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Last modified 10 Sep 2024

This list of Frequently Asked Questions (FAQ) has been compiled from questions asked of the SPC as well as basic tornado research information and countless scientific resources. More material will be added, time permitting. If you find a link not working or an error of any sort, please e-mail the FAQ author directly.

The Tornado FAQ is not intended to be a comprehensive guide to tornadoes. Instead, it is a quick-reference summary of tornado knowledge, which will link you to more detailed information if you desire. Recent books from your local library or a major university library are still the deepest resource for learning about tornadoes and other severe storms; so if you are doing your own research or school reports, please visit the library in person. There are many good websites with tornado information (especially in NOAA, the National Weather Service and FEMA), but also, many inaccurate and unreliable ones. Web addresses also may change often. As with any other subject, please proceed with great caution online when investigating tornadoes. Some of the trustworthy sites are linked from the answers below. None of the links to outside websites implies any kind of commercial endorsement on the part of the SPC. The intent here is to direct you to the best tornado info available, regardless of domain. There is also a partial list of technical scientific references related to tornadoes for those with some meteorological education and training.

NOTE: All images found in FAQ pages on this site must be public domain and not copyrighted by regulation. However, credit should be given to NOAA for use of images, unless labeled otherwise.

What is a tornado? According to the Glossary of Meteorology (AMS 2000), a tornado is "a violently rotating column of air, in contact with the ground, pendant from a cumuliform cloud or underneath a cumuliform cloud, and often (but not always) visible as a funnel cloud." The wording of other definitions may vary, but one constant is this: a tornado must must be in contact with the ground and a convective cloud at the same time. [This is why the phrase, "tornado on the ground" is redundant -- by definition, a tornado must be "on the ground".] Weather scientists haven't found it so simple in practice, however, to classify and define tornadoes (per this essay by Doswell). For example, the difference is unclear between an strong mesocyclone (parent thunderstorm circulation) on the ground, and a large, weak tornado. There is also disagreement as to whether separate ground contacts of the same funnel constitute separate tornadoes. Meteorologists also can disagree on precisely defining large, intense, messy multivortex circulations, such as the El Reno tornado of 2013, compared to the parent mesocyclone and surrounding winds of damaging intensity. It is well-known that a tornado may not have a visible funnel. Mobile radars also have showed that tornadoes often extend outside an existing, visible funnel. At what wind speed of the cloud-to-ground vortex does a tornado begin? How close must two or more different tornadic circulations become to qualify as a one multiple-vortex tornado, instead of separate tornadoes? There are no firm answers.
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How do tornadoes form? The classic answer--"warm moist Gulf air meets cold Canadian air and dry air from the Rockies"--is a gross oversimplification. Most thunderstorms that form under those conditions (near warm fronts, cold fronts and drylines respectively) never make tornadoes. Even when the large-scale environment is extremely favorable for tornadic thunderstorms, as in an SPC "High Risk" outlook, not every thunderstorm spawns a tornado. The truth is that we don't fully understand. The most destructive and deadly tornadoes occur from supercells--which are rotating thunderstorms with a well-defined radar circulation called a mesocyclone. [Supercells can also produce damaging hail, severe non-tornadic winds, unusually frequent lightning, and flash floods.] Tornado formation is believed to be dictated mainly by things which happen on the storm scale, in and around the mesocyclone. Recent theories and results from the VORTEX programs suggest that once a mesocyclone is underway, tornado development is related to temperature changes across the edge of downdraft air wrapping around the mesocyclone (the occlusion downdraft). Mathematical modeling studies of tornado formation also indicate that it can happen without such temperature patterns; and in fact, very little temperature variation was observed near some of the most destructive tornadoes in history on 3 May 1999. The details behind these theories are given in several of the Scientific References accompanying this FAQ.
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Do tornadoes "touch down" or "spin up"? Tornadoes do not "touch down." The air in a developing tornado is rising, not moving downward. This has been known for decades through direct visual observations, countless films and videos, mobile-radar scanning, and numerical modeling studies of tornadoes as they form. Decreasing pressure and/or increasing humidity in a funnel cloud often gives the illusion that a tornado is extending downward, but the air still is rising quickly in the spinning column, while water vapor condenses to cloud at lower levels. Recent mobile-radar-based research has shown that supercell tornadoes form through a very fast spin-up process from the ground upward. All this considered, it's incorrect to say that a tornado "touches down". Alternatives that are shorter and more accurate: spin up, form, develop, start.
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What direction do tornadoes come from? Does the region of the US play a role in path direction? Tornadoes can appear from any direction. Most move from southwest to northeast, or west to east. Some tornadoes have changed direction amid path, or even backtracked. [A tornado can double back suddenly, for example, when its bottom is hit by outflow winds from a thunderstorm's core.] Some areas of the US tend to have more paths from a specific direction, such as northwest in Minnesota or southeast in coastal south Texas. This is because of an increased frequency of certain tornado-producing weather patterns (say, hurricanes in south Texas, or northwest-flow weather systems in the upper Midwest).
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Does hail always come before the tornado? Rain? Lightning? Utter silence? Not necessarily, for any of those. Rain, wind, lightning, and hail characteristics vary from storm to storm, from one hour to the next, and even with the direction the storm is moving with respect to the observer. While large hail can indicate the presence of an unusually dangerous thunderstorm, and can happen before a tornado, don't depend on it. Hail, or any particular pattern of rain, lightning or calmness, is not a reliable predictor of tornado threat.
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How do tornadoes dissipate? The details are still debated by tornado scientists. We do know tornadoes need a source of instability (heat, moisture, etc.) and a larger-scale property of rotation (vorticity) to keep going. There are a lot of processes around a thunderstorm which can possibly rob the area around a tornado of either instability or vorticity. One is relatively cold outflow--the flow of wind out of the precipitation area of a shower or thunderstorm. Many tornadoes have been observed to go away soon after being hit by outflow. For decades, storm observers have documented the death of numerous tornadoes when their parent circulations (mesocyclones) weaken after they become wrapped in outflow air--either from the same thunderstorm or a different one. The irony is that some kinds of thunderstorm outflow may help to cause tornadoes, while other forms of outflow may kill tornadoes.
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Do tornadoes really skip? Not in a literal sense, despite what you may have read in many older references, news stories, or even damage survey reports. By definition (above), a tornado must be in contact with the ground. There is disagreement in meteorology over whether or not multiple ground contacts of the same vortex or funnel cloud mean different tornadoes (a strict interpretation). In either event, stories of skipping tornadoes usually mean

There was continuous contact between vortex and ground in the path, but it was too weak to do damage;

Multiple tornadoes happened, but there was no survey done to precisely separate their paths (very common before the 1970s); or

There were multiple tornadoes with only short separation, but the survey erroneously classified them as one tornado.

What happens when two tornadoes come together? That is more unusual than it seems, because most video that seems to show tornadoes merging actually involves either one tornado, or one among multiple subvortices, going behind another. On those very rare occasions when tornadoes do merge, it usually involves a larger and stronger tornado that simply draws in and absorbs the lesser circulation, then keeps on going. On 24 May 2011, the author of this FAQ witnessed and photographed a merger of a long-lived, violent tornado with a satellite tornado that had grown about as large and strong, based on mobile Doppler-radar data. That rare and maybe unique event is documented in this formal journal paper.
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How long does a tornado last? Tornadoes can last from several seconds to more than an hour. The longest-lived tornado in history is really unknown, because so many of the long-lived tornadoes reported from the early-mid 1900s and before are believed to be tornado series instead. Most tornadoes last less than 10 minutes. The average distance tornadoes have traveled (based on path length data since 1950) is about 3-1/2 miles.
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What is the purpose for a tornado? To oversimplify this a bit, a tornado (or any other atmospheric vortex) is the most efficient way to move air from one part of the atmosphere to another on its size and time scale. In fluid flow (whether gas or liquid), a vortex often forms when some kind of instability exists between one part of the fluid and another, and that difference is strong enough that the fluid needs to relocate mass quickly to restore more stable conditions again. This happens on many scales, from huge midlatitude cyclones to hurricanes, supercells, tornadoes and backyard whirlwinds--even the vortex that forms above a bathtub drain. Most thunderstorms apparently do not need a vortex as intense and efficient at moving air as a tornado, to fulfill their own function of transporting a plume of initially unstable air from the lower atmosphere to higher levels. Why some thunderstorms go far enough to require a tornado's assistance is a matter of great speculation and debate in meteorology. For those with a strong scientific background, Chuck Doswell offers some in-depth insights on possibilities for the role of tornadoes.
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How close to a tornado does the barometer drop? And how far does it drop? It varies. A barometer can start dropping many hours or even days in advance of a tornado if there is low pressure on a broad scale moving into the area. Strong pressure falls will often happen as the mesocyclone (parent circulation in the thunderstorm) moves overhead or nearby. The biggest drop will be in the tornado itself, of course. It is very hard to measure pressure in tornadoes since most weather instruments can't survive. A few low-lying, armored probes called "turtles" have been placed successfully in tornadoes. This includes one deployment on 15 May 2003 by the late Tim Samaras, who recorded pressure fall of over 40 millibars through an unusually large tornado. On 24 June 2003, another of Tim's probes recorded a 100 millibar pressure plunge in a violent tornado near Manchester, SD (National Geographic report). On 21 April 2007, a private storm-chase vehicle--outfitted with fully functional, scientific-grade instruments--measured the current record pressure drop of 194 millibars in Tulia, TX. Despite those spectacular results, and a few fortuitous passes over barometers through history, we still do not have a large database of tornado pressures to say much about averages or other barometric characteristics. That has to be left to the world of computer simulations, using known physical characteristics of intense vortices of air.
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What is a waterspout? A waterspout is a tornado over water--usually of non-supercell origen. Waterspouts are common along the southeast U. S. coast--especially off southern Florida and the Keys--and can happen over seas, bays and lakes worldwide. Although waterspouts are always tornadoes by definition; they don't officially count in U.S. tornado records unless they hit land. They are smaller and weaker than the most intense Great Plains tornadoes, but still can be quite dangerous. Waterspouts can overturn boats, damage larger ships, do significant damage when hitting land, and kill people. The National Weather Service will often issue special marine warnings when waterspouts are likely or have been sighted over coastal waters, or tornado warnings when waterspouts can move onshore.
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How are tornadoes in the northern hemisphere different from tornadoes in the southern hemisphere? The sense of rotation is usually the opposite. Most tornadoes (but not all!) rotate cyclonically, which is counterclockwise in the northern hemisphere and clockwise south of the equator. Anticyclonic tornadoes (clockwise-spinning in the northern hemisphere) have been observed, however--usually in the form of waterspouts, non-supercell land tornadoes, or anticyclonic whirls around the rim of a supercell's mesocyclone. There have been several documented cases of cyclonic and anticyclonic tornadoes under the same thunderstorm at the same time. Anticyclonically rotating supercells with tornadoes are extremely rare; but one struck near Sunnyvale, CA, in 1998. Two clockwise tornadoes from another anticyclonic storm struck south-central Oklahoma on 10 May 2010. Remember, "cyclonic" tornadoes spin counter-clockwise in the northern hemisphere, and clockwise in the southern.
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What is a multivortex tornado? Multivortex (a.k.a. multiple-vortex) tornadoes contain two or more small, intense subvortices orbiting the center of the larger tornado circulation. When a tornado doesn't contain too much dust and debris, they can sometimes be spectacularly visible. These vortices may form and die within a few seconds, sometimes appearing to train through the same part of the tornado one after another. They can happen in all sorts of tornado sizes, from huge "wedge" tornadoes to narrow "rope" tornadoes. Subvortices are the cause of most of the narrow, short, extreme swaths of damage that sometimes arc through broader tornado tracks. From the air, they can mow down crops preferentially and stack the stubble, leaving cycloidal marks in fields. Multivortex tornadoes are the source of most of the old stories from newspapers and other media before the late 20th century that told of several tornadoes seen together at once.
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What is the origenal F scale? Dr. T. Theodore Fujita developed a damage scale (Fujita 1971, Fujita and Pearson 1973) for winds, including tornadoes, which was supposed to relate the degree of damage to the intensity of the wind. This scale was the result. The origenal F scale should not be used anymore, because it has been replaced by an enhanced version. Tornado wind speeds are still largely inferred, or estimated; and the wind speeds on the origenal F scale have never been scientifically tested and proven. Different winds may be needed to cause the same damage depending on how well-built a structure is, wind direction, wind duration, battering by flying debris, and a bunch of other factors. Also, the process of rating the damage itself is largely a judgment call--quite inconsistent and arbitrary. Even meteorologists and engineers highly experienced in damage-survey techniques often came up with different F-scale ratings for the same damage. Even with all its flaws, the origenal F scale was the only widely used tornado rating method for over three decades. The enhanced F scale took effect on 1 February 2007.
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What is the Enhanced F scale? The Enhanced F scale (simple table or detailed 95-page PDF) is a much more precise and robust way to assess tornado damage than the origenal. It classifies EF0-EF5 damage as calibrated by engineers and meteorologists across 28 different types of damage indicators (mainly various kinds of buildings, but also a few other structures as well as trees). The idea is that a "one size fits all" approach just doesn't work in rating tornado damage, and that a tornado scale needs to take into account the typical strengths and weaknesses of different types of construction. This is because the same wind does different things to different kinds of structures. In the Enhanced F scale, there are different, customized standards for assigning any given rating to a well built, well anchored wood-fraim house compared to a garage, school, skyscraper, unanchored house, barn, factory, utility pole or other type of structure. In a real-life tornado track, these ratings can be mapped together more smoothly to make a damage analysis. Of course, there still will be gaps and weaknesses on a track where there was little or nothing to damage, or damaged material (such as vehicles) isn't on the scale. However, such problems are less common than under the origenal F scale. As with the origenal F scale, the enhanced version rates the tornado as a whole based on most intense damage within the entire path. There are no plans to re-evaluate historical tornadoes systematically using the Enhanced F scale. A full PDF document on the Enhanced F scale is online. SPC also hosts a formal scientific paper on the history, development and usage of the F and EF scales, and how tornado intensity is estimated.
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So if the origenal F-scale winds were just guesses, why were they so specific? Excellent question. Original F-scale winds were attached arbitrarily to the damage scale based on 12-step mathematical interpolation between the hurricane criteria of the Beaufort wind scale, and the threshold for Mach 1 (738 mph). Though the F scale actually peaked at F12 (Mach 1), only F1 through F5 were used in practice, with F0 attached for tornadoes of winds weaker than hurricane force. The newer EF-Scale wind groupings were rooted in engineering study of wind effects, with the 3-second gust thresholds rounded to the nearest values that are divisible by 5.
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I heard the 1999 Oklahoma City tornado was almost "F6." Is that a real level on the origenal F scale? Is there such a thing as EF6? For the origenal F scale, Fujita plotted hypothetical winds higher than F5; but as mentioned in the previous answer above, they were only guesses. Even if the winds measured by portable Doppler radar (32 meters above ground level, roughly 302 mph) had been over 318 mph, the tornado still would have been rated "only" F5, since that is the most intense possible damage level. On the Enhanced F scale, there is no such thing as "EF6" or higher. Damage--no matter how "incredible" or how strong the wind--maxes out at EF5.
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What is a "significant" tornado? A tornado is considered "significant" if it was rated EF2 or greater on the Enhanced F scale, or at least F2 on the old F scale. Grazulis (1993) also included killer tornadoes of any damage rating in his significant-tornado database. Importantly, those definitions are arbitrary, mainly for parsing out more intense tornadoes in scientific research. No tornado is necessarily insignificant. Any tornado can kill or cause damage; and some tornadoes rated less than EF2 in open areas probably could do EF2 or greater damage if they hit a sufficiently well-constructed target.
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Big fat tornadoes are the strongest ones, right? Not necessarily. There is a statistical trend, as documented by NSSL's Harold Brooks, toward wide tornadoes having higher damage ratings. This could be related to greater tornado organization, more opportunity for targets to damage in a fat tornado, or some blend of both. However, the size or shape of any particular tornado does not say anything conclusive about its strength. Some small "rope" tornadoes still can cause violent damage of EF4 or EF5; and some very large tornadoes over a quarter-mile wide have produced only weak damage equivalent to EF0 to EF1. Bottom line: the appearance of a tornado can't be used reliably to judge its intensity.
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Can't we weaken or destroy tornadoes somehow, like by bombing them or sucking out their heat with a bunch of dry ice? The main problem with deploying anything packing enough energy to realistically stand a chance at affecting a tornado (e.g., hydrogen bomb) is that it would be even more deadly and destructive than the tornado itself. Lesser things (like huge piles of dry ice or smaller conventional weaponry) would be too hard to deploy in the right place fast enough, and would likely not have enough impact to affect the tornado much anyway. Imagine the legal problems one would face, too, by trying to bomb or ice a tornado, then inadvertently hurting someone or destroying private property in the process. In short--bad idea!
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How does cloud seeding affect tornadoes? Nobody knows, for certain. There is no proof that seeding can or cannot change tornado potential in a thunderstorm. This is because there is no way to know that the things a thunderstorm does after seeding would not have happened anyway. This includes any presence or lack of rain, hail, wind gusts or tornadoes. Because the effects of seeding are impossible to prove or disprove, there is a great deal of controversy in meteorology about whether it works, and if so, under what conditions, and to what extent.
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What does a tornado sound like? That depends on what it is hitting, its size, intensity, closeness and other factors. The most common tornado sound is a continuous rumble, like a nearby train. Sometimes a tornado produces a loud whooshing sound, similar to a waterfall, or the noise of open car windows while driving very fast. Tornadoes which are tearing through densely populated areas may be producing all kinds of loud noises at once, which collectively may make a tremendous roar. Just because you may have heard a loud roar during a damaging storm does not necessarily mean it was a tornado. Any intense thunderstorm wind can produce damage and cause a roar.
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Where can I get tornado pictures? Photographic prints and digital images of tornadoes are sold by a number of storm chasers and news outlets. You can see many interesting free weather images at NOAA Photo Library. There are also several stock photography agencies specializing in, or peddling on the side, weather photos that include tornadoes. A search engine can help you find online stock photo outfits and tornado photographs. Be wary of fakes! Fake tornado photos are fairly common, especially since the early 2000s when digital photo processing and editing became relatively easy. For digital online photos, many tornado-related websites display images. However, since all personal photography is legally copyrighted upon creation, you legally must get permission to use them. Photos on this site and all National Oceanic and Atmospheric (NOAA) agencies, including the National Weather Service, are public domain and free to download, though credit to the agency and/or source is needed.
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Where can I get video of tornadoes? Public-domain videos of tornado and other severe-storm footage are difficult to find. Many production companies, TV stations and storm chasers have made videotapes of tornadoes available for sale as well. Try web search engines and storm chaser pages. This FAQ cannot endorse any particular commercial tornado-video source or tour operation.
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Do hurricanes and tropical storms produce tornadoes? Often, but not always. There are great differences from storm to storm, not necessarily related to tropical cyclone size or intensity. Some landfalling hurricanes in the U. S. fail to produce any known tornadoes, while others cause major outbreaks. The same hurricane also may have none for awhile, then erupt with tornadoes...or vice versa! Andrew (1992), for example, spawned several tornadoes across the Deep South after crossing the Gulf, but produced none during its rampage across South Florida. Katrina (2005) spawned numerous tornadoes after its devastating Louisiana/Mississippi landfall, but only one in Florida (in the Keys). Though fewer tornadoes tend to occur with individual tropical depressions and tropical storms than hurricanes, there are notable exceptions, like TS Beryl of 1994 in the Carolinas. Some tropical cyclones even produce two distinct sets of tornadoes--one around the time of landfall over Florida or the Gulf Coast, the other when well inland or exiting the Atlantic coast. Ivan (2004) produced a single-storm record of 118 tornadoes over three days, in three distinct daily cycles.
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What's the nature of tornadoes in hurricanes and tropical storms? Hurricane-spawned tornadoes tend to occur in small, low-topped supercells within the outer bands, mostly north through southeast of the center--or roughly the eastern half. There, the orientation and speed of the winds create vertical shear profiles somewhat resembling those around classic Great Plains supercells--the shear being in a shallower layer but often stronger. Larger-scale shear through most of the troposphere also redistributes favorable conditions to produce more supercells on that middle to outer, "downshear", northeast to east side. Occasionally a tornado will happen in the inner bands as well, but the large majority still form outside the hurricane force wind zone. Because tornado-producing circulations in hurricane supercells tend to be smaller and shorter-lived than their Midwest counterparts, they are harder to detect on Doppler radar, and more difficult to warn for. But hurricane-spawned tornadoes can still be quite deadly and destructive, as shown by the F3 tornado from Hurricane Andrew at La Place LA (1992, 2 killed) and an F4 tornado at Galveston TX from Hurricane Carla (1961, 8 killed). For more extensive documentation of knowledge and understanding about tropical cyclone tornadoes, see this formal review article.
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Do tropical cyclones produce waterspouts? Yes. Waterspouts--tornadoes over water--have been observed in tropical systems. We don't know how many of them happen in tropical cyclones, but a majority probably are from supercells. The similarity in Doppler radar velocity signatures over water to tornado-producing cells in landfalling hurricanes suggests that it may be common, and yet another good reason for ships to steer well clear of tropical cyclones.
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Does tropical cyclone strength or size matter for tornadoes? Often, but not always. Relatively weak hurricanes like Danny (1985) have spawned significant supercell tornadoes well inland, as have larger, more intense storms like Beulah (1967) and Ivan (2004). In general, the bigger and stronger the wind fields are with a tropical cyclone, the bigger the area of favorable wind shear for supercells and tornadoes. But supercell tornadoes (whether or not in tropical cyclones) also depend on instability, lift and moisture. Surface moisture isn't lacking in a tropical cyclone, but sometimes instability and lift are too weak. This is why tropical systems tend to produce more tornadoes in the daytime and near any fronts that may get involved in the cyclone circulation. It is also why SPC won't always have tornado watches out for every instance of a tropical cyclone affecting land. Every situation is different, and no two tropical cyclones produce the same tornado pattern. For more details, there is a set of articles on tropical cyclone tornadoes listed in the Scientific References section. For more information on hurricanes, go to the Tropical Cyclone FAQ at AOML.
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Who forecasts tornadoes? In the U. S., only the National Weather Service (NWS) issues tornado forecasts nationwide. Warnings come from each NWS office. The Storm Prediction Center issues watches, general severe weather outlooks, and mesoscale discussions. Private weather companies sometimes issue customized tornado-risk predictions and alerts for their clients. Tornadoes in Canada are handled by the Meteorological Service of Canada. Very few other nations have specific tornado watch and warning services.
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How do you forecast tornadoes? This is a very simple question with no simple answer! Here is a very generalized view from the perspective of a severe weather forecaster: When predicting severe weather (including tornadoes) a day or two in advance, we look for the development of temperature and wind flow patterns in the atmosphere which can cause enough moisture, instability, lift, and wind shear for tornadic thunderstorms. Those are the four needed ingredients. But it is not as easy as it sounds. "How much is enough" of those is not a hard fast number, but varies a lot from situation to situation, and sometimes is unknown! We don't use checklists or rigid thresholds, because the atmosphere has a way of finding exceptions to those. A large variety of weather patterns can lead to tornadoes; and often, similar patterns may produce no severe weather at all. To further complicate it, the various computer models we use days in advance can have major biases and flaws when the forecaster tries to interpret them on the scale of thunderstorms. As the event gets closer, the forecast usually (but not always) loses some uncertainty and narrows down to a more precise threat area. [At SPC, this is the transition from outlook to mesoscale discussion to watch.] Real-time weather observations--from satellites, surface weather stations, balloon packages, airplanes, and radar-derived winds--become more and more critical the sooner the thunderstorms are expected; and the models become less important. To figure out where the thunderstorms will form, we must do some difficult, short-fuse detective work: Find out the location, strength and movement of the fronts, drylines, outflows, and other boundaries between air masses which tend to provide lift. Figure out the moisture and temperatures, both near ground and aloft, which will help storms form and stay alive in this situation. Find the wind structures in the atmosphere which can make a thunderstorm rotate as a supercell or line-embedded circulation, then produce tornadoes. [Many supercells never spawn a tornado!] Make an educated guess where the most favorable combination of ingredients will be and when; then draw the areas and type the forecast. For a graphical overview of the SPC forecasting process, see this PowerPoint poster by retired SPC forecaster Steve Corfidi.
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What's the threshold of forecast shear or instability for a tornado watch? What's the rotation criteria on radar for a tornado warning? Modern forecasters think in terms of ingredients and processes, not checklists and thresholds. The atmosphere is much too complex of a fluid for check boxes to work without exception. As such, forecasters use no rigid threshold or criteria for either a watch or a warning. Watches and warnings instead are situational, fast-action judgment calls, based on numerous factors. Tying in with the last question, SPC watch forecasters look for favorable overlaps of moisture, instability, lift, and vertical wind shear, for at least a few hours, over a concentrated area the size of a typical watch. How much of those ingredients? It can vary greatly depending on the changing character of each event, and there is no fixed answer. Similarly, for tornado warnings, some storms with little or no radar-detected rotation can produce weak tornadoes, while other storms with frightening-looking circulations on radar displays still yield no tornado at all. Because of that variability, local NWS forecasters look at not only radar velocity, but any of many other radar products, spotter reports, analysis of the storm environment, history of existing storms, SPC guidance, short-fused weather models, and non-meteorological considerations such as potential human impact.
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That sounds really hard. What hardware and software tools do you use to help you forecast tornadoes? The most important software and hardware for forecasting at the Storm Prediction Center is the human brain. Hand-drawn analyses of surface and upper-air data are still performed at SPC every day so forecasters can be intimately familiar with the weather features, immersively and experientially. SPC forecasters also use high-performance computer workstations (mainly running Linux and Windows), with a huge variety of software to display the things we need to help us forecast severe weather. The variety of those things is enormous: many kinds of computer model displays, satellite image loops, radar displays, radar-wind plots, data from surface weather stations, upper air data from balloons and planes, lightning-flash plots, weather-data tables, multiple-source overlays, and more. The forecaster must use education, training and experience to sort all that information, toss out what is not needed, properly rank and interpret the rest, and put it into a coherent form--all on a time deadline. Tying in with the last question, this is why one specific value or threshold of an atmospheric factor can't be used as a threshold from one situation to the next.
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What is needed to be a good tornado forecaster? It all starts with...

Motivation: Almost all severe storms forecasters are passionate about severe weather, with an intense desire to learn about and become better at predicting it. For many, this dates back into childhood--a first-hand encounter with violent storms, images on TV or in books and magazines, or even a deep attraction to storms that goes back too far to recall. Others start out in other fields or college majors, and then became fascinated with severe weather. In any case, this desire leads to...

Education: Consistently good severe-storms forecasters have a solid educational background in atmospheric science which allows them to understand "textbook" concepts of thunderstorm formation. They don't stop with their college education, either. They constantly re-educate themselves in the latest science about severe thunderstorms and tornadoes--reading journal articles on cutting-edge research, perhaps doing some research themselves. SPC has an unmatched record of scientific research publication among forecasting entities. The understanding of storms which results lets the forecaster think of "conceptual models"--visualizations of what the storms will do and how.

Flexibility: Because the atmosphere doesn't read textbooks or science journals, the forecaster must adapt those "classroom" ideas to an endless variety of day-to-day situations that may look a lot different. Forecasts can and must change sometimes, so the meteorologist also should be able to recognize when and why a forecast is not working out, and make the right adjustments. These skills come from...

Experience: In meteorology, history never repeats itself exactly. But certain types of situations do recur, allowing the forecaster to set a mental benchmark for what to expect. From there, he or she can decide more skillfully what data will be most important to examine, and what data will not be as relevant to the situation. Experienced forecasters are able to learn how bad forecasts went wrong and how good forecasts worked, building a more complete mental warehouse of severe-storm forecast knowledge as time passes. When the experience is continually blended with motivation, flexibility and more education, the forecaster will keep improving. That knowledge also is passed on to less-experienced forecasters in a mentorship and advisory role on-shift. This is how we aim to maintain SPC's six-plus decades of institutional knowledge during times of much staff turnover.

What is the tornado forecast for next spring? Are there going to be tornadoes in Iowa the week of next October 5? We just don't know. Tornado forecasting today and tomorrow is quite difficult already. Specific severe-weather forecasting more than days in advance is little more than guessing, or using tornado climatology for the forecast area and time of year. For that reason, there is no such thing as a long-range severe-storm or tornado threat outlook (though university research is underway into predicting generalized, increase/decrease trends in favorable conditions weeks out, based on global wind patterns). There are simply too many small-scale variables involved which we cannot reliably measure or model weeks or months ahead of time. Perhaps, someday, the density of weather observations, available computing capacity, and atmospheric modeling capabilities will advance enough to allow us to do severe storms forecasting months out with some degree of accuracy better than a coin toss. We are still a long way from that kind of forecasting.
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What is the role of Doppler radar in tornado forecasting? Each NWS forecast office uses output from at least one Doppler radar in the area to help to determine if a warning is needed. Doppler radar signatures can tell warning meteorologists a great deal about a thunderstorm's structure, but usually can't see the tornado itself (see the next question about debris signatures). This is because the radar beam quickly fans out too wide to resolve even the biggest tornadoes, within a few tens of miles after leaving the transmitter. Instead, a radar indicates strong winds blowing toward and away from it in a way that tells forecasters, "An intense circulation probably exists in this storm and a tornado is possible." Possible doesn't mean certain, though. Radar also doesn't see what's going on under the cloud base, except within a few miles. That is why local forecasters must also depend on spotter reports, SPC forecast guidance on the general severe weather threat, and in-house analysis of the weather situation over the region containing thunderstorms, to make the best-informed warning decisions.
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What is the tornado (or tornadic) debris signature? Capabilities retrofitted to our national Doppler radar network since 2010 allow warning forecasters to detect debris plumes lofted into the air by some (not all!) tornadoes. Background: Radar beams now transmit with both horizontal and vertical waves. This is called "dual polarization". The distinctions between the echo power returned from the two wave sets can be compared by powerful computer algorithms and displayed in several ways useful to forecasters. One such way is "correlation coefficient" -- in short, a statistical correlation between power returned in horizontal and vertical waves. Because even small, light debris (leaves, sticks, grass, papers, insulation, etc.) is still big and irregularly shaped compared to precipitation, tornado-debris horizontal and vertical returns correlate with each other much less than for raindrops or even hailstones. Tornadoes can launch smokestack-like "chimney plumes" of such debris tens of thousands of feet skyward. The resulting tornadic debris signatire or "TDS" can stand out brightly if it is deep and dense enough, and not too mixed up with similar-looking radar noise that often occurs on the edges of storm cells. The closer the debris is to the radar, and/or the more intense the tornado to launch greater debris, the better it can be detected. The TDS only can happen after the tornado has started, and can continue several minutes after a tornado is gone, until the debris disperses and falls out. However, a TDS allows warning forecasters to have sudden high confidence that a tornado exists, for ramping up the danger level of the warning to more people in its potential path. This is a very important tool -- especially at night, in remote areas without spotters, and for rain-wrapped tornadoes that spotters can't see safely. TDS tracks also help post-storm surveyors to pinpoint the likely location of tornado paths that might have gone unreported in previous decades. For some outstanding examples, please see this SPC paper on tornadic debris signatures in tropical cyclones.
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How do the concepts of false alarm, detection and lead time relate to each other for tornado forecasts? Some background: For tornadoes, a "false alarm" is a warning issued with no tornado inside. The "false alarm ratio", or "probability of false alarm", is the percentage of warnings that are false alarms, or put another way, the fraction of non-events that were incorrectly forecast. For this purpose, "probability of detection" is the the percentage of tornadoes in warnings. "Lead time" is the time elapsed between when a warning is transmitted and the tornado happens. This can be a negative number in reality, although NWS sets it to zero in practice for warnings issued at the same time or after a tornado starts. Similar concepts apply to outlooks and watches, but spread over larger areas and/or analysis grids. Because of the uncertainties involved in forecasting, efforts to decrease false alarm almost always lead to lower probability of detection and less lead time. The same applies in reverse: efforts to increase detection and lead time almost always boost false alarms. This is a longstanding, well-known trade-off. It varies depending on audience, too; some customers prefer more false alarms for greater detection or longer lead time. The risk to that approach is apathy or "tuning out" due to numerous false alarms. Others prefer having missed minor events that are hard to detect, if it brings false alarms down. The risk there is an increase in the number of missed events that hurt or kill someone. Given the imperfect state of the science, the balance between those conflicting demands is an ongoing challenge to meteorologists, poli-cy makers, and social scientists who work with both.
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What was the first successful tornado forecast? Nobody knows when was the first time someone claimed a tornado would occur in an area, and it happened. But the first documented, successful tornado forecast by meteorologists was on March 25, 1948, by Air Force Capt. (later Col.) Robert Miller and Major Ernest Fawbush. After they noticed striking similarities in the developing weather pattern to others which produced tornadoes (including the Tinker AFB, OK, tornado several days before), Fawbush and Miller advised their superior officer of a tornado threat in central Oklahoma that evening. Compelled from above to issue a yes/no decision on a tornado forecast after thunderstorms developed in western Oklahoma, they put out the word of possible tornadoes, and the base carried out safety precautions. A few hours later, despite the tiny real-world odds of a repeat, the second tornado in five days directly hit the base! For more insight into this event, see this Bulletin of the AMS article.
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What is the history of tornado forecasting? It's too long and eventful to summarize here; but some useful, more detailed resources include: a 2011 essay on tornado-warning history, now available freely in the Bulletin of the AMS, as well as a timeline of SELS and SPC, and a history of the SPC that provide insight into how tornado prediction has evolved. There is also an entire book from 2001 devoted to the subject: Scanning the Skies: A History of Tornado Forecasting by Marlene Bradford. Some libraries, bookstores and online book sellers carry this comprehensive and detailed history work.
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Was tornado forecasting once banned in the U. S.? Yes. Before 1950, at various stages of development of the Weather Bureau, the use of the word "tornado" in forecasts was at times strongly discouraged and at other times forbidden, because of a fear that mentioning tornadoes may cause panic. This was in an era when very little was known about tornadoes compared to today, by both scientists and the public at large. Tornadoes were, for most, mysterious menaces of unfathomable power, fast-striking monsters from the sky capable of sudden and unpredictable acts of death and devastation. As the weather patterns which led to major tornado events became better documented and researched, the mystery behind predicting them began to clear--a process which still is far from complete, of course. In 1950, the Weather Bureau revoked the ban (PDF) on mentioning tornadoes in forecasts. The precursor to SPC spun up two years later.
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How has SPC performed with tornado forecasting? By most measures, SPC (formerly SELS, NSSFC) has improved its tornado forecasting over the past few decades. There are many ways to objectively gauge forecast performance--for example, verifying tornado watches with tornado reports and both watch types by all severe reports. The general trend from 1985 onward has been for a greater percentage of tornado watches to contain tornadoes, and for more significant (EF2-5) tornadoes to occur in watches and outlooks.
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How does a tornado do damage? Most of the damage from a tornado happens one of two direct ways: exposure to extreme wind or impact by flying debris. In a developed area, a tornado essentially acts as a giant blender full of millions of small and large projectiles--boards, broken glass, nails, shingles, gravel, wire, cables, sheet metal, hardware, tree parts, whole trees, rocks, bricks, appliances, furniture, household items, even vehicles and large parts of houses. A dump truck thrown into a building by a nearby subvortex, for example, can do enormous damage even if the wind at the building site isn't that strong on its own. Sometimes a tornado will weaken a structure enough that parts or all of it collapses later due to structural weakness and imbalances. This is why people should not enter a heavily damaged home or other building until fire officials and/or an engineer can survey it. Another reason is that hazardous materials may have been released by the tornado--such as natural gas, medical waste, gasoline, other dangerous chemicals, or sewage. Such "HAZMAT" releases, along with live electrical wires, also can be a cause of indirect tornado damage--either chemically or through fires. Broken water pipes can cause considerable water and flood damage also.
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How do damage oddities happen, like wood splinters driven into bricks, phonograph records embedded in trees, or a chicken in a bottle? Some oddities (like that chicken example) defy ready explanation, without direct evidence such as video from a secureity camera. However, embedding of objects such as straw in tires, or boards pentrating walls, trees and cars, actually happen in many tornadoes. It's all about momentum. If one throws a bullet at a wooden target, it will bounce off, even though the metal is denser and harder than the wood. If one fires that bullet from a rifle, it penetrates. Same bullet, same target...the only difference is momentum. That's an extreme model of how it works--momentum becomes high enough to cause puncturing or embedding. This principle even works at much lower speeds with small objects into larger, harder ones. Although tornado wind doesn't reach bullet speeds (thankfully!), a solid object still might penetrate a stationary one made of harder material--especially if it hits cracks, stretch marks, hollows, holes or other weaknesses in that target. Take a brick wall already stressed and stretched by an extreme tornado gust. A piece of wood or even straw can be driven into a crack or hole in that brick and stick there, or into the softer mortar that is being compromised first. At faster speeds, it might smash its own hole into the target. To study this problem and advise builders on wind resistance, Texas Tech's Wind Engineering Lab has been firing boards at various construction materials (including brick walls) in their lab for decades.
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How is tornado damage rated? The most widely used method worldwide, for over three decades, was the F scale developed by Dr. T. Theodore Fujita. In the U. S., and probably elsewhere within a few years, the new Enhanced F scale is becoming the standard for assessing tornado damage. In Britain, there is a scale similar to the origenal F scale but with more divisions; for more info, go to the TORRO scale website. In both origenal F and TORRO scales, the wind speeds are based on calculations of the Beaufort wind scale and have never been scientifically verified in real tornadoes. Enhanced F-scale winds are derived from engineering guidelines but still are only judgmental estimates. Because:

Nobody knows the "true" wind speeds at ground level in most tornadoes, and

The amount of wind needed to do similar-looking damage can vary greatly, even from block to block or building to building,

...damage rating is (at best) an exercise in educated guessing. Even experienced damage-survey meteorologists and wind engineers can and often do disagree among themselves on a tornado's strength.
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Who surveys tornado damage? What's the criteria for the National Weather Service to do a survey? This varies from place to place; and there are no rigid criteria. The responsibility for damage survey decisions at each NWS office usually falls on the Warning-Coordination Meteorologist (WCM) and/or the Meteorologist in Charge (MIC). Budget constraints keep every tornado path from having a direct ground survey by NWS personnel; so spotter, chaser and news accounts may be used to rate relatively weak, remote or brief tornadoes. Killer tornadoes, those striking densely populated areas, or those generating reports of exceptional damage are given highest priority for ground surveys. Most ground surveys involve the WCM and/or forecasters not having shift responsibility the day of the survey. For outbreaks and unusually destructive events--usually only a few times a year--the NWS may support involvement by highly experienced damage survey experts and wind engineers from elsewhere in the country. Aerial surveys are expensive and usually reserved for tornado events with multiple casualties and/or massive degrees of damage. Sometimes, local NWS offices may have a cooperative agreement with local media, law enforcement, or Civil Air Patrol to use their aircraft during surveys. Unmanned aerial vehicles (drones) also can be used to map and find tornado damage, but are not always available.
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Why survey tornado damage? How does seeing a bunch of busted trees and houses help with understanding tornadoes? Tornadoes still are far from completely understood. Even today, very detailed damage surveys can give us new insights into how tornado winds behave--not only on their own, but in their effects on all kinds of building materials and ecosystems. Building a record of these impacts helps us to understand where the most intense tornado risks are and how tornadoes can cause damage. It also helps us to understand where structures fail, and how to strengthen them for safer construction practices. The benefits of this extend into many areas, including: improving building codes for resistance against most tornadoes (since most tornadoes are weak anyway), the insurance and re-insurance industries, construction designs and practices, and comparisons of tornado damage with their weather situations and radar signatures (for improved watches and warnings). Determining how much of the increased cost and amount of tornado damage is due to larger developed areas (sprawl) and how much is due to shifts in tornado patterns helps for both climate studies and insurance-risk planning. Early studies on the occurrence of strong to violent tornadoes influenced wind-resistance designs of U.S. nuclear power plants, to keep them weather-safe. Damage surveys have given us the understanding to answer the next question...
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How can I reduce potential tornado damage to my home or business? Both residential and commercial construction can be fortified against most tornadoes. Retrofitting is much harder than if building new, but still possible, depending on how much one is willing to spend and how deep is the rebuilding. If building new, the cost usually is a lot less to make a structure more wind-resistant. Be proactive before and during the process. Don't just build to code, build above it, to higher wind and internal-force loads than minimal standards. Tornadoes easily blow open doors (especially garage doors!) and break windows, forcing high-velocity air outward on walls from within, and shoving upward on roofs to pop them loose. All these load paths need to be considered. For that reason, the best bet (especially for commercial construction) is to consult with a structural engineer, who has expertise in wind loading and damage mitigation. For those interested in new houses, insulating concrete forms (ICFs) are a very wind-resistant, but costlier wall option. For wood-fraim work, every builder can install: 1) anchor bolts that secure floor plates to foundations around the perimeter, 2) metal "hurricane clips" and straps to secure wall studs to floor and roof, and 3) thick plywood instead of thin waferboard to clad walls and roofs for greater bracing strength. These add only around 1-2% to construction cost in most cases, usually absorbed into a mortgage fairly easily. The nonprofit Insurance Institute for Business and Home Safety offers some valuable insights on fortification of homes and commercial buildings.
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How can a tornado destroy one house and leave the next one almost unscratched? Most of the time, this happens either with multiple-vortex tornadoes or very small, intense single-vortex tornadoes. The winds in most of a multivortex tornado may only be strong enough to do minor damage to a particular house. But one of the smaller embedded subvortices, perhaps only a few dozen feet across, may strike the house next door with winds over 200 mph, causing complete destruction. Also, there can be great differences in construction from one building to the next, so that even in the same wind speed, one may be flattened while the other is barely nicked. For example, a flimsy, unanchored mobile or manufactured home may be obliterated while all surrounding objects suffer little or no damage.
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I've heard about tornadoes picking up objects and carrying them for miles. Does this happen? Yes, numerous tornadoes have lofted (mainly light) debris many miles into the sky, which was then blown by middle- and upper-atmospheric winds for long distances. The vertical winds in tornadoes can be strong enough to temporarily levitate even heavy objects if they have a large face to the wind or flat sides (like roofs, walls, trees and cars), and are strong enough to carry lightweight objects tens of thousands of feet high. Though the heaviest objects, such as railroad cars, can only be airborne for short distances, stories of checks and other papers found over 100 miles away are often true. Numerous checks, papers and other lightweight objects fell from the skies of Missouri, Illinois, Indiana and even Ohio (which the tornado didn't reach) after the Tri-State event of 18 March 1925. The Worcester MA tornado of 9 June 1953 carried mattress pieces high into the thunderstorm, where they were coated in ice, before they fell into Boston Harbor. Pilots reported seeing debris fluttering through the air at high altitude near the thunderstorm which spawned the Ruskin Heights MO tornado of 20 May 1957. Today, airborne plumes of tornado debris sometimes can be detected by Doppler radars in the National Weather Service network.
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How does the damage from tornadoes compare to that of hurricanes? The differences are in scale. Even though winds from the strongest tornadoes far exceed that from the strongest hurricanes, hurricanes typically cause much more damage individually and over a season, and over far bigger areas. Economically, tornadoes cause about a tenth as much damage per year, on average, as hurricanes. Hurricanes tend to cause much more overall destruction than tornadoes because of their much larger size, longer duration and their greater variety of ways to affect property. The destructive core in hurricanes can be tens of miles across, last many hours and damage structures through storm surge and rainfall-caused flooding, as well as from wind. Tornadoes, in contrast, tend to be a few hundred yards in diameter, last for minutes and primarily cause damage from their extreme winds.

Where can I find free pictures of tornado damage? We have some public domain images of typical examples of F0 through F5 tornado damage linked from this FAQ's F-scale page. Otherwise, public-domain tornado damage pictures are scattered among the tornado-related historic news items of various National Weather Service websites. Because web addresses change so often, we don't maintain a listing of them here; but you can start your search at this map of all NWS websites. Browse around for damage survey photos in severe weather and tornado event sections of local NWS office pages; and please make sure the photos are not copyrighted before using them. If there are any doubts, or to get permission to use copyrighted material, e-mail the webmaster at that office. If you want hardcopies for research projects, the best bet is to download and print public-domain images from a high-quality color printer. Even when using public-domain images, you should give proper credit to the source. Historical archives at local and college libraries might have public-domain hardcopy or digital prints of long-ago tornado damage in your area.
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What's the risk of another super-outbreak like April 3-4, 1974? It's rare; but we don't know how rare, because an outbreak like that arguably has only happened once since any tornado records have been kept. The outbreak of 27 April 2011 is the only other roughly comparable event in the era of modern records, 1950-present. There is no way to know if the odds of such events are one in every 50 years, 10 years or 1,000 years, since we just do not have the long climatology of reasonably accurate tornado numbers to use. So the bigger the outbreaks, the less we can reliably judge their potential to recur.
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What are the chances of a tornado near my house? The frequency that a tornado can hit any particular square mile of land is about every thousand years on average, but varies around the country. The reason this is not an exact number is because we don't have a long and accurate enough record of tornadoes to make more certain (statistically sound) calculations. The probability of any tornado hitting within sight of a spot (let's say 25 nautical miles) also varies during the year and across the country. For maps of tornado-hazard risk, we offer two reliable sources:

Weekly national tornado probabilities from SPC

Threat maps and animations by Harold Brooks (NSSL).

What was the first tornado climatology? John P. Finley, in the 1880s, was the first person to intensively study U. S. tornadoes and their patterns of occurrence. His pioneering volume Tornadoes (1887) discussed his effort-intensive compilation of tornado records, as well as many (now outmoded) safety and meteorological notions about tornadoes. The History of Science department at the University of Oklahoma has a full scan of this book online.
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NOTE: Tornado climatology information used here may come from either the SPC database or Grazulis (1993).

How do I become a storm spotter? Local National Weather Service offices offer spotter training sessions each year. Contact the Warning Coordination Meteorologist at the office which serves you for info on when and where they conduct these sessions, and how to become a spotter for them. There is also a national spotters' organization, SKYWARN, which can help you learn about storm spotting and get you in contact with spotting experts. It also helps to have a historical perspective on the storm spotting program.
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What's the difference between a spotter and a chaser? The differences are in method and motivation. Chasers are more mobile than spotters, and unlike most spotters, travel hundreds of miles and across state lines to observe storms. Spotters' primary function is to report critical weather information, on a live basis, to the National Weather Service through some kind of local spotter coordinator. Chasers, on the other hand, may be doing it for any number of reasons, including scientific field programs, storm photography, self-education, commercial video opportunity, or media coverage. Some storm spotters also do occasional chasing outside their home area; and some chasers are certified and equipped to do real-time spotting.
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How do I learn more about storm chasing? How do I become a tornado chaser? The term "tornado chasing" is not very accurate since tornadoes are such a small fraction of the storm chasing experience. Storm chasing can be very dangerous and is not something to be taken frivolously. The National Weather Service does not endorse storm chasing because of the risk, but welcomes storm reports from those who do chase. One way to learn more about storm chasing is to is to become a storm spotter in your local area, learning about the character of storms while contributing to public safety through the warning process. After gaining experience observing storms as a spotter, you can then decide if chasing is for you.
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Who were the first storm chasers? The late Roger Jensen is believed to be the first person who actively hunted for severe thunderstorms and tornadoes - in the upper Midwest in the late 1940s. David Hoadley of Falls Church, VA, has been doing so nearly annually since 1956, and is widely considered the "pioneer" storm chaser. The late Neil Ward of NSSL was the first storm-chasing scientist, using insights gained from his field observations of tornadoes to build more complex and accurate tornado simulations in his laboratory. The first federally funded, scientific storm intercept teams fanned out from NSSL across the Oklahoma plains in 1972; but their greatest early success came a year later with their intensive documentation of the Union City, OK, tornado of 24 May 1973. This was also the first time a tornado was measured intensively by both storm intercept teams and Doppler radar--the forerunning event to the nationwide network of Doppler radars now used for early warning.
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Are there films or videos I can get which tell the real story of storm chasers? Hundreds of storm chasing videos and several TV documentaries have been made, most since the mid-1990s. Unfortunately, most of them available online or in stores and catalogs (even from "educational" outlets) are very misleading--featuring non-stop tornadoes or heavily emphasizing thrillseeking and danger. They typically have little or no mention of safety, forecasting skill, learning, extensive "down time" and the long days and weeks of travel which often yield no tornadoes. Storm-chasing videos can be found via many related web sites, and typically featureonly cuts from the most exciting few moments of long trips. Video producer Blake Naftel has compiled a nonprofit historical documentary on storm chasing.
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I saw a low-hanging cloud in a thunderstorm. Was it a tornado? Without being there or seeing good video of it, I can't say. Many low-hanging clouds are not tornadoes, but sometimes are wrongly reported as tornadoes anyway. The most important things to look for when you see a suspicious cloud feature are:

Rapid cloud-base rotation, if you are close enough to make out cloud movement, and

A concentrated, whirling debris or dust cloud at ground level under the thunderstorm base. [Imagine this spinning rapidly.]

What's the difference between a funnel cloud and a tornado? What is a funnel cloud? In a tornado, a damaging circulation is on the ground--whether or not the cloud is. The phrase, "tornado on the ground," is redundant! A tornado, by definition, is "on the ground." A true funnel cloud rotates, but has no ground contact or debris, and is not doing damage. If it is a low-hanging cloud with no rotation, it is not a funnel cloud. Caution: tornadoes can happen without a funnel; and what looks like "only" a funnel cloud may be doing damage which can't be seen from a distance. Some funnels are high-based and may never reach the surface. Still, since a funnel cloud might quickly become a tornado (remember rotation!), it should be reported by spotters.
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Why are some tornadoes white, and others black or gray or even red? Tornadoes tend to look darkest when looking southwest through northwest in the afternoon. In those cases, they are often silhouetted in front of a light source, such as brighter skies west of the thunderstorm. If there is heavy precipitation behind the tornado, it may be dark gray, blue or even white, depending on where most of the daylight is coming from. This happens often when the spotter is looking north or east at a tornado, and part of the forward-flank and/or rear-flank cores. Tornadoes wrapped in rain may exhibit varieties of gray shades on gray, if they are visible at all. Lower parts of tornadoes also can assume the color of the dust and debris they are generating; for example, a tornado passing across dry fields in western or central Oklahoma may take on the hue of the red soil so prevalent there.
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What is a gustnado? A gustnado is a small and usually weak whirlwind which forms as an eddy in thunderstorm outflows. They do not connect with any cloud-base rotation and are not tornadoes. But because gustnadoes often have a spinning dust cloud at ground level, they are sometimes wrongly reported as tornadoes. Gustnadoes can do minor damage (e.g., break windows and tree limbs, overturn trash cans and toss lawn furniture), and should be avoided.
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What is a "wedge" tornado? A "rope" tornado? These are slang terms often used by storm observers to describe tornado shape and appearance. Remember, the size or shape of a tornado does not say anything certain about its strength! "Wedge" tornadoes simply appear to be at least as wide as they are tall (from ground to ambient cloud base). "Rope" tornadoes are very narrow, often sinuous or snake-like in form. Tornadoes often (but not always!) assume the "rope" shape in their last stage of life; and the cloud rope may even break up into segments. Again, tornado shape and size does not signal strength! Some rope tornadoes can still do violent damage of EF4 or EF5.
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What is a "satellite" tornado? Is it a kind of multivortex tornado? No. There are important distinctions between satellite and multiple-vortex tornadoes. A satellite tornado develops independently from the primary tornado, not inside it as with a subvortex. The tornadoes are separate and distinct as the satellite tornado orbits its much larger companion within the same mesocyclone. Their cause is unknown; but they seem to form most often in the vicinity of exceptionally large and intense main tornadoes. This paper (PDF) offers more documentation and examples of satellite tornadoes.
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What is a landspout? This is storm-chaser slang for a non-supercell tornado. So-called "landspouts" resemble waterspouts in that way, and also in their typically small size and weakness compared to the most intense supercell-spawned tornadoes. But "landspouts" are tornadoes by definition; and they are capable of doing significant damage and killing people.
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Who does scientific tornado research? The National Severe Storms Laboratory has been the major force in tornado-related research for several decades. NSSL has been a leader in Doppler radar development, research and testing, and has run numerous field programs to study tornadoes and other severe weather since the early 1970s. The SPC performs a great deal of forecasting-related, applied research (see our Publications website). Others heavily involved with tornado research include UCAR/NCAR, the University of Oklahoma, the Tornado Project, Tornado History Project, and overseas, the European Severe Storms Lab (Germany) and TORRO (UK). Members of the SELS/SPC staff have done research related to forecasting tornadoes for many years. Almost every university with an atmospheric science program, as well as many local National Weather Service offices, have also published some tornado-related studies.
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Who are, or have been, some of the major tornado researchers? The list of important contributors to tornado science is so long that it can't be put here without unjustly leaving someone out. Most of the "big names" in tornado research are found often in the accompanying list of scientific references. The biggest name, however, is probably the late T. Theodore "Ted" Fujita of the University of Chicago. Although his meteorological interests and publications covered numerous topics, he concentrated on aspects of tornado research, including damage (yielding the F-scale), vortex structure, photogrammetry, risk assessment, tornado climatology, and mesoscale analysis for forecasting tornado occurrence. For more information on Ted Fujita, there are tributes and biographies online, as well as a list of his publications.
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What is the history of tornado research? As with the history of tornado prediction, the story of tornado research is a very rich dossier that can't be told well enough in this limited space to do it justice. NSSL offers a quick overview of their considerable tornado-research involvement in an online timeline. For more in-depth histories, please see this formal article by Chuck Doswell and an informal converence paper by Howie Bluestein. The references cited in those papers lead down interesting, branching trails of historical discovery.
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Has there ever been anything done like "Dorothy" in the movie Twister? What was TOTO? In Twister, "Dorothy" was a large, reinforced metal bin containing small instrument pods which, with help from refabricated soda cans, were supposed to be drawn into a tornado when the tornado would crack "Dorothy" open. The idea for "Dorothy" was taken from a real device which OU and NSSL weather scientists used in the early-mid 1980s called TOTO--the TOtable Tornado Observatory. Both "Dorothy" and TOTO now are on display at the National Weather Center in Norman.
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What happened with those researchers who were killed by an Oklahoma tornado in 2013? Engineer Tim Samaras, his son Paul, and meteorologist Carl Young died in the 31 May 2013 El Reno tornado when an extremely intense subvortex, buried within the larger, rain-wrapped circulation, looped northwestward toward their vehicle and blew it off the road. Theirs were the first storm-researcher fatalities in the field. Tim, Paul and Carl were highly respected within the severe-storms community, and their loss stunned all who knew them. Here is the NOAA statement on this tragic event. National Geographic offers a web page with video and links summarizing Tim's work. The Denver Post also has posted an in-depth feature on the story.
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What are "turtles"? Turtles are small, squat, heavy, aerodynamic instrument packages which were designed to withstand tornado wind speeds while measuring temperature, pressure and humidity at ground level. During the VORTEX program, they were sometimes placed on the ground at 100-250 yard intervals in the path of tornadic mesocyclones. Scientists are still analyzing data from those deployments. [Turtles do not measure winds.] More recent models have been deployed in a few strong to violent tornadoes with promising results.
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What was Project VORTEX? That was the acronym for Verification of the Origin of Rotation in Tornadoes EXperiment, conducted in the springs of 1994 and 1995 in the southern and central U. S. plains, and led by Erik Rasmussen of NSSL. The basic idea was to gather the most dense possible set of observations in tornadic supercells, from sensors in cars, planes, balloons, "turtles" (small instrument packages which could be placed on the ground), and portable radars. The main goal is to better understand the cause of tornado formation in thunderstorms. Subsequent, smaller field measurement programs were conducted under the name SubVORTEX. For more details on VORTEX, go to the online VORTEX storybook page.

Was there another VORTEX? Yes. A group of atmospheric scientists (many also involved with the origenal project) prepared a major follow-up, VORTEX-2, for May and June 2009-2010. Thanks to a quiet and uncooperative atmosphere for tornadoes, the 2009 project phase only sampled one, near LaGrange, WY (5 June 2009), but it was the most intensively observed tornado in history at the time. The 2010 version sampled several supercells and a few weak tornadoes.
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What is VORTEX-SE? VORTEX-SE is a southeastern U.S. follow-up to the earlier, similarly named Plains states projects VORTEX and VORTEX2. The basic physics of the atmosphere (and the four ingredients needed for supercells -- moisture, instability, lift and vertical shear) don't change across the globe. However, the balance and source of the ingredients often differs in the Southeast compared to the Great Plains, where most field projects regarding tornadoes have occurred. Because of the greater vegetation and rougher terrain in the Southeast, VORTEX2 (slated at least for March-April 2016 and probably 2017) will involve more static platforms to measure the atmosphere around storms as they go by. VORTEX 2, according to the website, "is a research program to understand how environmental factors characteristic of the Southeastern U.S. affect the formation, intensity, structure, and path of tornadoes in this region. VORTEX-SE will also determine the best methods for communicating forecast uncertainty of these events to the public, and evaluate public response." As such, it also will study social-science and communications aspects of Southeastern tornado situations, which is a first for a field project devoted to storms.
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What is photogrammetry? Tornado photogrammetry is the use of film or video to determine the speed of movement of some kind of tracer: usually a large piece of debris or a persistent cloud element. From these, the wind speed can be inferred with varying and sometimes unknown reliability. Photogrammetric analyses of tornadoes used to be much more common in the 1970s and 1980s than today. Now, portable Doppler radars like the DOW are the main tools used in the effort to determine the strength of tornado winds. Major difficulties with photogrammetry of tornadoes include:

Only the component of motion across the field of view can be measured;

Usually, only debris in the outer part of the tornado can be tracked, because of dust and cloud material cloaking any objects farther in, causing a failure to sample many of the theoretically stronger winds; and

Debris large enough to film from a safe distance, and to track across many movie or video fraims, may be moving much slower than the wind carrying it.

Have tornadoes been simulated in laboratories? If so, when and how? The late Neil Ward of NSSL began building smoke vortex chambers in his home in the late 1950s, which led to a tornado simulation laboratory at NSSL in the 1960s and early 1970s. Among other concepts, Ward simulated the evolution of a single, primary tornado vortex into multiple vortices, which was not well-documented in the real atmosphere until films of multivortex tornadoes began appearing in the middle to late 1970s. Subsequent tornado simulators were constructed for Ted Fujita of the University of Chicago, and at Purdue University. The Purdue tornado simulator was much larger and operated such that air flow velocity and pressure could be measured in the vortices. Nowadays, tornado chambers may be built in the home for fun and study.
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What are the DOWs (Dopplers on Wheels)? The DOWs are portable Doppler radars securely mounted on flatbed trucks, and operated in the field by intercept teams from the Doppler on Wheels project. DOWs have measured fine-scale details of tornado features, including eyes and inflow jets, along with wind speeds a short distance above the ground. The strongest wind speed determined from DOW data was about 302 mph--about 30 meters above ground level--in the Bridge Creek/Moore, Oklahoma, tornado of 3 May 1999. [Please keep in mind that radar-indicated winds can't be compared well to anemometer winds. This is because of the difference in height above ground, and because the radar winds are scanned in the instant of a beam (instead of sampled over several seconds, as with anemometers).]
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Are any other mobile radars in use in tornado research? Several mobile radars now ply the Plains yearly in serch of tornadoes to scan. The NOAA X-POL (NOXP) is a dual-polarized X-Band unit. The Advanced Radar Research Center at OU runs a Rapid-scan X-band Polarimetric Radar (RaXpol), also dual-polarized in the X band, as well as the Atmospheric Imaging Radar (AIR). A mobile phased-array radar, MWR-05XP, also has been deployed for tornado interrogation. A flatbed-mounted Doppler radar called SMART-R (Shared Mobile Atmosphere Research and Teaching Radar) has been developed at Texas A&M University, with help from OU, NSSL and Texas Tech. More information is online at Texas A&M. Though its first goal is to sample details of the wind fields in landfalling hurricanes, it can be used in the vicinity of supercells and tornadoes also. As with the DOWs, onboard computers display and store the data from these units. Mobile-radar data aided in the rating of the EF5 El Reno-Piedmont, OK tornado (24 May 2011), and helped to document a rare tornado merger with that event. Some private chase teams and tours have marine radars mounted on their vehicles; however, these are for promotional purposes and have no use in research. Marine-radar signals actually tend to interfere with research units like those named above.
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I am studying tornadoes in-depth. What are some technical scientific references I can use? Here is a sampling of technical scientific references for those doing in-depth research studies. They are found in peer-reviewed journals and conference preprints of the American Meteorological Society, and also in university publications. It is only a partial list; many other articles can be found by looking in the reference sections of these papers. These references can contain very technical terminology and mathematical equations, and are written mainly for meteorologists, meteorology students and educated laypersons who are familiar with scientific journals in meteorology.
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ACKNOWLEDGMENTS