A tornado is a rapidly spinning column of
air that develops in association with severe
thunderstorms. It can vary in size from only
a few metres to over 1,000 metres in diameter
and its track may be from a few hundred metres
to more than 100 kilometres. It can last from
only a few minutes to an hour or more.
Inside the air column, there is a very powerful
updraught which causes very low atmospheric
pressure at the surface. Immediately outside
the column, winds are rotating (anticlockwise
in the northern hemisphere, clockwise in the
southern) at speeds that can be higher than
90 metres per second (200 miles per hour).
Fig 1: Tornado
south of Dimmitt, Texas June 1995 (Image
courtesy of NOAA)
How do tornadoes
form?
Fig 2: Synoptic
situation for tornado formation
Tornadoes are difficult to predict accurately as scientists
still do not fully understand exactly how they are
formed. Generally speaking, if cold, dry air comes
into contact with warm, moist air, there is the potential
for tornadoes to develop. This happens on a cold front
and several tornadoes may develop on a particularly
active front. Figure 2 shows
a particular case, but is representative of the kind
of synoptic situation that could lead to tornadoes
forming. The warm, moist air is moving northwards from
the Gulf of Mexico while cold, dry air is moving south-eastwards
from the Rockies.
Severe thunderstorms known as 'supercell thunderstorms'
can form near to the front. These have extremely
powerful updraughts that can reach up to the top
of the cloud. There is a large change of wind direction
and an increase in wind speed with increasing height
near the front. This 'vertical wind shear' causes
the updraught to rotate, causing a spinning motion
in the lower atmosphere.
Rapidly rising air (convection) within the storm
cloud moves this spinning motion (rotation) towards
the vertical.
This rapid rotation extends downwards towards the
surface, initially as a funnel cloud. If conditions
are favourable, it will reach the surface (touch
down) as a tornado.
Fig 3: Tornado formation
Fig 4: Tornado funnel
Where do
tornadoes form?
The short answer to this is that they could form anywhere
where the atmospheric conditions are favourable. There
is a lot of information and publicity about tornadoes
in the USA and there is no doubt that some of those
are very dangerous and cause considerable damage and
loss of life. However, when area is taken into account,
they are more frequent over the British Isles. The
difference is that very few British tornadoes cause
significant damage or loss of life.
Fig 5: Location of
'Tornado Alley' Courtesy of NOAA
The area known as the Mid-West, between the Rockies
in the west and the Appalachians in the east, has the
dubious distinction of being termed 'Tornado Alley'.
This is where the greatest number of the most damaging
tornadoes occur and there is a good reason why this
should be the case. The area around the Gulf of Mexico
is hot and humid. At the same time, the air that passes
over the Rockies can be cold and, perhaps more importantly,
is very dry. It is this combination that can lead to
the development of enormous cumulonimbus
clouds growing up to nearly 18,000 metres (60,000
feet). Occasionally, the clouds may group together
into what is known as 'supercells', and it is from
these that the most dangerous tornadoes are likely
to form - along with intense thunderstorms and damaging
hail showers. The cumulonimbus clouds that develop
over the British Isles rarely extend above 12,000 metres
(40,000 feet) and are consequently less 'energetic'
than those over America.
The only continent where tornadoes have not been reported
is Antarctica. They have occurred in every US state,
including Alaska, though very rarely west of the Rockies.
Tornadoes often occur in groups, known as 'outbreaks',
and the greatest number on any one occasion was during
the Super Outbreak on 3-4 April 1974 when 148 were
reported from 11 states. As a comparison, there were
105 tornadoes reported in England within a few hours
on 21 November 1981.
When do tornadoes
form?
Theoretically, tornadoes can form at any time of year;
one struck Selsey in West Sussex on 8 January 1998.
However, they are most likely during spring and early
summer. They are less frequent during late summer and
early autumn, then there is an increase for a time
from mid to late autumn, though well below the springtime
frequency.
What is the
effect of tornadoes and can they be predicted?
For their size, tornadoes are the most destructive
weather features of all. The direct impact and
twisting effects of the winds themselves can
cause considerable damage, but this is made much
worse by the flying debris that accompanies them.
Straws have been driven through wood and small
pieces of wood into concrete walls.
The most powerful tornadoes occur in the USA
and can cause a broad swathe of total destruction
along their path (Fig 6). Nevertheless, given
the right conditions in the British Isles,
they can become powerful enough to uproot trees,
damage houses and overturn vehicles, as happened
in Birmingham on 28 July 2005 (Figs
7 and 8).
Tornadoes have been studied most in the USA
and the British Isles. This study has been more
practical in the US, since they can be so destructive
there. The type of weather situation favourable
for their development has been discovered. This
is known as the climatology of tornado formation.
Images from satellites and radars can show
features that indicate if tornadoes are about
to form or have already done so. Numerical
models are being developed that may also help.
The important result of this is that warnings
of the likelihood of tornadoes in a region
can now be made and this has allowed people
to take precautions. A system of watches and
warnings, similar to those for hurricanes,
has been developed.
Fig 9: Synoptic
situation on 28 July 2005 at 1200 GMT,
prior to the Birmingham tornado event
How strong
can tornadoes be?
It is rarely possible to measure the winds around
a tornado directly, so its strength is assessed by
both its size and the damage it causes. There are two
methods in use. In America, the Fujita or F-scale has
been developed. This has a range from F0 to F5, with
F5 being the most powerful and destructive. The damage
shown in Figure 6 was caused
by an F5 tornado. On this scale, the Birmingham tornado
was F2.
The Fujita
or F-scale as used in America
Fujita
intensity
Description
of tornado and wind speeds
Description
of damage (for guidance only)
F0
Light
damage (<73 m.p.h.)
Some
damage to chimneys; branches broken off trees;
shallow-rooted trees pushed over; sign boards
damaged.
F1
Moderate damage (73-112
m.p.h.)
Peels surface off roofs;
mobile homes pushed off foundations or overturned;
moving autos blown off road.
F2
Considerable
damage (113-157 m.p.h.)
Roofs torn
off frame houses; mobile homes demolished; boxcars
overturned; large trees snapped or uprooted; light-object
missiles generated; cars lifted off ground.
F3
Severe damage (158-206
m.p.h.)
Roofs and some walls
torn off well-constructed houses, trains overturned;
most trees in forest uprooted; heavy cars lifted
off ground and thrown.
F4
Devastating
damage (207-260 m.p.h.)
Well-constructed
houses levelled; structure with weak foundations
blown off some distance; cars thrown and large
missiles generated.
F5
Incredible damage (261-318
m.p.h.)
Strong frame houses lifted
off foundations and swept away; automobile-sized
missiles fly through the air in excess of 100 metres
(109 yards); trees debarked; incredible phenomena
will occur
In Britain, a scale based on the Beaufort wind scale
is used, which is known as the TORRO or T-scale. This
has a range from T0 to T10. The Birmingham tornado
was measured as T3-T4.
TORRO tornado
scale values as used in the UK
TORRO
intensity
Description
of tornado
& wind speeds
Description
of damage (for guidance only)
T0
Light
tornado 17-24 m/s (39-54 m.p.h.)
Loose light
litter raised from ground level in spirals. Tents,
marquees seriously disturbed; most exposed tiles,
slates on roofs dislodged. Twigs snapped; trail
visible through crops.
T1
Mild tornado 25-32 m/s
(55-72 m.p.h.)
Deckchairs, small plants,
heavy litter becomes airborne; minor damage to
sheds. More serious dislodging of tiles, slates,
chimney pots. Wooden fences flattened. Slight damage
to hedges and trees.
T2
Moderate
tornado 33-41 m/s (73-92 m.p.h.)
Heavy mobile
homes displaced, light caravans blown over, garden
sheds destroyed, garage roofs torn away, much damage
to tiled roofs and chimney stacks. General damage
to trees, some big branches twisted or snapped
off, small trees uprooted.
T3
Strong tornado 42-51
m/s (93-114 m.p.h.)
Mobile homes overturned/badly
damaged; light caravans destroyed; garages and
weak outbuildings destroyed; house roof timbers
considerably exposed. Some of the bigger trees
snapped or uprooted.
T4
Severe
tornado 52-61 m/s (115-136 m.p.h.)
Motor cars
levitated. Mobile homes airborne/destroyed; sheds
airborne for considerable distances; entire roofs
removed from some houses; roof timbers of stronger
brick or stone houses completely exposed; gable
ends torn away. Numerous trees uprooted or snapped.
T5
Intense tornado 62-72
m/s (137-160 m.p.h.)
Heavy motor vehicles
levitated; more serious building damage that for
T4, yet house walls usually remaining; the oldest,
weakest buildings may collapse completely.
T6
Moderately-devastating
tornado 73-83 m/s (161-186 m.p.h.)
Strongly-built
houses lose entire roofs and perhaps also a wall;
more of the less-strong buildings collapse.
T7
Strongly-devastating
tornado 84-95 m/s (187-212 m.p.h.)
Wooden-frame houses wholly
demolished; some walls of stone or brick houses
beaten down or collapse; steel-framed warehouse-type
constructions may buckle slightly. Locomotives
thrown over. Noticeable de-barking of trees by
flying debris.
T8
Severely-devastating
tornado 96-107 m/s (213-240 m.p.h.)
Motor cars
hurled great distances. Wooden-framed houses and
their contents dispersed over long distances; stone
or brick houses irreparably damaged; steel-framed
buildings buckled.
T9
Intensely-devastating
tornado 108-120 m/s (241-269 m.p.h.)
Many steel-framed buildings
badly damaged; locomotives or trains hurled some
distances. Complete debarking of any standing tree-trunks
T10
Super tornado
121-134 m/s (270-299 m.p.h.)
Entire
frame houses and similar buildings lifted bodily
from foundations and carried some distances. Steel-reinforced
concrete buildings may be severely damaged.
How
destructive can tornadoes be?
The greatest number of fatalities occurred
on 18 March 1925, when 695 people were killed
in the Tri-state Outbreak that affected Missouri,
Illinois and Indiana. This was caused by a single
F5 tornado that travelled a distance of 219 miles
(352 km) at a speed frequently in excess of 60
m.p.h. (97 k.p.h.). It crossed both farmland
and towns, with most of the casualties occurring
in the towns. Six other tornadoes also developed
that day. Damage has been estimated at around
$16.5 million in 1925 money. There were several
reasons why the casualty rate was so high and
the damage so extensive.
The power and track of the tornado.
The type of buildings. Most houses were
built of wood, which would have disintegrated
when struck by such a tornado, the wooden
planks then turning into deadly missiles.
These days, mobile homes are among the most
vulnerable dwellings.
The lack of shelters.
The lack of warning.
The greatest number of tornadoes in recorded
history occurred on 3-4 April 1974 when 148 'twisters'
touched down in 13 US states and killed 330 people.
Casualties would have been considerably higher
but for the Tornado Warning system that was in
place by then.
Fig 10: Tornado
Cordell. Oklahoma May 1981. Image courtesy
of NOAA
The great majority of information about tornadoes
originates from the USA. This does not mean that they
are necessarily weaker or less frequent elsewhere in
the world; simply that there is considerably more interest
in America, with much more research work being done
there. This may change in the future.
The following links provide further information on
tornadoes:
