Journal of Natural Disaster Science, Volume 24, Number 2, 2002, pp61-71

Destruction mechanism of Oklahoma Tornadoes in 1999
Junji KATSURA
Professor Emeritus of Kyoto University
20-9 Matsubara-cho, Otsu, Shiga 520-0831 Japan
Phone +81-77-533-0968
Harold CONNER
Professor Emeritus of the University of Oklahoma
Oklahoma Professional Engineer No. 11991
2820 Castlewood Dr., Norman, Oklahoma 73072-7525 USA
Phone +1-405-321-1622, Fax +1-405-321-8912(Received for 11 Oct., 2002)

ABSTRACTOn the trace of a devastated zone by a tornado we often catch a strange sight that a heavily damaged house isadjacent to a sound house and a partial half of a roof is lost leaving the last half perfect Also tales on persons who
were flying through the air and softly landed 100 yards away are heard about. These phenomena are hard to beunderstood without the small but strong vortexes which exist in a large rotational stream in diameter of the funnel
cloud.
In the evening of May 3 in 1999, Oklahoma State was attacked by 62 tornadoes killing 42 people. Articles
from the newspapers and the video tapes from the television stations were analyzed and arranged in time
sequence. A scene of falling thick debris in the KFOR-TV video aided in associating a surface friction model thatconsists of many child vortexes with axis slanted to the current direction in the parent vortex. This flow systembrings destructive power in the right side of the proceeding direction with two actions; one is stretch of the childvortexes and the other the concentrated down draft. If this idea is applied to the disaster by the tornado running
24 miles from Bridge Creek to Midwest City, there can be found a general tendency that the fierce action of small
vortexes was more remarkable in the earlier stage and the gusty downdraft was developed in the later stage.
A downburst disaster was identified at Stroud 40 miles apart from the main tornado site.

1. PREFACE
Oklahoma is known as one of the states hit by a great numberof tornadoes in the U.S. The group of tornadoes, generated in theevening of May 3, 1999, was a large-scale one covering 21 out of
77 counties in the state, including massive tornadoes. Though theexact number is unknown, NWS (National Weather Service) counted 62 tornadoes, which were generated from a dozen supercells.
These tornadoes killed 42 people, completely destroyed 3,016
homes, and damaged totally 12,000 buildings including partly
damaged homes, apartment houses and public buildings in thestate. They say that this is a disaster that might occur once in halfa century even in Oklahoma.
Since the damaged area of this large-scale disaster is too vast
to investigate, we tried to describe the phenomena of the winds
near the ground judged by the damages on the ground, mostly
focusing on the main tornado. With the articles from newspapers,
TV news programs, and the web site www.oklahoman.com
arranged in time-series, we tried to study the cause and effect ofthe phenomena on the ground. From June 1 through 4, almost amonth after the disaster, we visited the devastated area from
Bridge Creek to Midwest City, which covered most of the path themain tornado went through, and then Tanger Outlet mall in Stroud.

2.

WEATHER CONDITIONS BEFORE THE OUTBREAK

At least 10 tornado warnings were issued in the north-central
part of Oklahoma in the morning of May 3, 1999, and 44 out of 77
counties were under a tornado watch by 4:00 p.m. NWS’s regularupper observation evaluated that the risk seemed to be decreased at
11:15 a.m., but the situation became serious afterwards.
According to Dennis McCarthy, a NWS forecaster, a dry air mass
spread from north to south at the border near Childress, Texas. In
the east existed warm moist air up from the Gulf of Mexico riding
on south-southeasterly winds, intersecting the dry line. The dry air
mass sucked the moist air, creating a trigger for thunderstorms.
At midday, NWS released an extra weather balloon. Over 3,000
feet, winds shifted from southeast to due south and increased from
20 mph to 40 mph. At 10,000 feet, winds shifted to southwest anddropped to 35 mph. At 20,000 feet, winds shifted further to thewest, increasing to 50 mph. Jet stream over 20,000 feet fromsouthwest helped to create a powerful lifting and circulation of thewinds below. As soon as the dry line hit the thunderstorm line,
unstable lifting occurred, which produced supercells and powerful
thunderclouds with strong updrafts at stunning speed. This is the
typical condition for a tornado outbreak. At 3:49 p. m., NWS
upgraded the risk evaluation again to high, and at 4:45 p.m. a tor-

KEY WORDS: tornado disaster, suction vortex, Reynolds stress, parent vortex, child vortex, downdraft, downburst, Fujita-Pearson scale

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nado warning was issued in three counties, Comance, Caddo andGrady.
3.

THE OUTBREAK OF TORNADOES

Fig.1 shows the spots and times the tornadoes were reported
on the day of outbreak, limited within the area on the map.
Before the outbreak, KFOR-TV (CH4) had already prepared
for tracking the coming tornadoes with the helicopter and cars.
The first report was from near Cyril at around 5:00 p.m., that a 7year-old girl was injured in Stecker. At 5:21 and 5:28 p.m., a tornado was also found northwest of Cyril. At that time, the helicopter and car of CH4 started tracking the tornado that touched
down. It is quite possible that these tornadoes are the same one
judging from the direction. Two tornadoes that were reported in
Roosevelt and Lake Tomsteed, over 50 miles west-southwest from

Fig. 1

Cyril, seemed to be generated from a different supercell. At 5:40
p.m., another tornado was reported in the south of Anadarko.
Just before 5:47 p.m., a tornado was found through the rightwindow of CH4’s car, which was running to the east along US62
near Verden. The picture from the helicopter shows a multi-cell
vortex, with a few vortexes rounding each other on a circle, which
was often seen afterwards. At 5:55 p.m., a large tornado wasfound two miles north of Laverty, which we suppose was different
from the one CH4 tracked, judging from its location. At 6:07 p.m.,a multi-cell vortex tornado was found in the west of Chickasha,and the KWTV (CH9) team at just started tracking tornadoes
encountered one around there. The Wheeler family of Chickasha
who took refuge in the basement felt the strange sensation in theears caused by the sudden drop of air pressure. At 6:19 when the
tornado was still active, two hangars in the Chickasha Municipal
Airport, three miles north-northwest of the city, were damaged andReported tornado spots and times

DESTRUCTION MECHANISM OF OKLAHOMA TORNADOES IN 1999

planes inside the hangars were turned upside down. Wustrek’shome, 100 yards south of the airport, was safe except that the roofof his pool was pulled up. A crop-spraying service building along
US81 was broken and a trailer park was damaged. The Wustrek
family felt as if a shell attacked them, but they were safe.
A characteristic of the tornadoes around here is that they weregenerated from a single super-cell and disappeared in so many
places and so repeatedly that they were uncountable. This characteristic can be applied to the later cases.
4. THE MAIN TORNADO
4.1 Grady County
At 6:26 p.m., a tornado was found crossing US92 which runs
from Amber to the south (Fig.1). If we suppose this tornado was
the one that damaged the airport, it is calculated that it moved six
miles in seven minutes, or over 50 mph, which is too fast to betrue, so it is inferred that they were not the same tornado but werein the same group because a part of the wing of an airplane anddocuments blown away from the airport reached South Oklahoma
City 30 miles away. At 6:37 p.m. CH4’s helicopter saw a tornado
moving parallel to I-44, and at 6:43 p.m., CH9’s helicopter pilot
saw it was going due east of Amber to the northeast. The tornado
moved six miles in this 17 minutes, or 20 mph. Ed Tilman in
Amber saw three small tornadoes were wrapped by one big tornado in the east. The relationship between them and the main tornado is unknown. A tornado was also reported at 6:40 p.m. in
Middleburg, eight miles to the east-southeast, but this is clearly adifferent one. A tornado was reported also at 6:51 around theplace where the main tornado crossed I-44 later. The main tornado
is supposed to have been three miles before Bridge Creek at thatmoment.
After crossing E1280th Road east of Amber, the speed of thetornado increased, became almost 50 mph and attacked the mobile
home community in Bridge Creek at around 6:57 p.m. Fig.2, arun-time curve of the main tornado after crossing US92 south ofAmber, clearly shows the acceleration. When stretched, a vortex
tends to stick to the ground more strongly. The radius of revolution becomes shorter, the atmospheric pressure at the center drops,
and the revolving speed increases to gain centrifugal force to balance with the low pressure. An aerial photograph taken the following day shows the bare ground from which grass was torn off.
This phenomenon is not yet clarified, but we can imagine that thesame thing may happen when we vacuum grass with a powerful
vacuum cleaner, which reminds us of Fujita’s (1971) suction vortex. All that we could see there on July 2 was the wreckage of asmall factory and what seemed to have been trailer houses scattered far away, and it was unbelievable that 235 houses weredestroyed and 11 people died. The important point is that the helicopter pilots of CH4 and CH9, consistently following the main tornado, could not catch the scene of the devastation. Though it wasquite natural that they could not go close to the dangerous tornado,
they must have been south-southeast of the tornado across I-44 atthat moment. Therefore it may be inferred that a tornado cannot beseen in case of heavy touch down. What we can see of a funnel
cloud is only the part where water vapor becomes drops by thedecrease of atmospheric pressure at the center of the tornado.

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When the whole tornado becomes like a cloud, you cannot recognize its shape.
Let us consider one of the news reports that relates to physicalphenomenon of a strong wind. When the tornado came above him
and blew away one mobile home and another just a few yards
away from him, Wesley Early could do nothing but grab a crosstie
and rush into the ditch. A few seconds later, 30 tons of gravel,
having been put down on the premise last year, flew around andscratched his back. Desperately keeping his eyes closed with thedebris hitting him, he was lifted in the air, not breathing landed
100 yards away from the original place. He returned in a hurry
and helped Mrs. Vandiver, who was dragged from her home andwas injured in her legs and head.
Regarding the expression “gravel flew around,” the mechanism of this phenomenon is as follows: a small vortex of a few
yards in radius, not so long as half a mile, is generated. It istremendously powerful and produces a strong flow especially justabove the ground, and the negative pressure at the core absorbs ahuman body and lifts it up. When the vortex is gradually scatteredand the absorbing power decreases, it slowly releases the body
down to the ground. This phenomenon is not extraordinary. One
of us has met two cases – Keiko Yoshino and her mother in
Mobara, Japan in 1990 and Abdu Joli in Gopalpur, Bangladesh in
1996 – and this is the third case. In three cases, all of them fainted,
and we are not sure whether this is because of a sudden drop of
atmospheric pressure or terror.
Ridge Crest Baptist Church at Bridge Creek was partly
destructed after 60 children had left, and a brand-new car climbed
the altar. Through the window of his house that was safe, Pastor
Duckett saw houses were blown up. If this was also caused by avortex, it leads to the conclusion that a larger vortex than the onethat lifts a person exists. It is a well-known fact that a large vortex
is divided into many small ones as it disappears. As long as theheating mechanism works that keeps the strong updraft blowing
high in the sky, various sizes of vortexes can exist at the sametime. We cannot tell, however, whether the vortexes exist from the
beginning or they are generated around objects in the air.
The zigzagging center of the tornado crossed I-44 at around
7:09 p.m. At just 7:00 p.m., when it almost reached I-44, 40 houses were broken, a farm damaged and scores of livestock injured at
northwest Newcastle, which was next to I-44 to the east. We canFig. 2

Run-time diagram of fiercest tornado

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not tell whether these damages are caused by the same main tornado or not. At the same time, a woman driving a sports car, who
failed to take shelter beneath the overpass because there were too
many other cars, was sucked up in the air and miserably killed.
Perhaps at another overpass, a mother was also killed, after askinga man from the same town to look after her 11-year-old son.
4.2 Cleveland County
After crossing I-44, the tornado went northeast with its power
decreasing, but regained its power again just before reaching
Pennsylvania Avenue in South Oklahoma City at 7:15 (Fig.3). It
attacked Country Place Estates, south of SW134 Street, anddestroyed 60 houses and killed some people. After passing a small
forest, tearing off the roof of Eastlake Elementary School and damaging Eastlake residential area to the north, it went almost straightto the east 30 degrees north at 20mph, passing a similar residential
area and a prairie, completely destroyed all the houses within thearea of 100 yards wide except the prairie out, and reached a golfcourse in the northeast of City of Moore (Fig.3 and Photo.1).
When the tornado was still around I-35, CH9’s helicopter
arrived above the Eastlake residential area and took a close-up picture of the destruction (Photo.2). The picture shows almost no projecting things within about 50 yards at the center of 100-yardswide area. If this is the radius of the vortex core, it is smaller thanhalf a mile vortex in diameter, which will be considered later.
At 5:30 p.m., over 400 people were at Westmoore HighSchool before Western Avenue for a graduation ceremony. The

Fig. 3center of the tornado that brushed the southeast corner greatly
damaged the roof and windows of the school building, but injured
no people. When it reached Moore, the revolving flow became thestrongest in this area, passing SW119 Street before Santa Fe
Avenue to the north, and houses on the south side of SW119 werecarried away leaving only the concrete slab. It crossed Santa Fe
Avenue, and just before Jane Way near Crouch Corner, a family
who took refugee in the bathtub was taken away with it. A 14year-old boy and his parents were carried a few yards away to different directions. The boy and his father were injured and themother was killed. After completely destroying Kelley Elementary
School, the tornado reached the point where I-35 branches off toShield Blvd (Photo.3). A family was going to take shelter under
the overpass, and while the father paid attention to his two children, the mother was carried away and was killed.
After crossing I-35, the tornado attacked Highland District.
Photo. 3 shows that a typical revolving flows with unstable winddirection damaged this area; cars were carried one block away. At
this time, CH4’s car reached the crossing of I-35 and SE89 Street(Fig.3), and videotaped the scene in the south. Photo. 4, takenfrom the videotape, shows the debris falling thick. Where was thedebris sucked up into the air? Judging from the thickness of thedebris, it seems that we can exclude distant places such as West
Moore and the circumference of the main vortex’s core. It seems
natural that objects around Kelly Elementary School were torn off
by small vortexes, flown into the circulating main vortex, carried
from the east to the north, and fell between the water tower andTornado path through Cleveland County

DESTRUCTION MECHANISM OF OKLAHOMA TORNADOES IN 1999

Photo. 1

Tornado trail from Pennsylvania Avenue to Midwest City via South Oklahoma, Moore,
Southeast Oklahoma and Del Cities (from Dairy Oklahoman May 8 1999)

Photo. 2

Disaster of Eastlake Residential Area, seen also in Photo. 1, ( from broadcast by KWTV)

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J. KATSURA AND H. CONNER

Photo. 3 Disaster in Moore, tornado coming from top to left side
bottom (Photo by Roger Bondy, from The Norman
Transcript May 6 1999)

Photo. 4

Falling thick debris between Moore water tower and SE89th (from video by KFOR-TV)

DESTRUCTION MECHANISM OF OKLAHOMA TORNADOES IN 1999

SE89th Street, which is far away from the course of the main vortex.
Although a detailed structure of the tornadic vortex standing
still on the ground was investigated by Rotunno (1984) , we placed
focus on the ground surface friction of a counterclockwise rotating
cylindrical air mass in which the full system of the vortexes wasbuilt. When the cylinder imitated by the parent vortex horizontallymoves keeping its shape, the rotational axis deviates toward rightside of the line the cylinder goes on, because the frictional stressbecomes stronger at the right side and weaker at the left.
Therefore, the flow is faster at the narrow right side of the axis,
and slower at the wide left side. Let us consider a small vortex at
the bottom of the side surface of cylinder, with its top slanted
along the direction of cylinder rotation (Fig. 4). If the velocity
increments induced by the child vortex with the same rotational
component as the cylinder is u’ in the rotational direction and w’ inthe upward direction, u’>0, w’<0 outside and u’<0, w’>0 inside thecylinder, and the Reynolds stress(ρ: air density) acts as
the friction.
If a lot of such child vortexes are around the parent vortex, thechild vortex density becomes thicker in the right side of the proceeding direction where the friction is stronger, and thinner in theleft side. Also, the child vortexes usually adhere behind the protuberances on the ground and sustain the frictional force of the rotational motion of the parent vortex. When these vortexes clingaround the parent vortex and stretched by the parent vortex flow,
the angular momentum of the child vortexes is conserved, speeding up the rotation and lowering the pressure at the child vortex’s
center. The flow falls from the rear to the right side to compensate
the momentum consumed by friction, and the flow rises from theright to the front side. A child vortex often sucks up, conveys
everything and releases it when the friction decays. This idea canbe applied to the cases in Bridge Creek and Eastlake residential
area (Photo.2), and describes the damage in Del City and Midwest
City, which is mentioned later.

Fig. 4

Systematic flow pattern in right side of tornado-rotational stream

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The tornado went straight over the prairie, missing First
Baptist Church, which was used as a relief center later, and thewater tower at its northeast corner by 300 yards to the left. Then it
destructed facilities in Lakeside Golf Course, and reachedSoutheast Oklahoma City from Moore.
4.3 Oklahoma County
Entering Southeast Oklahoma City, the tornado graduallyturned to the left, damaged a group of apartments, and crossed thecounty border at SE89th and Sunnylane Rd. Then the tornado
reached the residential area in the northern part of Southeast
Oklahoma City, slanted the southeast edge of Del City acrossSE44th Street and brushed the northwest edge of Tinker Air Force
Base along Sooner Road, reaching Midwest City. Then the tornado went almost due north, entered the center of the city across I40, went through the residential area in the north, and the damage
ended before Reno Avenue just before 8:00 p.m. (Fig. 5).
As far as we look at the distribution map of damaged areas
made by the computer team of Oklahoma City (The Sunday
Oklahoman May 9 1999), there seems to be nothing strange about
the damage. Since this shows only the final condition of the tornado, let us check in detail. First, in Del City, the tornado killed several people, and Oakbrook Drive, just north of SE44th Street, wasthe center of the damage. CH4’s helicopter reported the destruction next day (Photo.5). Seeing carefully, we notice a slight difference from Eastlake (Photo.2). The tornado completely destroyed
homes 100 yards wide in both areas, but in Del City there
remained many projecting things and little debris could be seen on

Fig. 5Tornado path through Oklahoma county

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J. KATSURA AND H. CONNER

Photo.5

Disaster of Oakbrook Dr., tornado coming from top to bottom , compared with Photo.2(from video KFOR-TV, not Midwest City but Del City)

Photo.6

Disaster of Arkadelphia in Arkansas, strong wind blowing from WSW(from Shifting Herald March 6 1997)

the roads. Therefore it is thought that small vortexes hardly caused
the destruction and winds that blew almost straight were the main
factor of the destruction. What matters here is whether a parent
vortex causes downward wind without a hard child vortex. The
uplifting flow at the parent vortex center in Fig.4 is also the driving
power of the tornado rotating system on the ground. If the up-drift
is slow, it is inferred that the parent vortex itself became too weak

to give energy to the child vortexes.
The tornado crossed Sooner Road, broke a part of the lodging
in Tinker Air Force Base, went through the field and wood of
about 800 yards in the southwest corner of Midwest City andattacked a small town along I-40 that runs northwest to southeast.
Mike Penderson, a Cracker Barrel employee, helped about 30 people into the restaurant’s back kitchen and then went to the front toDESTRUCTION MECHANISM OF OKLAHOMA TORNADOES IN 1999

look at the tornado. Facing southward, he saw a massive debris
wall coming to the restaurant. He went in and told the people toget down. 15 seconds later he felt his ears “pop”. Then silencecame and as he looked around the only thing that had survived was
the kitchen. Though a rental truck was blown into the dining
room, no one was injured. This is a little different from the Moore
case, in which they heard in the shelter first the sound of the hitting
debris and then a sound as if a train were passing; after thosesounds they noticed that nothing remained around them. The fallof a cold air mass might cause the destruction of the restaurant.
The tornado slightly attacked the market on 15th Street andcame to Will Rogers Rd. There it blew up the bathtub in which
York and his two sons were and threw them out. This is similar to
the Moore case. The debris of the destructed north residential area
simply moved northward. This directional clearness is very similar to the disaster in Arkadelphia, Arkansas, on March 3, 1997
(Photo.6). Each case coincides with Fujita’s (1993) theory that thecause is the strong downward wind which appears in the right sideof the line that a tornado goes on.
5. OTHER STRONG WINDS
5.1 The Tornadoes in Choctaw
As the Fig.6 shows, at 8:10 p.m. immediately after the main
tornado disappeared, a new tornado was found at the center of
Choctaw, which was nine miles east-northeast of the place where

Fig. 669

the main tornado vanished. It went through to the northeast by
three miles and disappeared at 8:15 p.m. The new tornado
destroyed some 20 homes and stores, ripped off the roof of thelibrary and soaked the books. It is easily inferred from the distancethat it was generated from the same supercell as that of the main
tornado.
5.2 The Tornado in Dover
At 9:20 p.m. a tornado attacked the town of Dover in King
Fisher County, 40 miles north-northeast from Oklahoma City,
which damaged one-third of the town from southwest to west andnorthwest, leaving 17 homes completely destroyed. A woman was
crushed to death under the walls fallen from two sides in her bedroom of a single-storied brick house.
5.3 The Tornado in Crescent and MulhallThis tornado was first found at 9:29 p.m., four miles southwest from Cimarron City in southwest Logan County. At around9:50 p.m., it attacked a village a little away from the center ofCrescent and destroyed 25 homes. It caused some incidents that
equal those of Bridge Creek: Patrick Fuller’s mobile home completely disappeared; Paul Keispert was sucked out of his house into
the air and landed 100 yards away, which is the fourth example ofsoft landing. Then, going to the northeast, the tornado attacked thelittle town of Mulhall, in the northeast of Logan County, shortly
after 10:00 p.m. It destroyed 80 out of 115 buildings, the school,

Disaster sites by strong winds in Oklahoma State

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J. KATSURA AND H. CONNER

Photo. 7

Disaster of Tanger Outlet Mall in Stroud (from broadcast by KWTV)

Photo. 8

Microburst disaster near Little Rock Airport in Arkansas, March 1997

three churches and a silo—everything on its way. This tornado isalso considered to have come from the south of Caddo County, notso far from where the main tornado came. One went to the northeast a little to the east and the other a little to the north. It was second only to the main tornado in power. Quite coincidentally,another tornado took almost the same course an hour later.
5.4 The Downburst in Stroud
The characteristic of the articles of the damage in Stroud, 44
miles east-northeast of Midwest City is that there is no descriptionof how the people escaped. The closest tornado was reportedly
generated at 9:27 p.m. at four miles southwest of Meeker, which isanother 23 miles southwest of Stroud. So the people might feelsecure. But the sudden strong wind shortly after 10:30 p.m. thatshut off the electricity surprised them. Fortunately no one wasinjured as the strong wind missed the center of the city. Though ahospital, a storehouse and some homes were destroyed, the mostdamaged was Tanger Outlet Mall. This shopping mall occupies
200,000ft 2 at the north of the west exit of I-44, which runs east to
west along the north side of the town. In that vast lot the one-storied market that is one mile long is placed like the letter U(Photo.7), its open side facing I-44 and the center space being theparking area. The downburst brushed by the road from the southwest and blew out the west wing to the parking lot and the otherpart of building outward, their exterior and interior together. A
glimpse of the destruction was enough to convince us that it resulted from a different type of downdraft from that of Midwest City,
which was gradually ending. We have ever seen the same type as
this in the suburb near Little Rock Airport in 1997 (Photo. 8). But
we do not have a definite explanation of where the falling cool air
perhaps with momentum of a jet stream comes from. The nearest
tornadoes were in Shawnee or Meeker, but we cannot exclude theDESTRUCTION MECHANISM OF OKLAHOMA TORNADOES IN 1999

71

possibility of the downburst coming from the main tornado. There
are two evidences: first of all, 10 miles north-northwest from
Stroud is Cushing, and photographs and other things blown up
from Moore and South Oklahoma were scattered from Cushingsouthward to Agra (Fig.6); secondly, a tornado was generated at
midnight in Sapulpa, 35 miles away on I-44 from Stroud toward
Tulsa. Moore, Stroud and Sapulpa are almost in a straight line andwe have seen in some past large tornadoes the cycle of ascending
and descending currents—such as ascending in Moore, descending
in Davenport-Stroud and ascending again in Sapulpa.

of Texas, the courses of the tornadoes were divided into the northwest ones and the southeast ones from the center of Oklahoma
City, where skyscrapers are. It is impossible to know the relationship between the courses and the surface roughness of the groundor the heat radiation. But the measurement of the surface roughness itself is not so difficult. While tracing the course of the helicopter by GPS, the laser shot vertically downward tells us the distance. Considering the condition of landing for a vortex that has
the vertical axis, it is natural to think the effect of the surface
roughness is not small.

6. CONCLUDING REMARKSACKNOWLEDGMENTS

It was fortunate that the dead were only 42, considering thepower of the tornadoes. It is still difficult to predict exactly where
and when a tornado breaks out. But the NEXRAD system enables
us to observe the movement of a thundercloud in real time and to
broadcast the result at once. Without this system more people
might have died. Live broadcasting on TV from helicopters,
though it is rather dangerous because we cannot know when a tornado breaks out near the helicopter, can attract the attention ofpeople. In considering security against tornadoes, examination
after disaster is important. NOAA’s research on the courses of thetornadoes has Fujita scales calculated by NWS. The Fujita scale isinferred from the degree of the destruction and converted into windspeed for objectivity. This wind speed, however, is different fromthe one used in a wind tunnel because small but strong vortexes
involved in the large vortex of a tornado speeds up the current.
When we actually see destruction sites, it is often impossible evento decide the course of the flow. Clearly small vortexes help thedestruction. When some stronger flows extend them, their speed
becomes unstable. We must understand the system of the destruction caused by them.
Another interesting fact of the tornadoes is that though the
updraft which caused super-cells was observed only on the border

The study for this report is based on the collaborative study agreement
between the University of Oklahoma and Kyoto University made in 1992.
We express our gratitude to Professor Michael Iver Wahl, College ofArchitecture of the University of Oklahoma, who examined the damage by
the Arkansas tornado in 1997, and to Dr. Yasuo Okuda, who collected theinter-net information and asisted us on the field survey, as well as to
KFORTV and KWTV, who showed us valuable videos and to the newspapers that permitted us to reprint their aerial photographs. We are deeply
grateful to Dr. Donald W. Burgess of National Severe Storm Laboratoryfor his encouragement to publish this report.

REFERENCES
Fujita T.T.,1971, Proposed mechanism of suction spots accompa nied bytornado, 7th conf . On Severe Local Storms, Kansas City, American
Meteorological Society, 208-213.
Fujita T. T. and Smith B. E. , 1993, Aerial survey and photography of tornado and microburst damage, Tornado: It’s structure, dynamics, prediction and hazards edited by Church C., Burgess D., Doswell C. andDavis-Jones R., Geographical Monograph, Vol.79, 479-499.
Rotunno R. 1984, An Investigation of a Tree-Dimensional Asymmetric
Vortex, Journal of Atmospheric Science, Vor.41 No.2, pp.283-298.