher) had a maximum width of 300 feet (91 m).^[21] Also, tornadoes typically change shape during the course of their lifespan, further complicating any attempt to classify how dangerous a tornado is as it is occurring.^[22]

Appearing to reach the ground

It is commonly and mistakenly thought that if the condensation funnel of a tornado does not reach the ground, then the tornado can not cause substantial damage. This is another deadly myth. A tornado appears to be on the ground only when its condensation funnel descends to the surface, but this is misleading. The circular, violent surface winds, and not the condensation funnel, is what both defines the tornado and causes the tornado's damage. Spotters should keep sight of swirling debris directly under any visible funnel or rotating wall cloud, even if such structures appear to not descend entirely to the ground.^[22][23] Additionally, tornadoes can be wrapped in rain, and thus may not be visible at all.^[23]

Direction of travel

It has been thought in the past that tornadoes moved almost exclusively in a northeasterly direction.^[6] This is false, and a potentially deadly myth which can lead to a false sense of security, especially for unaware spotters or chasers. Although the majority of tornadoes move northeast, tornadoes can arrive from any direction. A deadly F5 tornado that hit the city of Jarrell, Texas in 1997 moved to the southwest - directly opposite of commonly expected storm motion. Additionally, tornadoes can shift without notice due to storm motion changes or effects on the tornado itself from factors such as its rear flank downdraft.^[11]

Geographical and temporal influences

Geographical scope

Areas worldwide where tornadoes are most likely, indicated by orange shading

It is often thought that tornadoes only occur in North America.^[24] The majority of tornadoes do occur in the United States; however, tornadoes have been observed on every continent except Antarctica.^[25] Australia, United Kingdom, Russia, Bangladesh, Philippines and other areas apart from the United States and Canada experience tornadoes on a regular basis.^[26]

Near rivers, valleys, mountains or other terrain features

There are many misconceptions involving the effect of terrain features—bodies of water, mountains, valleys, and others—on tornado formation and behavior. Most of these beliefs stem from the idea that tornadoes cannot cross or form near these terrain features. While most modes of tornadogenesis are poorly understood,^[22][27] no terrain feature can prevent the occurrence of a tornado.^[6]

Small bodies of water such as lakes and rivers are insignificant obstacles to tornadoes. Violent tornadoes have formed over rivers and lakes—including the 1878 Wallingford tornado and the 1899 New Richmond tornado—as well as crossing over them after forming elsewhere. More than a dozen tornadoes have crossed the Mississippi River in recorded history.^[28] Regarding mountains, tornadoes have been observed on terrain as high as 12,000 feet (3,700 m) above sea level, and have been known to pass up a 3,000-foot (910 m) ridge unaffected.^[1][29]

These myths have been debunked in the past. The devastating Tri-State Tornado crossed two major rivers along a record 219-mile (352 km) or longer path.^[20] In 1944, a violent tornado cut a continuous path at least 60 miles (97 km) through heavily forested and mountainous territory in West Virginia, killing at least 100 people.^[30] A hill known as Burnett's Mound on the southwest end of Topeka, Kansas was purported to protect the city from tornadoes, according to an old legend. In 1966, an F5 tornado passed directly over the hill through downtown, killing 18 people and causing $100 million (1966 USD) in damage.^[31] During the Super Outbreak, violent tornadoes crossed dozens of rivers, including the Ohio, as well as crossing over mountains and ridges hundreds of feet high.^[32]

Attraction to mobile homes and/or trailer parks

This mobile home was destroyed by a relatively weak EF0 tornado.

The idea that manufactured housing units, or mobile homes, attract tornadoes has been around for decades. This may appear to be true at first from looking at tornado fatality statistics: from 2000–2008, 539 people were killed by tornadoes in the US, with more than half (282) of those deaths in mobile homes.^[33] Only around 6.8% of homes in the US are "manufactured/mobile homes".^[34]

However, it is highly unlikely that single-story structures such as mobile homes can have a substantial effect on tornado development or evolution. More people are killed in trailer parks because mobile homes are less able to withstand high winds than permanent structures. Winds which can demolish or roll a mobile home may only cause roof damage to a typical one- or two-family permanent residence.^[35] Another likely contributing factor to the continued propagation of this myth is confirmation bias: whenever a new instance of a tornado hitting a mobile home park occurs, media outlets report on it more extensively, ignoring damage to the surrounding area which may not have produced as many casualties.^[36]

Downtown areas

See also: List of tornadoes striking downtown areas of large cities

Tornadoes are extremely rare in Utah, but downtown Salt Lake City was struck by this F2 tornado in 1999, which killed one person.

Some people believe that, for various reasons, large cities can not be struck by tornadoes. More than 100 tornadoes have struck downtown areas of large cities in recorded history. Many cities have been struck twice or more, and a few—including Lubbock, Texas, St. Louis, Missouri, Atlanta, Georgia, and London, England—have been struck by violent tornadoes (F4 or stronger).^[21][37]

Tornadoes may seem rare in downtown areas because downtown areas cover such a small geographical area. Considering the size of a central business district is very small compared to the city limits, tornadoes will strike outside of the downtown area more often.^[1]

The misconception, like most, has a small basis in truth. Research has been done in a few metropolitan areas suggesting that the urban heat island effect may discourage the formation of weak tornadoes in city centers, due to turbulent warm air disrupting their formation. This does not apply to significant tornadoes, however, and it is possible that the presence of tall buildings may actually intensify storms which move into downtown areas.^[1]

During winter

Because they generally require warm weather to form, tornadoes are uncommon in winter in the mid-latitudes.^[38] However, they can form, and tornadoes have even been known to travel over snow-covered surfaces.^[39] Deadly tornadoes are no exception: from 2000 to 2008, 135 of the 539 US tornado deaths occurred during meteorological winter (December through February).^[33] Tornadoes in winter may be more dangerous, since they tend to move faster than tornadoes at other times of the year.^[40]

See also

• List of tornadoes and tornado outbreaks
• Tornado records
• Shape of tornadoes
• Size of tornadoes
• List of tornadoes striking downtown areas
• Tornadoes of 2011
• List of F5 and EF5 tornadoes

References

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2. ^ Miller, Daniel J.; Doswell, Charles A., III; Brooks, Harold E.; Stumpf, Gregory J.; Rasmussen, Erik (1999). "Highway Overpasses as Tornado Shelters". National Weather Service WFO Norman, Oklahoma. p. 2. http://www.srh.noaa.gov/oun/?n=safety-overpass. Retrieved June 29, 2009. 
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21. ^ ^{a b} Data from the Storm Prediction Center archives, which are accessible through SeverePlot, free software created and maintained by John Hart, lead forecaster for the SPC.
22. ^ ^{a b c} Doswell, Moller, Anderson et al. (2005). "Advanced Spotters' Field Guide" (PDF). United States Department of Commerce. http://www.weather.gov/os/brochures/adv_spotters.pdf. Retrieved 2006-09-20. 
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24. ^ Williams, Jack (2004-05-28). "Answers: Do tornadoes occur outside the USA". USA Today. http://www.usatoday.com/weather/resources/askjack/2004-05-19-global-tornadoes_x.htm. Retrieved 2009-06-11. 
25. ^ Perkins, Sid (2002-05-11). "Tornado Alley, USA". Science News. pp. 296–298. Archived from the original on August 25, 2006. http://web.archive.org/web/20060825011156/http://www.sciencenews.org/articles/20020511/bob9.asp. Retrieved 2006-09-20. 
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27. ^ Biggerstaff, Michael I.; Wicker, Louis J.; Guynes, Jerry; Ziegler, Conrad; Straka, Jerry M.; Rasmussen, Erik N.; Doggett, Arthur IV; Carey, Larry D.; Schroeder, John L.; and Weiss, Chris (September 2005). "The Shared Mobile Atmospheric Research and Teaching Radar" (PDF). Bulletin of the American Meteorological Society (American Meteorological Society) 86 (9): 1263–1274. Bibcode 2005BAMS...86.1263B. doi:10.1175/BAMS-86-9-1263. http://www.nssl.noaa.gov/projects/smartradars/pubs/SR-BAMS.pdf. Retrieved June 29, 2009. 
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36. ^ Grazulis, Thomas P (2001). "Tornado Myths" (Google Books). The Tornado: Nature's Ultimate Windstorm. University of Oklahoma Press. p. 152. ISBN 0806132582. http://books.google.com/books?id=N6Tiz_7VmJoC&printsec=frontcover#PPA146,M1. Retrieved 2009-02-15. 
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External links

• Tornado Myths and Facts National Climatic Data Center
• Tornado Myths (Tornado Pictures Website)
• Tornado Myths and Tornado Reality
• LaPorte, Texas Emergency Management (PDF)