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Tornadoes
Tornadoes are among the most violent storms on the planet. They are small in size and short-lived but
can be highly destructive. Because of their severe nature, the measurement and observation of
tornadoes is difficult. A small number of armoured probes called ‘turtles’ have been placed
successfully in tornadoes, but there is still very little known about these storms. Tornadoes consist of
elongated funnels of cloud that descend from the base of a well-developed cumulonimbus cloud,
eventually making contact with the ground below. In order for a vortex to be classified as a tornado, it
must be in contact with both the ground and the cloud base. Rotating violent winds inside the funnels
can exceed 100 m per second and pressure can reach 25 mbar per 100 m. In comparison, a largerscale cyclone has a pressure gradient of about 20 mbar per 100 km.
How tornadoes form
The standard explanation for how tornadoes form is that warm, moist air meets cold, dry air. However, tornadoes
differ from other low-pressure systems, such as tropical cyclones, hurricanes and typhoons, as these develop over
warm seas whereas tornadoes occur over a relatively small area of land. Tornadoes develop when a number of
conditions occur simultaneously. Figure 1 shows how tornadoes form in North America:
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A fast northerly flow of humid air from the Gulf of Mexico with temperatures over 24°C at ground level.
A cold, dry air mass moving at speeds of over 80 km/h from Canada or the Rocky Mountains.
Jet-stream winds racing east at over 384 km/h.
Figure 1. How tornadoes form.
As warm Gulf air moves upward, a strong updraft is formed. This is spun around by fast-moving, cold polar air and
then twisted in another direction at higher levels by the jet stream. This produces a corkscrew motion, with warm air
rising on the leading side and cool air descending on the trailing side. Once formed, tornadoes are further fuelled by
energy from electrical discharges of lightning within clouds and from latent heat released by heavy rainfall from the
cloud. The key ingredients in tornado formation therefore are the combined effects of moisture, lift, wind shear and
instability.
The size and strength of a tropical cyclone is not always related to the severity of a subsequent tornado. Relatively
weak hurricanes like Danny (1985) have spawned significant supercell tornadoes inland, as have larger, more
intense storms like Beulah (1967) and Ivan (2004). In general, the bigger and stronger the wind fields are within a
tropical cyclone, the bigger the area of favourable wind shear for supercells and tornadoes. But supercell tornadoes
(whether or not in tropical cyclones) must 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.
Most tornadoes, but not all, rotate cyclonically – anticlockwise in the northern hemisphere and clockwise south of the
equator. Many thunderstorms form under those conditions (near warm fronts and cold fronts), but never even come
close to producing tornadoes. Even when the large-scale environment is extremely favourable for tornadic
thunderstorms, not every thunderstorm spawns a tornado. The most destructive and deadly tornadoes occur from
supercells. These are rotating thunderstorms with a well-defined, low-pressure system called a mesocyclone.
Many tornadoes have a short life. Tornadoes can last from several seconds to more than an hour. The convectional
activity that creates the source cloud is itself highly variable, and a single cloud can spawn a number of different
tornado vortices. These vortices can occur either simultaneously or in sequence beneath different areas of the cloud,
as parts of the cloud develop and decay. The tornado vortices usually move with the parent cloud, often with the
funnel twisting sinuously across the ground below. Once contact with the ground is made, the track of a tornado at
ground level may frequently extend for only a few kilometres, though there are examples of sustained tracks
extending over hundreds of kilometres. The diameter of the funnel is rarely more than 200 m so therefore track length
and width are limited.
Tornadoes, being associated with extreme atmospheric instability, occur as a result of both seasonal and locational
factors. Favoured areas are temperate continental interiors (Figure 2) in spring and early summer, when the air may
be unstable, although many parts of the world may be affected by tornado outbreaks at some time. The Great Plains
of the USA , including Oklahoma, Texas and Kansas, have a high chance of tornadoes, and are particularly prone at
times when cool, dry air from the Rockies overlies warm, moist Gulf air. 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 tornado-producing weather patterns, such as hurricanes in south Texas or northwestflow weather systems in the upper Mid West.
Figure 2. Locations of tornadoes in the USA.
Some landfall hurricanes in the US fail to produce any known tornadoes, while others cause major outbreaks. The
same hurricane also may cause no tornadoes for a while, then may erupt with tornadoes. 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 landfall.
Tornado damage
About one thousand tornadoes hit the US yearly. On average, tornadoes kill about 60 people per year – most from
flying or falling (crushing) debris. A tornado’s impact as a hazard is extreme. There are three damaging factors at
work:
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Firstly, the winds are often so strong that objects in the tornado’s path are simply removed or very severely
damaged.
Secondly, strong rotational movement tends to twist objects from their fixing, and strong uplift can carry
some debris upwards into the cloud.
Finally, the very low atmospheric pressure near the vortex centre is a major source of damage.
Tornadoes can have a devastating effect on buildings. When a tornado approaches a building, external pressure is
rapidly reduced and, unless there is a nearly simultaneous and equivalent decrease in internal pressure, the walls
and roof may explode outwards in the process of equalizing the pressure differences.
Most of the time, tornado damage happens either with multiple-vortex tornadoes or very small, intense single-vortex
tornadoes. The winds in most of a multiple-vortex tornado may only be strong enough to do minor damage to a
particular house. But one of the smaller sub-vortices, perhaps only a few metres across, may strike the house next
door with winds over 322 km/h, causing complete destruction. Also, there are 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.
Although winds from the strongest tornadoes far exceed that from the strongest hurricanes, hurricanes typically
cause much more damage 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 greater variety of ways to damage property. The destructive
core in hurricanes can be tens of kilometres across, last many hours and damage structures through storm surge and
flooding caused by rainfall, as well as from wind. Tornadoes, in contrast, tend to be a few hundred metres in
diameter, last for minutes and primarily cause damage with their extreme winds.
Analysing damage
Dr. T. Theodore Fujita developed a damage scale for winds, including tornadoes, which relates the degree of
damage to the intensity of the wind. Unfortunately, the disadvantage of using the Fujita scale (Figure 3) is that it
does not take into account differences in building structure and materials.
Figure 3. Fujita Tornado Damage Scale.
Category F0
Light damage (<117 km/h)
Some damage to chimneys; branches
broken off trees; shallow-rooted trees
pushed over; signs damaged.
Category F1
Moderate damage (118–180
km/h)
Peels surface off roofs; mobile homes
pushed off foundations or overturned;
moving autos blown off road.
Category F2
Considerable damage (181–253
km/h)
Roofs torn off frame houses; mobile
homes demolished; boxcars overturned;
large trees snapped or uprooted; lightobject missiles generated; cars lifted off
ground.
Category F3
Severe damage (254–332 km/h)
Roofs and some walls torn off wellconstructed houses, trains overturned;
most trees in forest uprooted; heavy cars
lifted off ground and thrown.
Category F4
Devastating damage (333–418
km/h)
Well-constructed houses levelled;
structures with weak foundations blown off
some distance; cars thrown and large
missiles generated.
Category F5
Incredible damage (419–512
km/h)
Strong frame houses lifted off foundations
and swept away; automobile-sized
missiles fly through the air in excess of
100 metres; trees debarked; incredible
phenomena will occur.
A new Enhanced F-Scale will be used from 2006. The Enhanced F-scale will be a much more precise way to rank
tornado damage than the original, because it will classify damage F0–F5 as calibrated by engineers across over 20
different types of buildings. The idea is that, when rating tornado damage, the scale of a tornado 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 buildings. In the Enhanced F-scale, there are different, customized standards for
assigning any given F-rating to a well-built, well-anchored wood-frame 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 proper damage analysis.
Managing tornadoes
The main problem with anything that could realistically stand a chance at affecting a tornado (e.g. a hydrogen bomb)
is that it would be even more deadly and destructive than the tornado itself. Lesser things (like huge piles of dry ice)
would be too hard to deploy in the right place fast enough, and would likely not have enough impact to affect the
tornado.
Figure 4. A tornado in action.
The Indiana/Kentucky tornado, November 2005
Indiana’s deadliest tornado for more than 30 years flattened a mobile home park outside Evansville and struck other
communities early in November 2005, killing at least 16 people and injuring 200 others. The ‘twister’ was the
deadliest to strike the state since the 3 April 1974 ‘super outbreak’ of 21 Indiana tornadoes that killed 47 people.
The tornado slashed a 48-km path through Warrick County, from north of Newburgh to south of Boonville, before the
damage stopped near the town of Tennyson. The worst damage was at the Eastbrook Mobile Home Park on
Evansville’s southeast side where at least twelve people died. Other victims were thought to be trapped in the debris.
About 100 of the park’s 320 occupied homes were destroyed and 125 others damaged. Five others were confirmed
dead in adjoining Warrick County, east of Evansville, where the Ohio River city of Newburgh was struck after 2 am.
The National Weather Service posted severe thunderstorm warnings for sections of northern Ohio. The damage path
through Newburgh, 13 km east of Evansville, was about 1.2 km wide, and extended for roughly 32 km. Emergency
sirens sounded, but most people did not hear them because the tornado happened in the middle of the night. No
deaths were reported in Kentucky. Some 25,000 homes were without power, mostly in Warrick County. There also
were reports of natural gas leaks.
Figure 5. Map of Indiana’s tornado locations.
Past tornadoes in Indiana
Indiana is in what is considered to be ‘Tornado Alley’, a swath of states extending from the Southeast to the Plains.
Although the state lacks the high frequency of tornadoes seen in places like Kansas and Oklahoma, it makes up for it
in tornadic intensity.
Tornadoes can occur in any month but March–June is considered tornado season in Indiana. Historically, the most
destructive tornadoes strike in March and April. June holds the record for the most tornadoes in Indiana on any given
day (37) and for the most in a single month (44). Both records were set in 1990 (also the year for the most tornadoes
in the state, 49).
From 1950 to November 2001, 1,024 tornadoes caused more than $1.7 billion (around £1 billion) in damage in
Indiana and killed 223 people. Indiana was one of three midwestern states in the path of the deadliest tornado in
American history. On 18 March 1925, the Tri-State Tornado travelled a record 352 km on the ground from Missouri
through Illinois and into Indiana where it struck Posey, Gibson and Pike counties. The town of Griffin lost 150 homes,
and 85 farms in the area around Griffin and Princeton were devastated. About half of Princeton was destroyed with
losses totalling nearly $2 million (around £1.2 million). The funnel finally dissipated just outside Princeton, three and a
half hours after it had begun. Nearly 700 people died, 74 of them in Indiana. Murphysboro, Illinois, lost 234 people, a
record for a single community.
On Palm Sunday in 1965, eleven tornadoes struck 20 counties in central and northern Indiana, killing 137 people.
More than 1,700 people were injured and property damage exceeded $30 million (£17 million). It was Indiana’s worst
tornado disaster. The tornadoes that devastated Indiana were part of an outbreak in which nearly 50 tornadoes
struck the Great Lakes region on 11–12 April, causing 271 deaths and more than 3,400 injuries.
The most destructive tornado outbreak of the twentieth century was the ‘Super Outbreak’ of 3–4 April 1974. During a
sixteen-hour period, 148 tornadoes hit thirteen states, including Indiana. The path of destruction stretched 4,000 km.
More than 300 people died and more than 5,000 were injured. The most notable tornado in this group destroyed
much of Xenia, Ohio. In Indiana, 21 tornadoes struck 39 counties, killing 47 people. Seven of the tornadoes produced
damage rated F5 on the Fujita scale, the maximum possible, and 23 more were rated F4. This was one of only two
outbreaks with over 100 confirmed tornadoes, the other being with Hurricane Beulah in 1967 (115 tornadoes).
Figure 6. Table of Indiana’s tornado disasters.
April 13, 1852
New Harmony
16 killed
May 14, 1886
Anderson
43 killed
March 23, 1913
Terre Haute
21 killed
March 11, 1917
New Castle
21 killed
March 23, 1917
New Albany
45 killed
March 28, 1920
Allen through Wayne counties
39 killed by three tornadoes
April 17, 1922
Warren through Delaware counties
14 killed
March 18, 1925
‘Tri-State Tornado’, Posey, Gibson
and Pike counties
74 killed
March 26, 1948
Coatesville destroyed
20 killed
May 21, 1949
Sullivan and Clay counties
14 killed
April 11, 1965
‘Palm Sunday Outbreak’, 11
tornadoes, 20 counties
137 killed
April 3, 1974
‘Super Outbreak’, 21 tornadoes hit 39 47 killed
counties
June 2, 1990
37 tornadoes hit 31 counties
Figure 7.
Fatalities in Indiana.
8 killed
Tornadoes in the UK
Birmingham
The UK experiences more tornadoes per square kilometre than the USA but most are small events, rarely making the
headlines. However, on Thursday 28 July 2005, roofs were ripped off houses, cars hurled across roads and trees
uprooted as a tornado struck high street shops and suburban streets in Birmingham in the West Midlands. Winds of
over 209 km/h tore through the Small Heath district. At least 12 people were injured and 1,000 buildings were
damaged. At one stage, paramedics set up an emergency field centre to cope with cuts and broken bones. In the first
hour after the tornado, the West Midlands dealt with 240 incidents, using 25 fire engines and 100 firefighters.
The devastation centred on Ladypool Road in Sparkbrook and parts of King’s Heath and Balsall Heath, where the
tornado ploughed a narrow trail of damage across residential roads and a park. The sudden onslaught occurred at
2.30 pm. Many attempts to run for cover were thwarted by falling debris. Traffic chaos followed immediately after the
winds had gone. Drivers tried vainly to find ways through the stricken area. King’s Heath High Street was sealed off
after a car park attendant’s wooden hut was blown from one side to the other.
The violent weather, on a meteorological ‘faultline’ where warm and very wet conditions were pushing north against
much colder, dry air, then moved eastwards, causing a second but much smaller tornado in Peterborough. Air around
the upright cyclone began to spin and was drawn towards the centre, increasing the tornado’s speed.
Damage ran into millions of pounds. The government announced that it would grant emergency financial aid to
Birmingham city council and could consider grants of up to 85 per cent. Cornwall received similar such assistance
after its flash floods.
Some 20 properties were wholly or partly demolished. A dozen roads were closed off because of dangerous
structures and debris. Leaflets in English and Urdu, giving people practical advice on safety, were distributed. The
stricken area is less than a kilometre from the scene of a storm in 1999 when high winds blew over a roadside tree
on to several vehicles, killing three people. The King’s Heath area was also struck by a similar tornado in 1931, when
there was one fatality.
Figure 8. Tornado damage.
A second tornado struck Birmingham in October 2005, within a kilometre of where the July tornado struck. The roof
of a house was ripped off and a road was closed because of fallen debris. This occurred when a low-level blanket of
hot and humid air met a wall of high-level cold air on its way from the Arctic, drawing the moist, hot air higher.
The reason why Birmingham gets more than its share of tornadoes is to do with the topography of the city. It is
located on a plateau, surrounded by lower-lying land, notably Worcester and Evesham to the south. When the wind
blows from the south, air is forced northwards. As it reaches the Birmingham plateau, it can either go around the
plateau or be forced upwards, which is what happened when the tornadoes were formed. Large storm clouds formed
over the city, trapping a vast amount of energy within them.
The Bristol Twister
Less than a week after the first tornado in Birmingham, residents in Bristol experienced a giant twister over the
southern suburbs. A waterspout is a tornado over water. Although waterspouts are always tornadoes by definition,
they don’t officially count as tornadoes unless they hit land. They are smaller and weaker than the most intense Great
Plains tornadoes, but can still be quite dangerous. Waterspouts can overturn small boats, damage ships and do
significant damage when hitting land. The Met Office confirmed that the twister was up to 300 m in length and came
close to causing widespread damage. The twister swept across a 24 km area, including the suburbs of Whitchurch,
Hartcliffe and Keynsham, lasting for about fifteen minutes. The twister was technically not a tornado because it failed
to touch the ground.
Conclusion
Tornadoes are among the world’s most violent, extreme weather conditions. They are a complex phenomenon and
are difficult to study. They are particularly common in parts of the USA and also in the UK. Nevertheless, they can
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occur in many locations. The UK holds the record for the greatest density of tornadoes per km , although they tend to
be small scale. The extent of damage is related to the strength of the tornado, as well as the materials that buildings
are constructed of. Management of tornadoes is very difficult and few solutions have yet been seriously considered.