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Page history last edited by Karen 11 years, 8 months ago











Karen Woodley

Stephany Lutz - lutz4@illinois.edu

Donna Yeung - yeung1@illinois.edu

Nicole Hall - nhall5@illinois.edu

Tom Skawski - skawski1@illinois.edu






Opening Statement


Tornadoes are fascinating natural disasters that many people recognize, but may not completely understand. They are violent rotating columns of air that can be powerful enough to throw a car hundreds of yards, demolish buildings, and flatten entire cities. Tornadoes are deadly too, and much of the United States’ history is littered with stories that are evidence of their lethal nature. For instance, the Tri-State Tornado of 1925 holds the record for deaths from a single tornado with 689 people losing their lives in the wake of this tornado.[1]


Tornadoes have a large range in width, duration, and destruction. These twisters have been observed on every continent except for Antarctica, although most instances of tornadoes occur in the United States. (United States Tornado Watch) Nearly a third strike in Texas, Oklahoma, Nebraska and Kansas, which is dubbed “Tornado Alley”.  It is located in the central United States where the main ingredients necessary for tornado formation come together most often.  This force of nature is an incredible event targeted by many storm chasers because of its visual appeal and at times, catastrophic results. It is truly one of Mother Nature’s most awesome displays of power.


Figure 1: The main ingredients necessary for tornado formation come together most often in the Central United States at Tornado Alley.

Tornadoes are typically formed by supercell thunderstorms, but can also form in thunderstorms along squall lines, near the ends of thunderstorm bow echoes, and within land-falling hurricanes. By definition they must touch the ground, where they are often surrounded by a swirl of debris swept up from violent winds. Their destruction and severity is judged by the Fujita Scale (F-Scale), or the more recently updated Enhanced Fujita Scale (EF-Scale) which is used to estimate a tornado’s wind speed based upon a damage survey. At the most intense degree, an EF-5, they can cause houses to be lifted off foundations and spread their remains across fields as seen in the image to the left. These violet twisters can turn cars and even train boxcars into missiles tossing them into the air like toys. The most severe tornadoes tend to stay on the ground longest, with an average track of about 34.9 miles for an F5 tornado while F0-F2 tornadoes typically last for less than 6 miles. On average, most tornadoes are weak and last less than 10 minutes, but the strongest of all tornadoes have recorded winds of 318 mph and the longest observed path was 219 miles long. [2]

Figure 2: The destruction from an tornado spreads rubbage and debri around the surrounding areas.

While tornadoes can cause catastrophic damage and numerous fatalities, there are many safety measures that can be taken in order to keep safe. In the event of a tornado warning, which means a tornado has been spotted by a trained storm spotter or a Doppler radar has indicated rotation in a thunderstorm capable of producing a tornado, one should immediately seek shelter. When the tornado sirens sound to alert the public of this tornado, individuals should try to go underground in basements or shelters or the innermost and lowest room of a building. Each year, there can be about a 1200 tornadoes and although a low percentage of these are extremely severe, on average around 70 people per year are killed by tornadoes. With increased precaution and knowledge about what to do before, during and after a tornado, we can protect ourselves against this amazing natural disaster. [2]


Figure 3: A strong tornado touches lands near Wakeeney Kansas on June 9th, 2005.







Tornadoes are violently rotating columns of air that extend from a thunderstorm to the ground (not to be mistaken with funnel clouds, often the first stage of a tornado, which do not touch the ground).  They form most often in supercell thunderstorms (which also produce the most violent tornadoes) [3], but can also form in hurricanes or within squall line thunderstorms.  While the exact process of formation of tornadoes is not certain, the atmospheric conditions necessary for tornado formation are known, and there are theories about the formation itself (see “Formation”) [4].



Although tornadoes are the most violent kinds of storms with respect to wind speed, most tornadoes are weak: small, short-lived, and have relatively slow wind speeds [3].  They can be as wide as ½ mile or more and as narrow as 150 feet and can last anywhere between several seconds to an hour or more, but most last less than 10 minutes [2].  Tornado wind speeds range from about 58 to over 300 mph, with the fastest recorded wind speed coming from the May 3, 1999 Moore Oklahoma F-5 tornado (see Figure 4), which had wind speeds of 316 mph [4].  Tornadoes are ranked using the EF-scale (the Enhanced Fujita-scale, which is the updated version of the Fujita Scale [F-scale]), based on the life’s work of Dr. Theodore Fujita.  This scale is used to estimate tornadoes’ wind speeds based on the damage done by them, and goes from EF0 to EF5 (EF5 being the tornadoes that do the most damage) [2].










Figure 4: May 3, 1999 near Anadarko, OK: This tornado was part of the storm that created the F5 Oklahoma City tornado (Credit: OAR/ERL/NSSL)


75% of the world’s tornadoes occur in the United States with most of these tornadoes occuring within the famed “Tornado Alley” (outlined on the map in Figure 5).  This region is focused on the Central Plains of the U.S. and exists here as a result of the frequent interation of the conditions necessary for tornado formation [4]. 


Tornadoes can be very destructive and detrimental to cities.  On average, they cause about 70 fatalities and 1,500 injuries annually [5].  About $1.1 billion is spent every year cleaning up the damage produced by the estimated 1,200 tornadoes that occur on average every year in the United States.  Fortunately, due to the NEXRAD Doppler radar network, which began its use in the early ‘90s, the warning time for tornadoes has increased, with the current average warning time near 13 minutes.  Advances in technology, like Doppler radar and better scientific understanding of tornado formation and destruction, have prevented many potential fatalities and injuries every year [6].  It is important to be aware of the dangers of tornadoes to hopefully save your own life and help others around you in the case of severe weather in your area. 



Figure 5: Tornado Alley: where tornadoes in the United States are most likely to form (Credit: Rauber, Walsh, and Charlevoix, 2006)









               Tornadogenesis, or the process of the formation of a tornado, cannot yet be fully described by atmospheric scientists.  However, they do know the components needed to form a tornado, and from this they have created three main theories for the formation of tornadoes.

                The majority of tornadoes are formed within supercell thunderstorms; the type of thunderstorm that is responsible for the strongest and the most destructive tornadoes.  Tornadoes form most often within supercell thunderstorms because these storms always rotate due to the high vertical wind shear (winds that increases in speed and change direction with height) of the environment in which these storms grow (SEE FIGURE 7).   This is the foundation on which a tornado builds upon through a three step process of tilting, stretching, and squeezing [4].

Figure 6: High vertical wind shear can be seen with the arrows in this picture.  If you follow the arrows from the bottom up, you can see that the direction of the wind changes drastically changes from Northwest to Northeast.


In[s1]   the first step, the high vertical wind shear allows the rotation to begin. Since the wind speed increases with height in an environment ideal tornado formation, a vortex of air begins to roll near the surface of the earth (SEE FIGURE 7). This causes the air to rotate around an axis that is parallel to the ground.

Figure 7: The lower wind speeds at the surface and the high wind speeds aloft cause the air to roll along the surface.

As a thunderstorm passes over this horizontally aligned vortex of spinning air, the updraft tilts this circulation into a vertical position. The tilting of this vortex into the thunderstorm causes the updraft of the storm to begin to rotate as well (SEE FIGURE 8).  The rotating updraft of a supercell thunderstorm, called the mesocyclone, is the birthplace of the tornado (SEE FIGURE 9).


Figure 8: The updraft of the thunderstorm tilts the column of air into a vertical position.

Figure 9: The now vertical column of air becomes the rotating updraft, or mesocyclone.  This is where the tornado is created.


The next step in tornado formation is less certain.  However, scientists have agreed that the mesocyclone is stretched upwards which cause its rotational speed to increase by the conservation of angular momentum.  One theory called the Top-Down Theory states that the mesocyclone is squeezed as rear flank downdraft wraps around the mesocyclone and therefore contracts inward on the mesocyclone causing the rotational winds to spin faster. Evidence of this process can be seen on radar by identifying the “hook echo” (SEE FIGURE 10).  The theory dictates that as the rear flank down draft curls around the mesocyclone, constricting it the radius of rotation is reduced and its rotational velocity increases. 


Figure 10:  As the rear flank downdraft curls around the mesocyclone, the radius of the mesocyclone is reduced, and its rotational velocity increases due to the conservation of angular momentum.


Another theory is the Bottom-Down Theory.  This theory states that a horizontal circulation forms along the gust front of cool, descending air ahead of the storm that collides with warm, moist, ascending air that feeds the storms updraft. As this collision occurs, the warm moist air is thrust over the top of the cooler air and since they are headed in opposite directions, a rotating vortex of air develops between them. This circulation looks very similar to that shown in Figure 7 but it forms as a result of the storms own wind fields rather than because of the environmental conditions present before the storm developed (see top-down theory). Regardless, both theories are similar in that this horizontal circulation is then lifted to a vertical position by the strong updraft (SEE FIGURE 8).  This causes the mesocyclone to form, and as the mesocyclone is stretched vertically by the updraft, the rotational winds increase, and the tornado forms  [4].

Tornadoes strengthen by a process called “Vortex Breakdown”.  This process suggests that the tornado begins with a strong central updraft, but, because of extremely low pressure and centrifugal forces, a down draft begins to form in the center of the tornado.  This widens the tornado and causes the down draft and the updraft to work together to create suction vortices (SEE FIGURE 11). Although the larger tornado’s wind speeds slow down during this process, the suction vortices spin extremely fast as they rotate around the inside of this tornado [4] . To watch a video of this process in action, click here.



Figure 11: Because of extremely low pressure at the center of the central updraft, a central downdraft begins to form.  This causes the tornado to gain strength and create suction vortices, which can be seen in the above pictures, and look like smaller tornadoes.



Although the majority of tornadoes are created by supercells there are also some that are created by other means.  These tornadoes are called landspouts and typically form within a squall line.  They are created by a sharp change in wind speed and direction between airmasses of different temperatures.  This creates horizontal wind shear which can become strong enough to be stretched to form a tornado. These tornadoes are typically much weaker and cause much less damage. One unique aspect on landspouts is that they can often line up along the leading edge of a thunderstorm similar to the image in figure 12 [4].


Figure 12:  This images shows the unique tendency of landspouts to  line up along the edge of a leading thunderstorm.







On average, tornadoes claim about 70 lives per year in the United States. Through mid-October 2008, over 123 people have lost their lives to 35 killer tornadoes [7].  Although this number may seem small when compared to other natural disasters that cause fatalities, the severity of the storms that produce tornadoes and the destruction they bring are well known and feared by most.

The severity of a tornado was, for three decades, measured using a scale created by Dr. T. Theodore Fujita called the Fujita-scale or F-scale.  Although similar scales are used elsewhere in the world, like the British TORRO scale, the F-scale is the most recognized due to the large percentage of the world’s tornadoes that occur in the United States [8].  The F-scale is based on the damage done by the tornado and is used to predict the wind speeds of the tornadoes based off of this surveyed damage.  The table below shows the ranking system, which uses a scale that starts at 0 (weakest) and ends at 5 (most violent).  The ranking system is then related to an estimated wind speed and the typical damage patterns for a tornado of a given intensity. 

Although the scale has proved very useful, there were inherent problems in its design.  First, since the damage indicators (given in the 3rd column of Table 1) are generic, how would the ranking for a tornado change if it were to hit a part of town that had new construction with modern materials and better construction standards versus the same tornado hitting an older part of town with aged construction with old building standards and methods?  It is easy to see the scale may rank the tornado differently.  Secondly, the experience of the damage surveyor may lead to a different interpretation of the damage of a given tornado.  Finally, what if the tornado hits nothing; the scale cannot be used.  These limitations and others led to the development of the Enhanced F-scale which was introduced in the United States in February 2007 and is now used to measure the severity of tornadoes.  The Enhanced F-scale uses estimates calculated by engineers and information from the original F-scale to assign a numeric EF value to a tornado [8].  The EF scale is based off of 28 different damage indicators such as small barns, mobile homes, shopping malls, hardwood trees, and high rises.  (The other 23 damage indicators can be found here.)  The damage done to these indicators helps meteorologists assign more accurate EF values to the tornadoes.  The table below (Table 2) compares the original F-scale to the EF-scale and it can be seen that the original F-scale was overestimating tornado wind speeds.

Fujita Tornado Damage Scale (1971, Dr. T. Theodore Fujita)


Wind Estimate (mph)

Typical Damage



Light damage: Damage to chimneys, broken tree branches, billboards broken



Moderate damage: Shingles off roofs, mobile homes blown over, cars blown off the road



Considerable damage: Roofs torn off homes, mobile homes destroyed, trains overturned, trees pulled out of the ground, cars can be lifted off the ground, small objects thrown through the air



Severe damage: Roofs and walls torn off homes, trains pulled off tracks, forests destroyed, heavy cars and trucks thrown through the air



Devastating damage: Homes demolished, weak foundation homes blown away, cars and large objects thrown through air



Incredible damage: Homes torn off of foundations, cars thrown more than 100 meters from starting point, bark torn off trees

Table 1: The Fujita Scale as proposed by Dr. T. Theodore Fujita (CREDIT: SPC/NOAA)


Fujita Scale

Derived EF Scale

Operational EF Scale

F Number

Fastest ¼ Mile (mph)

3 Second gust (mph)

EF Number

3 Second gust (mph)

EF Number

3 Second gust (mph)










































Over 200

Table 2: A comparison of the original F-scale to the newly developed EF-scale.  (CREDIT: SPC/NOAA)


One important facet of using the EF-scale is to understand that the rank of a tornado cannot be determined as soon as the tornado forms.  In fact, in order to assign an EF value, surveys of the damage caused by the tornado have to be done [9].  Since these surveys rely on human interpretation, miscalculations of the tornado’s severity are common.  For example, if the storm occurred in an extremely rural area where its path merely consisted of corn stalks, the damage is going to seem much less than if the same tornado went through a small town and destroyed multiple homes.  Regardless of current technology, the EF ratings of tornadoes are still, at best, educated guesses because the true wind speed of a tornado during its destructive period is not known [8].  The only successful measurements of the wind speeds within a tornado have been done with portable Doppler radars but since there are only a few of these radars around the world, the EF-scale is the best estimate of the wind speed of any given tornado.


In order to ensure the EF value given to a tornado is as accurate as possible, the damage surveys are completed by trained professionals.  Because most tornadoes are weak, somewhere between EF0 and EF2, the budget of the National Weather Service keeps them from sending personnel from the National Weather Service itself and instead, the ground surveys of these storms are completed by Warning-Coordination Meteorologists or other forecasters who are not on duty at the time of a storm [8].  Tornadoes that cause considerable amounts of damage, hit densely populated areas, or are considered “killer” tornadoes (EF4 or EF5) are surveyed by highly experienced damage survey experts and even sometimes engineers from across the country and have the most extensive ground surveys of all tornadoes.  Although aerial surveys are rare because they are incredibly expensive, the National Weather Service has an agreement with local media or police to use their helicopters or planes during/after storms to complete a survey if needed [8].

The following pictures show what a specific EF rating tornado may look like and the damage that it may cause (Note: the destruction pictured is not necessarily caused by the pictured tornado, it is an example):

EF 0:






The picture above is of the “candy stripe” tornado, one of the most famous pictured EF 0 tornadoes. The damage caused by an EF 0 tornado is mainly broken tree branches falling into homes, cars, or the street.


EF 1:







  This picture is of the “Wizard of Oz” tornado, taken in Oklahoma right before this tornado dissipated. The damage caused by an EF 1 tornado can knock over some trees, break branches, and pull shingles off of homes.

EF 2:







The above picture was taken in Salt Lake City, Utah of an EF 2 tornado.  The damage caused by an EF 2 tornado can knock over most trees, tear the roofs off of homes, and even lift smaller cars off of the ground.



EF 3:









This photo, taken in Stoughton, Wisconsin was of a tornado with EF 3 strength.  The damage caused by an EF 3 tornado is considerable: tearing roofs and walls off of homes and carrying large cars and trucks through the air.

EF 4:








This EF 4 tornado, picture taken in Barnes County North Dakota, caused major damage.  An EF 4 tornado can flatten most homes and even blow away homes that have weak foundations.


EF 5:






Finally, this picture of an EF 5 tornado was taken in May 1999 in Oklahoma of one of the most recent EF 5 tornadoes to develop.  EF 5 tornadoes can destroy anything in their path, homes, cars, or trains.  Homes are even completely blown off of their foundations as seen in the photo above.





The following tables, graphs, and charts are statistics relating to the death and destruction of tornadoes:

Figure 13:  This graph shows the number of fatalities in the United States linked to tornadoes between the years 1940 and 2004. (CREDIT: Snodgrass Lecture)


Figure 14:  This map shows a preliminary look at the number of tornadoes reported across the United States between January 1, 2008 and October 22, 2008.  Although some tornadoes may be reported and shown on this map, a tornado may not have occurred and may therefore be disregarded when the final tornado report is released. (CREDIT: Snodgrass Lecture)





Figure 15:  This map shows the average number of tornadoes in each state across the United States.  Notice how the central United States contains the highest average number of tornadoes compared to the rest of the United States. Texas alone has had 6,000 tornadoes since 1950! This area of the US is called Tornado Alley because it is the location in the US where the four ingredients needed to make a tornado come together most often [See "Formation"] [4].  The state in which the most tornadoes occur per square mile is Oklahoma, which has nearly two times as many tornadoes per square mile than any other state in the US. (CREDIT: Snodgrass Lecture)


Figure 16:  This graph shows both the ten year average number of tornadoes and the actual number of tornadoes per year.  Around 1200 tornadoes a year is considered to be a “normal” year.  As of October 22, 2008, nearly 1400 tornadoes have occurred in the United States, proving this year to be an abnormal year in terms of the number of tornadoes forming.  (CREDIT: SPC/NOAA)



Figure 17:  This graph shows the number of tornado related fatalities in relation to other weather fatalities.  (CREDIT: SPC/NOAA)



Figure 18: This picture shows the path of the number one deadliest tornado in the United States: the Tri-State tornado.  This tornado occurred on March 18, 1925, traveled through Missouri, Illinois, and Indiana, and killed 695 people ( Top 25 Deadliest Tornadoes). (CREDIT: Curt Westra) 




The Storm Prediction Center keeps a running total of tornadic thunderstorm events each year including injuries, fatalities, number, location, and other climatological statistics on tornadoes.  The most current information of this year’s tornadoes can be found at this website.



Case Study




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[1] “United States Tornado History.” http://www.WildWildWeather.com/‌. Aon Corporation.  20 Oct. 2008 http://www.wildwildweather.com/‌united_states_tornado_history.pdf.


[2] Edwards, Roger. “The Basics About Tornadoes.” http://www.spc.noaa.gov/‌. 26 May 2008. Storm Prediction Center.  20 Oct. 2008 http://www.spc.noaa.gov/‌faq/‌tornado/.


[3]  Questions and answers about tornadoes: Basics. (2006, November 15). Retrieved November 2, 2008, from http://www.nssl.noaa.gov/‌primer/‌tornado/‌tor_basics.html


[4] Rauber, R. M., Walsh, J. E., & Charlevoix, D. J. (2005). Severe & hazardous weather: An introduction to high-impact meteorology (2nd ed.). Dubuque, IA: Kendall/‌Hunt Publishing.


[5] NOAAWatch: Severe weather. (n.d.). Retrieved November 2, 2008, from http://www.noaawatch.gov/‌themes/‌severe.php


[6] NOAAEconomics: Extreme events: Tornado. (2008, October 27). Retrieved November 2, 2008, from http://www.economics.noaa.gov/‌?goal=weather&file=events/‌tornado


[7] "SPC STATIJ Product Page." Storm Prediction Center. 26 Nov. 2008. 4 Dec. 2008 http://www.spc.noaa.gov/climo/torn/2008deadlytorn.html.

[8] "Tornado Damage." The Online Tornado FAQ. Fall 2006. Storm Prediction Center. 22 Oct. 2008 <http://www.spc.noaa.gov/faq/tornado/#damage>.


[9] "Tornado Intensity and Damage." Wikipedia. 10 Sept. 2008. 22 Oct. 2008 <http://en.wikipedia.org/wiki/tornado_intensity_and_damage>.


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