|
 |
 |
||||||||||||||
Experts answer tornado questions Here are answers from tornado experts at the National Oceanic and Atmospheric Administration, including the National Weather Service, to tornado questions from readers. Tornado safety Q: Where is the safest place to go if a tornado hits near or in an amusement park? First answer by: Charles A. Doswell III, research meteorologist, NOAA National Severe Storms Laboratory, Norman, Okla. A: This is a very difficult problem and one that scares many of us in the research community. Tornadoes harm people primarily through flying debris, usually generated from the destruction of structures. In situations like amusement parks, sporting events, or any other large gathering of people, the problems are:
I can imagine the panic-stricken crowd trampling one another in the attempt to get in their vehicles and drive away. This is a bad strategy, since it almost certainly will create a hopeless traffic jam and people would be caught in their vehicles, notorious death traps in a tornado. The only alternative is to require storm shelters at any such gathering place, at least in regions where tornadoes are possible, capable of holding a capacity crowd. The cost of building such shelters almost certainly would be viewed as prohibitive. The chances of someone experiencing this are very low, of course. People are much more likely to be struck by lightning or to have a motor vehicle accident going to or coming from the event. Although the probability of a disaster in this sort of situation is so low it probably does not make economic sense to build shelters at every conceivable gathering place, nevertheless it eventually will happen. If a tornado actually strikes a crowded amusement park or some sporting event (a violent tornado in Omaha narrowly missed a busy horse racing track; another tornado near Memphis just missed a crowed dog racing track), a major disaster is possible. The reader who asked the question was also quite right to be concerned about getting caught in the parking lot, so the bathroom probably was as good a choice as any among the limited available options. People should seek the sturdiest place of shelter they can find that can reduce the threat from flying debris. Engineered structures (like the roller coaster or a modern office building) are unlikely to fail in a tornado. Second answer by: William Alexander, meteorologist, NOAA Office of Meteorology, Silver Spring, Md. The safest thing in such a situation mirrors what to do if confronted with an approaching tornado in a traffic jam. Avoiding the cars was the smartest thing to do. However, to seek shelter in the bathroom may not have been, unless it's part of a reinforced permanent building. Aside from such a permanent structure, the best place to seek shelter is in the lowest spot of terrain possible--a ditch or ravine. The reason is, most serious injuries and fatalities from tornadoes are the result of missiles--things thrown at high speed. Those objects fly horizontally, so placing yourself below the rest of the ground level will allow most debris to pass over you. Q: Do you know of any companies in North America that manufacture underground tornado shelters (not made of concrete or brick)? I'm just not having any luck on the Web finding something like this. Can you help? Answered by: Tim Marshall, editor, STORMTRACK MAGAZINE, storm chaser, Dallas, Texas A: To my knowledge there is no underground tornado shelter manufacturer on-line. Most of the tornado shelters I have seen are steel tanks that are set in the ground. There are a lot of mom and pop shelter places in tornado alley, but I haven't seen one produced on a mass scale - like on an assembly line. Q: I have a new manufactured home, some people call it a mobile home, but is not a mobile home. It is on a permanent foundation, anchored in several locations that go though the walls, roof and through the other side. What safety concern should I have? The same as a regular home or mobile home owner in reference to tornadoes? The home was built with special features which include an emergency pressure relief valve. Answered by: Tim Marshall, editor, STORMTRACK MAGAZINE, storm chaser, Dallas, Texas A: Usually, houses already have an opening or many openings, such as broken windows, or even more serious damage, thus, I don't believe a pressure release valve will be of any benefit. Inflow winds that roar at ground level into the base of the tornado can cause damage way before the tornado even hits your house. This answer is similar to the one above about "differential pressure" doing damage in a home, with the addition that as a tornado approaches you must go through the radius of maximum winds (rmw) to get in the center where the lower pressure is. Detecting tornadoes Q: What is a Tornado Vortex Signature and how does it appear on a Doppler radar screen? Answered by: Greg Stumpf, research meteorologist, NOAA National Severe Storms Laboratory, Norman, Okla. A: Tornadic Vortex Signature (or TVS for short) is the signature of a tornado, or tornado-like vortex as seen in radial velocity fields of the Doppler radar. Radial velocity is defined as the wind speed either directly away from the radar beam or directly toward the radar beam (Doppler radar cannot measure winds that blow "perpendicular", or directly across the radar beam). The TVS usually shows up a very small area (about 1/2 to 1 mile in diameter) of strong radial velocities, where half of the velocities are toward the radar, and the other half are away from the radar. When different colors are used to depict velocities blowing away from the radar versus toward the radar, the TVS will show up as adjacent areas of contrasting colors on the radar screen. The National Oceanic and Atmospheric Administration's National Severe Storms Laboratory has developed "algorithms" (or computer programs) that can automatically detect TVSs using Doppler radial velocity data, leading to an increase in the time between the Tornado Warning, and the actual tornado occurrence on the ground. More about Doppler radar is in USA TODAY's Weather technology index. Q: I've heard about a guy who was able to predict a tornado by using a bunch of garden hoses all connected at a central spot. Evidently, he could hear sound changes, much as varying levels of water in a thin-necked bottle will produce different sounds if you blow across the top of it. Have you heard anything about this? If so, Where can I find more information? Answered by: Alfred Bedard, Jr., aerospace engineer, NOAA Environmental Teachnology Laboratory, Boulder, Colo. A: The system detects very low frequency sound waves well below the range of human hearing (near 1 Hertz) and uses an array of 4 sensors located on an area about the size of a football field. Because the sounds are weak we need to remove wind noise and hence the use of lengths of porous irrigation garden hose at each of the 4 sensors. Sound appears over the area covered by the "octopus" of hose essentially simultaneously while wind eddies do not and are averaged out. We have detected sounds using this system at ranges greater than 1,000 miles. Low frequency sounds travel in a detectable form for long distances. (more below) Q: About a month ago on ABC news Al Bedard from NOAA demonstrated a devise for detecting tornadoes. The devise consisted of an array of common garden hoses connected to a central sensor. It was able to give 30 minutes warning of a approaching tornado. I am trying to find out what he is using for a sensor. Answered by: Alfred Bedard, Jr., aerospace engineer, NOAA Environmental Technology Laboratory, Boulder, Colo. A: Our goal is to detect sounds at low frequencies near 1 Hertz in the presence of wind noise. The pressure amplitudes are quite small (about 1 microbar or the equivalent in pressure altitude of about 1 centimeter). Conventional microphones do not respond at such low frequencies and conventional microbarographs do not have enough sensitivity. We use a differential pressure sensor with a high pass pneumatic filter to get the sensitivity we need, but the filter insures that we do not let lower frequency, large amplitude pressure changes exceed the dynamic range of the sensor and yet respond to the frequencies we are interested in. Q: When I was a young person, I recall a rumor that the close proximity of a tornado could be "seen" by tuning your television (black and white in those days) to channel 13, darkening the screen, then turning to channel 2 to see if the screen was dark or bright (or something like this). Is there any method to use a television for such a purpose? Answered by: John Snow, dean of the College of Geosciences at the University of Oklahoma, Norman, Okla. A: You are referring to what was (or is called) the "Weller Method" of tornado detection (named after its proponent). This was a popular technique a decade or so ago, when people still had individual TV antenna's. The idea was to use the TV set as a lightning detector (a detector of the radio waves emitted by a lightning flash), and under some conditions it would work. The idea was that tornadic thunderstorms were very active lightning producers. However, the method had (has) several shortcomings. Not all tornadic storms produce large amounts of lightning. TV's are not all equally sensitive, and in fact some are made to filter out lightning signals. If you are connected to cable, it won't work. The bottom line is that the method provide completely unreliable in actual field tests. Did it work sometimes? Yes, but most of the time it did not -- it either indicated a tornadic storm when none occurred, or it did not indicate the presence of such a storm when in fact one was nearby. In meteorological terms, its success score was too low and its false alarm rate too high to be of use. Observing tornadoes for science Q: The people in the movie Twister made little balls to go up in a tornado. Have people tried to do that or is that some make-believe thing? Q: I was wondering if the device used in the movie Twister is actually possible or practical. You know, the "Dorothy" thingamajig with hundreds of sensors swirling around inside the tornado. If it is possible or practical, has it ever been done? I'm sure when the movie came out months ago you got lots of questions about it, but perhaps you could answer it again? Answered by: Joe Golden, NOAA senior scientist, National Oceanic and Atmospheric Administration, Silver Spring, Md. A: The "DOROTHY" device in the movie Twister was derived from a real-life invention by Dr. Al Bedard of NOAA's Environmental Technology in the late l970s. Dr. Bedard's idea was copied by the movie's producers, but did not have the small spherical sensors and transmitters packed inside; indeed, it was called the "TOtable Tornado Observatory" or TOTO, after Dorothy's pet dog in the film, "The Wizard of Oz." TOTO weighed about 350 pounds and had rugged weather sensors sticking up on the outside to measure wind, pressure and temperature, with batteries and recording equipment inside the metal cylinder. It was carried out on tornado chases by scientists at National Severe Storms Laboratory in Norman, Oklahoma, and Professor Howard Bluestein, University of Oklahoma, during the l980s, but they found it very difficult to deploy TOTO in the direct path of a tornado, unlike the frequent tornado intercepts depicted in Twister! Scientists thought at first that the feasibility of replicating the "DOROTHY" technology depicted in the movie was remote; however, in recent months, interactions with technical people in the Defense Department suggest that highly-advanced technologies developed during in the early 1980s may indeed allow the fabrication of a tornado-probing device such as "DOROTHY", but there may be better platforms available in Department of Defense to launch probes into a tornado SAFELY than placing a "DOROTHY" device directly in a tornado's path at close range on the ground. Meteorology of tornadoes Q: Does lighting really appear inside the eye of a tornado? I saw the movie Twister and never expected it to appear there. If so, what causes it to? Answered by: Ron Holle, research meteorologist, NOAA National Severe Storms Laboratory, Norman, Okla. A: If lightning ever occurs inside a tornado, it is extremely rare. There is no conclusive evidence that it has done so. Both lightning and tornadoes are caused by strong convection - cumulonimbus clouds - with strong updrafts in deep clouds. Lightning needs an updraft that reaches high enough for some water droplets to freeze and form ice, which occurs above about 15,000 feet in warm seasons and locations. Similarly, tornadoes need strong updrafts and other wind conditions in the convection. But the likelihood of them occurring simultaneously in time and space is extremely small. Q: A college friend of mine said he read somewhere that some past studies indicated that a tornado vortex could be treated as a plasma. Does a charge distribution (electron/ion) exist in a tornado vortex? Answered by: Vlad Mazur, research meteorologist, NOAA National Severe Storms Laboratory, Norman, Okla. A: Plasma is a fully ionized gas consisting of free electrons and positively charged ions which makes plasma electrically conductive. Ionization can be produced by heating to a high temperature, by an electrical discharge, or by light impact. A tornado vortex is a vortex of air. There are no processes involved in the tornado formation that can ionize the air in the vortex thus transferring it into plasma. Q: Do you know of any research on topographic features impacting tornado development or direction of travel? I live in the southeast and there appear to be recognizable "tornado alleys" in our area in which several tornadoes have followed the same path over a period of several seasons. Answered by: Harold Brooks, research meteorologist, NOAA National Severe Storms Laboratory, Norman, Okla. A: There have some studies that have looked at this question, but little conclusive evidence can be drawn. In general, tornadoes are rare enough events that it takes a very long time to create a large sample. Other factors, such as population density, often are involved with topography to make it difficult to separate effects in the observations. There appear to be areas that are tornado minima, such as the Ozark Mountains, but it is hard to draw definite conclusions. It is likely that topography could change the direction of the inflow into a thunderstorm, for instance, which could affect the storm's behavior. Recently, Patrick King of Environment Canada has shown evidence that lake breezes off of the Great Lakes appear to influence tornado distributions in Ontario, primarily by influencing where storms form. Many towns in the central United States that have not been struck by a tornado for many years have a story about some topographic feature, usually a river, or a hill, that "protects" the town from tornadoes. One of the most memorable of these "legends" was that Burnetts Mound protected Topeka, Kansas. On June 8, 1966, a violent tornado passed directly over Burnetts Mound, killing 16 people and causing $100 million in damage in Topeka. Q: What would the sea-level adjusted pressure in an F-5 tornado be? I realize that any meteorological instruments struck directly by such a storm would be most unlikely to survive. Do computer models give a meaningful answer? Is the lowest pressure in a F-5 tornado likely to be lower than any recorded reading in a hurricane? Answered by: Joe Golden, NOAA senior scientist, National Oceanic and Atmospheric Administration, Silver Spring, Md. A: A very good question. There are very few surviving pressure records following a tornado's passage, because most conventional instruments, including anemometers are destroyed. Eyewitnesses watching aneroid barometers during a tornado's passage directly overhead, have reported pressure drops of as much as five inches of mercury (normal atmospheric pressure is around 30 inches). However, such readings are taken under great stress and are therefore unreliable. Theoretical estimates based on assumed vortex flow models for an F5 tornado yield estimates around 100 millibars, or one-tenth drop from normal atmospheric pressure. During the recent NSSL VORTEX l994-95 experiment, heavy instrument packages called TURTLES were dropped in front of a large tornado, and one of them was passed over by the tornado core, and recorded a pressure drop. An important question is whether or not the pressure sensor makes it into the core of the tornado, and not just the edge. Q: What exactly is a cold-air funnel? Are they dangerous? Answered by: Harold Brooks, research meteorologist, NOAA National Severe Storms Laboratory, Norman, Okla. A: There has been little systematic study of "cold-air funnels." They are associated with cold pools of air aloft, typically with clear skies in the wake of cold fronts. The instability of the atmosphere may be sufficient to support deep convective clouds (towering cumulus) if there is enough moisture in the lowest part of the atmosphere. Often such situations occur with large values of vertical vorticity (a measure of the tendency of the atmosphere to rotate) in the 10-30,000 feet altitude layer of the atmosphere. While the mechanism of formation of cold air funnels is unclear, it is possible that it is simply due to the stretching of the vorticity by the air going up as part of the deep convection. Just as when an ice skater starts to rotate faster as he or she pulls in his or her arms, the stretching of vorticity intensifies the rotation in the storm. Cold air funnels rarely touch down, but can, in the right circumstances, cause minor damage to vulnerable structures. Tornadoes and the enviornment Q: Is there any pollution that is made by a tornado? If not, what environmental impacts can tornadoes have? Can they damage the environment? Answered by: John Snow, dean of the College of Geosciences at the University of Oklahoma, Norman, Okla. A: One can look at damage to the environment in many ways. Certainly, tornadoes can cause great destruction along their paths, but this damage is usually confined to a small area, typically a few hundred meters wide; the strong winds surrounding the tornado core (the funnel cloud) may do noticeable damage out to a kilometer or two (0.6 to 1.2 miles) to either side of the damage track. However, this is in fact a natural phenomena, dramatic and violent, but completely natural. I think where we get into "environmental problems" is when a tornado strikes a facility that contains potentially hazardous or toxic materials. There is a lot of this sort of stuff around; it ranges from farm chemicals to trash in the local landfill to medical waste awaiting disposal to radioactive materials in storage. Not only can this material be spread around the immediate site where the tornado strikes, a small (but important) fraction can be carried aloft into the thunderstorm and then transported along ways down stream. I have data suggesting that such material might go several hundreds of kilometers (hundreds of miles) before returning to Earth's surface. I should note that there is also some evidence linking tornadoes with the spread of diseases, such as histoplasmosis, which is due to a fungus found in the soil. The exact cause and effect is not clear, but it is likely due to the lofting and later fallout of small quantities of soil. I have also heard of a biology researcher investigating the possibility that this long range transport process could be responsible for the spread of certain types of small animals and plants across portions of the U.S. Q: How much power does the strongest tornado have? And, I know Twister is not true but can a tornado be a mile wide? Answered by: Joe Schaefer, director, NOAA/NWS Storm Prediction Center, Norman, Okla. A: The total energy in a tornado is relatively low. A typical tornado contains 10,000 kilowatt-hours, while a hurricane contains 10,000,000,000 kilowatt-hours. (For comparison, a Hydrogen Bomb also contains 10,000,000,000 kilo-watt hours.) However, because a tornado is so much smaller than a hurricane, the energy density (energy per unit volume) of a tornado is about 6 times greater for a tornado than for a hurricane. In terms of energy density, a tornado is the strongest of nature's storms. In a study of 34 years of tornado tracks (over 22,800) storms, it was found that the median tornado had a path length of 0.3 miles, and a width of 0.1 mile. But tracks 31 miles or longer were reported with 17 tornadoes, and 60 tornadoes had tracks that were at lease one mile wide! Q: I was in the April 19, 1996 Decatur, Ill. tornado. I have some questions about some of my observations. The house had the roof and rear wall removed, but the ceiling drywall and attic insulation came down and remained with the house, the roof trusses were not seen again at least not by me. An interior door and frame were pushed out of the wall and taken down the hall and beds that were made had the blankets and sheets removed. Many small items were not moved. Could differential air pressure explain some of these observations? Answered by: Tim Marshall, editor, STORMTRACK MAGAZINE, storm chaser, Dallas, Texas A: Actually wind pressure is what causes the damage to a house - not air pressure. The reasons why walls and roofs go, but the coffee maker on the kitchen table remains is that large objects have more force placed on them than small objects. If roofs and walls are not tied down properly, they can easily become airborne. Proper attachments - like straps and clips are needed to resist the uplift loads created on a house by the wind. Q: I was watching the news recently and heard something about cold-weather tornadoes. Is there such a thing and what exactly causes one? Answered by: Harold Brooks, research meteorologist, NOAA National Severe Storms Laboratory, Norman, Okla. A: Tornadoes typically occur in situations where the surface air is warm and moist, but not completely saturated. The warm, moist air rises, leading to the development of the parent thunderstorm. If the surface air is too cold or dry, it won't rise. Storms can form in those environments, but they require that the rising air starts from somewhere above the ground. Current conceptual models of the processes leading to tornadoes have focused on the role of the cooling of air associated with the evaporation of precipitation in downdrafts of thunderstorms. Again, if the environmental air is cold, it is difficult to evaporate water in it. An important, unknown question is how deep a layer of cold air would be required to retard the process of evaporation enough to prevent the development of low-level rotation. Significant tornadoes have been observed with temperatures near freezing (for example, the Altus, Okla., tornado of 22 February 1975), but they are rare and almost certainly involve processes going on above the surface layer leading to surface rotation. Q: Why do tornadoes go counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere? Why does it sometimes reverse? Q: Do tornadoes spin in opposite directions in the Southern Hemisphere than in the Northern Hemisphere? Answered by: Joe Golden, NOAA senior scientist, National Oceanic and Atmospheric Administration, Silver Spring, Md. A: All of the available observations suggest that the vast majority of Northern Hemisphere tornadoes do indeed rotate counter-clockwise, while those in the Southern Hemisphere tend to rotate in the clockwise direction (both are cyclonic). This is due in large measure to the statistical influence of the earth's rotation, the Coriolis Force, has on the larger scale flow regime that spawns tornadic thunderstorms; they tend to form within larger-scale cyclonic flows, i.e. large low-pressure areas with frontal zones. However, there is growing scientific evidence from videos of tornadoes and their close- cousins, waterspouts, and aerial damage surveys of tornado damage that a few percent (up to 10 percent) of Northern Hemisphere tornadoes rotate clockwise, or anticylonically, and may be very destructive. Q: Why are tornadoes round? Answered by: Bob Davies-Jones, research meteorologist, NOAA National Severe Storms Laboratory, Norman, Okla. A: Air revolves around a tornado in a circular orbit because it is being sucked in towards the low pressure or partial vacuum at the center and, at the same time, being centrifuged out of the tornado. These opposing forces are the same on all sides of the tornado. Hence, tornadoes have to be round by symmetry. Satellites orbit around the earth for a similar reason, except the inward force in this case is gravity. Tornado history Q: I'm looking for data on the 1990 Plainfield, Ill., tornado maps of its destruction, injuries, damage reports. A general area on were the tornado touched down would be helpful. Thanks for your help. I've been looking for awhile but not much break down on certain tornadoes. Answered by: Bob Maddox, NOAA Cooperative Institute for Mesoscale Meteorological Studies, Norman, Okla. A: The National Oceanic and Atmospheric Administration sometimes surveys significant storm events and for very unusual or severe events conducts a natural disaster survey. For Plainfield a Natural Disaster Survey, essentially an assessment of the event and National Weather Service forecast and warning procedures, was done. A very detailed damage survey of the tornado was done by Professor Ted Fujita of the University of Chicago. All of the pertinent details requested in this question are provided by Fujita in: "Plainfield Tornado of August 28, 1990," which is the first chapter in the American Geophysical Union Monograph 79. |
||||||||||||||
|
| | |||||||||||||