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International Journal of Modern Physics B
Vol. 28, No. 22 (2014) 1450175 (14 pages)
c World Scientific Publishing Company
DOI: 10.1142/S0217979214501756
Eliminating the major tornado threat in Tornado Alley
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R. Tao
Department of Physics, Temple University, Philadelphia, PA 19122, USA
rtao@temple.edu
Received 13 March 2014
Revised 25 May 2014
Accepted 27 May 2014
Published 9 June 2014
The 2013 devastating tornadoes in Oklahoma, Illinois and other states in Tornado Alley
raise an important question: Can we do something to eliminate the major tornado threats
in Tornado Alley? Violent tornadoes in Tornado Alley start from the clash-between
northbound warm air flow and southbound cold air flow. As there is no mountain in
Tornado Alley ranging from west to east to weaken or block the air flows, some clashes
are violent, creating vortex turbulence called supercells. These supercells are initially
in horizontal spinning motion at the lower atmosphere and then tilt as the air turns to
rise in the storm’s updraft, creating a component of spin around a vertical axis. About
30% of supercells develop into tornadoes, causing tremendous damages. Here we show
that if we build three east–west great walls in the American Midwest, 300 m high and
50 m wide, one in North Dakota, one passing Oklahoma to east and the third one in the
south Texas and Louisiana, we will weaken or block such air mass clashes and therefore
diminish the major tornado threat in the Tornado Alley forever. We may also first build
such great walls locally at some areas with frequent devastating tornado outbreaks and
then gradually extend them.
Keywords: Tornado; turbulence-vortex; atmosphere circulation.
PACS numbers: 92.60.Qx, 92.60.hk, 47.85.Gg, 92.60.kc
1. Introduction
The 2013 devastating tornado attacks in Oklahoma, Illinois, and many other states
in Tornado Alley remind us that tornadoes are an annually recurring threat (Fig. 1).
In view of the news that these tornadoes cost many lives, and incurred damages of
multi billion dollars,1 we raise an important question: Is it possible to eliminate or
at least diminish the major tornado threat in Tornado Alley? We want to emphasize
that we are not talking about how to shelter us from the tornado attacks. Instead,
we want to explore if there is anything we can do in order to eliminate the occurrence
of major tornadoes in Tornado Alley.
Some people may claim that such a question is beyond imagination as people
are powerless in facing violent tornadoes. However, as will be clear in this paper,
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Fig. 1. The Tornado at Moore, Oklahoma, 20 May 2013. (Source: Quinton Myers and Dalton
Myers).
Table 1. 2013 confirmed tornadoes in USA (enhanced
Fujita (EF) scale is used to rate the strength of tornadoes
in USA).
Total
EF0
EF1
EF2
EF3
EF4
EF5
811
413
288
79
19
8
1
Table 2. 2013 confirmed tornadoes in Europe
(Fujita scale is used to rate the strength of
tornadoes in Europe).
Total
F0
F1
F2
F3
F4
57
5
84
1
0
0
F5
we believe that human beings are not powerless on this issue. In fact, if we build
three east–west great walls in Tornado Alley, we will diminish or eliminate major
tornado threat there forever.
As shown in Tables 1 and 2, in 2013, there were 811 confirmed tornadoes in USA
and 57 in Europe.2 Also in 2013, there were three tornadoes in China: two of them
were triggered by hurricanes at southeast coast area and the third one occurred in
Gaoyou, a small city with nickname “Tornado Hometown” in China.
The tornadoes in USA are much more violent than those in Europe and China.
What causes such huge differences? What can we learn from these facts? What can
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Eliminating the major tornado threat
Fig. 2. Tornado Alley is a strip of land between the Appalachian Mountains and the Rocky
Mountains. (The figure adapted by author from www.tornadochaser.net).
we do to reduce or eliminate the major tornado threat in USA? All these will be
discussed in this paper.
2. Tornado Alley, Tornado Season and Tornado Formation
In the US, most devastating tornadoes occur in Tornado Alley (Fig. 2), which
is a strip of land between the Appalachian Mountains and the Rocky Mountains,
including the northern region of Texas, Oklahoma, Kansas, Nebraska, Iowa, Illinois,
Missouri, Arkansas, Louisiana, Indiana, South Dakota, North Dakota, the eastern
edge of Colorado, the southern edge of Minnesota, the west part of Tennessee,
Kentucky, and Ohio.3 – 7 In 2013, among 811 confirmed tornadoes in USA, most
of them occurred in Tornado Alley. Especially, the most violent tornado outbreaks
were in Tornado Alley.
In addition to Tornado Alley, Florida, the southeastern US coast area, also has
high number and density of tornado occurrences, too. However, the tornadoes in
Florida rarely approach the strength of those that occur in Oklahoma, Kansas, etc.
Second, Florida’s high tornado frequency is credited to their almost daily thunderstorms, as well as many tropical storms and hurricanes that affect the Florida
peninsula. In this paper, we will concentrate on Tornado Alley.
Tornadoes in Tornado Alley have a season which tends to move northward
from late winter to mid-summer. In the Gulf Coast region, Dixie Alley refers to
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West Tennessee, West Kentucky, North Mississippi and North Alabama, where the
tornado season occurs in late Fall from October through December. In the Southern
states, tornado season is typically from March to May. In the Southern Plains, it
lasts from May to early June. And in the Northern Plains, Northern states and
upper Midwest, peak season is in June or July.
This information about the tornado season indicates that the occurrence of
major tornadoes in Tornado Alley is related to the clashes between the northbound
warm and moist air flow and the southbound cooler and dry air flow.8,9
From the atmospheric circulation point of view, the Tornado Alley is inside the
Ferrell cell or “zone of mixing”,10 which lies between latitudes 30◦ and 60◦ .
The areas at the “thermal equator”, i.e., the sun’s zenith point, receive much
more solar energy per unit area than those at the polar region. Therefore, in the
tropic region, the sun heating the earth surface causes hot and moist surface air
to rise. The warm and moist air at high altitude flows in the poleward direction.
Due to the earth’s spin, the Coriolis force deflects the air flow to the right and
accelerates it. At about 30◦ N/S latitude, it descends in a high pressure area. Some
of the descending air travels equatorially along the surface, closing the loop of the
Hadley cell and creating trade winds. However, because of the baroclinic instability,
in the middle latitudes, the “zone of mixing”, we still have strong eddies that
transport warm air poleward. In Tornado Alley, these warm and moist air flows
move from south to north, ascending and tilting to east gradually. Similarly, in the
middle latitudes, there are cold air flows from the Polar Cell moving to the south,
descending and tilting to the west gradually (Fig. 3).
At a certain season, the warm air flow front clashes with the cold air flow front
at some place in Tornado Alley. Major tornadoes in Tornado Alley all start with
such clashes. In winter, because the northbound warm air flow is weak, the clash is
Fig. 3. In middle latitudes eddies transport warm air poleward and upward and cold air equatorward and downward. (Source: Ref. 10).
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(a)
(b)
(c)
Fig. 4. The intensive clash between the winds from the south and the winds from the north is the
source for formation of tornados in Tornado Alley. (a) Violent clash creates a vortex — supercell.
(b) Tilt and updraft creates a spin about a vertical axis leading to mesocyclone. (c) Further
stretching and strong vertical vorticity may lead to tornado. (The figures adapted by author from
Ref. 9 and National Weather Service).
minor and usually no major tornadoes occur in Tornado Alley. With the weather
getting warmer, the northbound warm air flow becomes stronger, the clash becomes
more and more intensified and we begin to see major tornadoes in Tornado Alley.
Especially as there is no mountain in Tornado Alley ranging from east to west to
weaken or block the air flows, some clashes are violent, creating vortex turbulence
with northbound winds near the ground and strong cold southbound winds aloft
[Fig. 4(a)]. Such violent vortexes, supercells, are initially in horizontal spinning
motion at the lower atmosphere, then tilt as the air turns to rise in the storm’s
updraft, creating a component of spin about a vertical axis [Fig. 4(b)].11 This
stage, the rotating circulation in a supercell associated with updraft, is called a
mesocyclone, which has a diameter of 5–20 km. Vortex stretching occurs in nearly
all supercells, but is not sufficient to create tornadoes. If the vortex stretching during
the vortex tilting intensifies the vertical vorticity enough to create a tornado, the
vortex size is getting much smaller as the rotation speed gets much faster. About
30% of supercells lead to tornadoes [Fig. 4(c)]. The diameter of tornadoes in the
US ranges from several meters to 4 km (Hallam, Nebraska, May 22, 2004) and they
cause tremendous damages.
The air flow clash location migrates with the season. In the winter, the thermal
equator is at 22.5◦ S latitude; therefore, the cold air flow moves much further south
before meeting the warm air flow as the warm air flow has farther to travel. As the
thermal equator migrates north with the season, the air flow encounter location
in Tornado Alley is also moving to the north. This is the reason why the location
of major tornado outbreak in Tornado Alley moves with the season. Figure 5 is a
typical tornado warming map presented for weather forecast around April. When a
strong warm moist air flow comes to Tornado Alley, the violent confrontation with
the cold air flow can extend several states, making tornado outbreaks at several
places in a very short period.
Turbulence research tells us that strong vortex-turbulence in air flows must
occur near the boundary. Therefore, strong supercells must be formed near the
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Fig. 5. When a strong warm moist air flow comes to Tornado Alley, the violent confrontation
with the cold air flow can extend several states, making tornado outbreaks at several places in a
very short period. (The figure adapted by author from the Weather Channel, weather.com).
ground first. Although it is possible to have some vortex formed in sky, say, 300 m
above the ground, such vortex must be much weaker than the ones formed near
the ground. In addition, the air pressure on the ground is much higher than that in
high sky. Therefore, even if the vortex in sky experiences updraft, stretching and
eventually becomes a vertical vortex, it is unlikely that such vertical vortex could
descend down to touch the ground because the high air pressure on the ground
prevents it from doing so. Roughly, in order to overcome the high pressure on the
ground and push a vortex with 100 m diameter down to the ground from 300 m
sky, it would require a force of 2.99 × 107 N. This is unlikely to occur.
3. Critical Wind Clash Speed for Tornado Formation
Let us find out what wind speed in clash may lead to tornadoes. The equation for
air motion is given by
∂V
DV
=ρ
+ (V · ∇)V = −∇P + F ,
(1)
ρ
Dt
∂t
where D/Dt = ∂/∂t + (V · ∇), ρ is the air density, P is the pressure, F is the
viscous force and gravity force. In the polar coordinate system (r, θ, z), we have
DVr
V2
1 ∂P
DVθ
Vr Vθ
1 ∂P
= θ −
+ Fr and
=−
−
+ Fθ .
Dt
r
ρ ∂r
Dt
r
ρr ∂θ
Hence we have
D(rVθ )
1 ∂P
=−
+ rFθ .
Dt
ρ ∂θ
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(2)
(3)
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Table 3.
Wind speed
(VS = VN )
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15
10
7.5
3
m/s
m/s
m/s
m/s
Maximum tangential velocity Vθ during air flow clash and stretching.
Vθ with D0
15
10
7.5
3
m/s
m/s
m/s
m/s
Vθ with D0 /2
30
20
15
6
m/s
m/s
m/s
m/s
Vθ with D0 /3
45
30
22.5
9
m/s
m/s
m/s
m/s
Vθ with D0 /4
60
40
30
12
m/s
m/s
m/s
m/s
Vθ with D0 /5
75
50
37.5
15
m/s
m/s
m/s
m/s
In tornado formation, the cylindrical surface with the maximum tangential velocity is one of the most important surfaces. Usually, ∂P/∂θ = 0 as pressure is
along the normal direction to the surface. If the viscous force is ignored, we have
rVθ almost unchanged during the updraft and stretching. This allows us to estimate
the maximum tangential velocity Vθ .
We denote the diameter of the initial vortex as D0 , the speed of south wind as
VS and the speed of north wind as VN . Then Vθ = (VS + VN )/2 initially [Fig. 4(a)].
If D0 = 300 m, VS = VN = 15 m/s, then the maximum tangential velocity of the
vortex, Vθ , will be 30 m/s as the vortex diameter is reduced to 150m during the
stretching process. This is EF0 scale tornado. When the diameter is reduced to
100 m, Vθ = 45 m/s, an EF1 scale tornado. When the diameter is further reduced
to 75 m, Vθ = 6 0m/s, an EF2 scale tornado. If the diameter is further reduced to
60 m, Vθ = 75 m/s, an EF4 scale tornado, which could cause deadly damages.
More results are in Table 3. The wind speed 7.5 m/s seems to be critical for
the tornado formation. When the wind speed is below 7.5 m/s (27 km/h), there is
almost no chance to form tornadoes. Therefore, reducing the wind speed is the key
to eliminate the formation of tornadoes.
As the major tornadoes in Tornado Alley are closely related to the warm and
cold air mass clashes, to prevent the occurrence of devastating tornadoes, we need
to eliminate such violent clashes. Is this possible? Nature actually tells us this is
possible.
4. What Can We Learn from Nature?
4.1. China
We note that the United States and China have similar geographic locations. In
particular, the Northern China Plain and the Eastern China Plain lay between
latitudes 30◦ and 45◦ , similar to our Tornado Alley. However, very few violent
tornadoes occur in this region in China.
As shown in Fig. 6, in China most mountains range from west to east, while
major mountains in North America, Appalachian Mountains and the Rocky Mountains, range from north to south. Mountains ranging north to south cannot block
air flows between north and south, but the mountains from east to west do. They
weaken or even block the violent clashes between the cold air flow and the warm
air flow and diminish the chance to generate devastating tornadoes.
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Fig. 6. The east–west mountain ranges in China eliminate the major tornado threat in the
Northern China Plain and the Eastern China Plain. Also Jiang-Huai Hills do not extend to ocean,
leaving a small area unprotected, such as Gaoyou, becoming “Tornado hometown” in China. (The
figure adapted by author from afe.easia.columbia.edu).
We particularly want to point out three mountains, which seem to play the
major role in protecting the Northern China Plain and the Eastern China Plain
from tornado threats. The first one is Yan Mountain which lies at the northern
boundary of these plains. Yan Mountain is more than 300 km long and connected
to Taihang Mountains. While some peak is as high as 2000 m above sea level, most
places are 400–1000 m above sea level. Yan Mountain effectively weakens the cold
air flowing south as evidenced by the average temperature difference between the
north side and south side of the mountain. The second is Nanling (Nan Mountains),
which is about 600 km long from west to east, between 24◦ N and 26◦ N. Nanling
consists of five separate mountains. Between these mountains there are low hills or
valleys, which are only several hundred meters above the plain. Nanling effectively
weakens the warm air flows from south. It also blocks the hurricanes from the
ocean. Because there are no mountain ranges to connect the five major mountains
in Nanling, some cold winds are able to pass through the valleys, but violent clashes
between the cold air flow and warm air flow are effectively diminished.
The third is Qinling (Tsingling) Mountain and its east extension, the Jiang-Huai
Hills. Qinling Mountain is a long mountain ranging from west to east across China.
However, as it comes to the Northern China Plain and the Eastern China Plain, it
only consists of hills, which are about 300 m above sea level. Even so, these hills
effectively weaken the cold air flow to south and make the weather quite different
at the north side and south side of the hills. Jiang-Huai Hills stretches 800 km from
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west to east.
We particularly want to point out that Jiang-Huai Hills do not extend to the
ocean, leaving a small area, north part of Jiangsu province unprotected. This small
area often has tornado outbreaks. For example, the city Gaoyou in this area has a
nickname “Tornado hometown”, which has tornado outbreaks repeatedly; for example, one tornado in 2013 and another in 2011. On average, Gaoyou has one tornado
in two years. There are other small cities in this area, where tornado outbreak
occurs once in three to four years.
These facts imply that Jiang-Huai Hills are extremely effective in preventing
tornadoes from formation in their surroundings. Without Jiang-Huai Hills, a quite
big area of the plain would become “Tornado Hometown” in China.
4.2. Shawnee Hills and Ozark Mountain
While there are no mountains in the US Tornado Alley to play the same role as Yan
Mountain, Jiang-Huai Hill and Nanling in China, there are two small mountains,
Ozarks Mountains in Missouri and Arkansas and Shawnee Hills in Illinois, which
significantly reduce tornado risk for some local areas.
Ozark Mountain consists of high and deeply dissected plateaus. There are four
primary physiographic sections: the Boston Mountains, Salem Plateau, Saint Francois Mountains and the Springfield Plateau. As seen from Fig. 7, the mountain hills
are south–north ranged. Most of them have 400–500 m elevation, while some peaks
Fig. 7.
Elevation map of Ozarks Mountains. (Source: Wikipedia).
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in Boston Mountains and Saint Francois Mountains have elevation about 750 m.
With Google satellite images, we have examined all these plateaus.
For most parts of Salem plateau and Springfield Plateau, these north–south hills
cannot block or weaken air mass flow between north and south because the deeps
and valleys between the hills are easy for south–north air flow to pass. In fact, for
example, as Joplin faces the north–south deeps and valleys formed by these hills,
the winds get more strength as they pass these valleys and deeps, making Joplin
very high tornado risk. On the other hand, some small sections of St. Francois
Mountains and Boston Mountains have the hills east–west connected. The connections seem to be 300 m above the ground. Therefore, they should be able to
block or weaken violent clashes between the cold air flow and the warm air flow in
their regions. In fact, these east–west connected sections indeed significantly reduce
tornado risks for Fayetteville, Harrison and Rolla in Arkansas and Missouri. As analyzed by www.homefact.com/tornadoes/, these cities are of low or very low tornado
risk, based on historical data with tornadoes that occurred within 30 miles of the
location. On the other hand, their neighbor cities, such as Mountain View, Joplin
and Hardy are of high risk of tornadoes because they are outside the protected zone
by these east–west connected mountain sections.
The devastating tornado outbreak in Washington County, IL on 17 November
2013 also reminds us about Shawnee Hills, which is a small mountain, 60 miles
east from Washington County, located in Southern Illinois. This small strip of land,
ranging from 5 to 40 miles wide and about 70 miles long, is characterized by higher
elevations between 120 m and 320 m. As shown in Fig. 8, most hills there are
along the south–north direction. However, some sections are east–west connected,
Fig. 8. Shawnee Hills in Southern Illinois. Some east–west connected hills are located at the south
border of Gallatin County. (The figure adapted by author from theshawneenationalforest.com).
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located at the south border of Gallatin County. Therefore, they should be able
to weaken the clashes between the cold air flow and the warm air flow in the
region. This is indeed the case. Shawnee Hills significantly reduces tornado risks for
Gallatin County. As analyzed by www.homefact.com/tornadoes/, Gallatin County
has very low tornado risk. We especially want to point out that most land in Gallatin
County is flat farm land and Shawnee Hills is just at its south boundary. On the
other hand, 60 miles west, Washington County has just suffered a horrible tornado
outbreak.
5. Conclusion: Building Great Walls to Eliminate Major Tornado
Threat in Tornado Alley
The above information is very encouraging. Although there are no mountains in
Tornado Alley to play the same role as these mountains in China, we can build
some great walls to weaken or eliminate the violent clashes between the cold air
flow and the warm air flow, hence to diminish the tornado threat. The walls should
run mainly from west to east, perpendicular to the air flows.
What height of such walls is needed to block or weaken the violent clashes
between cold air flow and warm air flow? The Jiang-Huai Hills have height of less
than 300 m above the ground, and so do many sections of Yan Mountains and
Nanling Mountains. Therefore, we estimate that the great walls should be about
300 m high from the ground. The information from Boston Mountains, St. Francois
Mountains, and Shawnee Hills also supports this estimation that a wall of 300 m
above ground will do a good job in reducing tornado risk.
How wide should the walls be? The wall height plays the most important role.
The wall width plays a secondary role. Jiang-Huai Hills have height 300 m from the
ground with their mountain tops less than 50 m wide, although the mountain base
is much wider. The man-made wall will have almost the same width from base to
top. Therefore, we estimate 50 m width should be enough for the wall. The 300 m
high wall will block air flows up to altitude of 300 m from the ground. In addition,
the wall will also provide friction for air flow just above 300 m height. The wider
the wall, the stronger the friction it provides. On the other hand, we do not want
such walls to change the weather too much. They only need to weaken the air flows
enough to eliminate the chance to produce tornadoes. Therefore, we suggest that
the great wall should be about 50 m wide.
To eliminate the tornado threat for the entire Tornado Alley, we may need to
build three great walls. The first one should be close to the northern boundary
of the Tornado Alley, maybe in North Dakota. The second one should be in the
middle, maybe in the middle of Oklahoma and going to east. The third one can be
in the south of Texas and Louisiana.
Such great walls may affect the weather, making the south side of the wall less
influenced by the north side and vice versa. On the other hand, because the walls
are only 300 m high and 50 m wide, their effect on the weather will be minor, as
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evidenced by Shawnee Hills in Illinois. In fact, with scientific design, we may also
use these walls to improve the local climate.
To build such great walls requires huge resources. It costs much more than
highway construction, but much less than building construction. In Philadelphia,
there is one skyscraper building, Comcast Center, just completed recently. The
Comcast Center is about 300 m high. From the cost of Comcast Center, we estimate
that to build one mile wall of 300 m height, we need about $160 million. On the
other hand, the damages caused by single tornado attack in Moore, Oklahoma on
20 May 2013 alone were multi-billion dollars. Therefore, it seems that the cost for
building such a wall is affordable.
While building the three great walls will eventually eliminate major tornadoes
in the entire Tornado Alley, it is unrealistic to expect such a huge project to start
in the near future because of the costs and other factors. On the other hand, it is
more realistic to build such great walls locally first, utilize any local natural hills
and then connect them piece by piece. For example, as Oklahoma seems to be
the state suffering from most devastating tornado attacks, we may build the first
great wall in the middle of Oklahoma and going to east. To do so locally, we must
remember that from air fluid dynamics, the area protected by the wall is roughly a
circle with the wall as its diameter (Fig. 9).
Also in developing any new city in Tornado Alley in future, we may consider
to build east–west skyscraper buildings first, then allocate the other parts of the
city surrounding the skyscraper buildings. In such a way, the skyscraper buildings
will serve as a wall, eliminating major tornado formation in their surroundings to
protect the whole city.
There is one important point we should point out here. It is well known in fluid
dynamics that for an incompressible fluid, the total flow rate is conserved because
∇v = 0. As a result, for an incompressible fluid, blocking the flow at one place will
lead the flow rate increase at other places. However, air is compressible and the
total flow rate thus is not conserved. Therefore, if the wind is blocked or weakened
Fig. 9.
The area protected by a 300 m high wall is roughly a circle with the wall as its diameter.
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Eliminating the major tornado threat
by a local wall at one place, the wind velocity at other places will basically not be
affected.
From a fluid dynamics point of view, tornadoes are violent turbulences. With
the global warming, the average kinetic energy of the atmosphere is increased and
thus the average Reynolds number is going up. With higher Reynolds numbers,
more violent turbulences will occur. Therefore, tornadoes may get worse with the
global warming.
We do not expect any construction of such great walls in the near future. However, we really hope that the above discussions are convincing that eliminating
major tornadoes in Tornado Alley is not an issue beyond our imagination. In addition, we also hope that this work will stimulate much research and investigation,
such as observations,11,12 simulations13,14 and lab tests,15 leading us to a perfect
solution for this important issue: How to eliminate the major tornado threats in
Tornado Alley.
Acknowledgments
This work is supported in part by a grant from US Naval Research Lab. I also wish
to thank Quinton Myers, Dalton Myers, Prof. J. Marshall, Prof. R. A. Plumb,
Wikipedia, www.tornadochaser.net, the Weather Channel and weather.com for
their help and support.
References
1. For example, see, http://www.srh.noaa.gov/oun/?n=events -20130520,
http://www.cnn.com/2013/11/18/us/midwest-tornadoes/ and
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