Localized Storms - Natural Hazards - page 99

Edmonton, Alberta, at 53.5°N. This is far north of
the zone where sharp temperature differences favor
tornado development. Here, in 1987, a F4 tornado
killed 27 people. Other areas of the world are sus-
ceptible to tornadoes. Surprisingly, a region such as
Queensland, Australia, reports 4.0 tornadoes per
10 000 people - more than Kansas. The Russian
Steppes are similar in relief to the Great Plains. Here,
however, there is no strong looping of the jet stream or
any nearby warm body of water supplying high
humidity. Great Britain and the Netherlands average
50 and 35 tornado-like vortices a year, respectively. In
Europe overall, about 330 vortices are reported each
year, of which half occur as waterspouts and fewer
than ten cause noteworthy damage. One of the
greatest tragedies on this continent occurred in Venice
on 11 September 1970 when a tornado sank a
waterbus near St Mark's Square and swept later
through a campground, killing 35 people. Surprisingly,
tornadoes are rare in Russia, Africa and the Indian
subcontinent. In the case of the last, tornadoes are
rarely reported outside the northern part of the Bay of
Bengal where they are associated with tropical
cyclones. However, the deadliest tornado occurred in
Bangladesh on 26 April 1989, killing 1300 people and
injuring 12 000 others.

(about 800 hPa), but how rapidly this change in
pressure occurs. The fact that the tornado is so
compact and has a sharp wall means that a pressure
reduction of tens of hPa s -1 could occur, although this
dip has been difficult to substantiate by measurement.
Air pressure in buildings affected by a tornado ends up
being much higher than the outside air pressure, and
cannot escape from the structure quickly enough
without causing an explosion. This exploding effect is
the main reason for the total devastation of buildings,
whereas adjacent structures or remaining remnants
appear untouched. The pressure reduction can be so
rapid that live objects appear burnt because of dehy-
dration. One of the major causes of death from a
tornado is not due to people being struck by flying
debris or tossed into objects, but to their receiving
severe burns. It is even possible that the defeathered
chickens described above could be cooked because of
this phenomenon.
Between 1916 and 2000, tornadoes killed 12 766
people in the United States. This death toll has
decreased by 200 people per decade because of
increased warning and awareness (Figure 4.13). In
April 1965, 257 people lost their lives as 47 tornadoes
developed along a cold front in the central United
States. The 'Three States' tornado of March 1925
created a path of destruction over 350 km long. Over
700 people were killed by the tornadoes spawned from
the movement of this single parent cloud. One tornado
in March 1952 destroyed 945 buildings in Judsonia,
Arkansas. Another tornado - in Rock Springs, Texas, in
April 1927 - killed 72 people, injured 240 others,
and wiped out most of the town of 1200 people in
90 seconds. In all, 26 per cent of the population
were either killed or injured. Cities are not immune,
although high-rise buildings tend to survive better than
single-unit dwellings. For example, in June 1966, a
tornado went through Topeka, Kansas, destroying most
homes and shops in its path but leaving multistoried
flats and office buildings undamaged. The worst
tornado disaster in the United States occurred during
a 16-hour period on 3-4 April 1974. A total of 148 tor-
nadoes was recorded from Alabama to Canada, killing
315 people and causing damage amounting to $US500
million. Six of the tornadoes were amongst the largest
ever recorded. Complete towns were obliterated, the
worst being Xenia, Ohio, where 3000 homes were
destroyed or damaged in a town with only 27 000
residents.

Torn ado destruction
(Whipple, 1982; Nalivkin, 1983)

Tornadoes are one of the most intense and destructive
winds found on the Earth's surface. The fact that they
most frequently form along cold fronts means that
they can occur in groups over wide areas. Their
destructive effects are due to the lifting force at the
funnel wall, and to the sudden change in pressure
across this boundary. The lifting force of tornadoes is
considerable, both in terms of the weight of objects
that can be moved, and the volume of material that
can be lifted. Large objects weighing 200-300 tonnes
have been shifted tens of metres, while houses and
train carriages weighing 10-20 tonnes have been
carried hundreds of meters. Tornadoes are also able to
suck up millions of tonnes of water, which can be
carried in the parent cloud. Tornadoes have been
known to drain the water from rivers, thus temporarily
exposing the bed.
The most destructive mechanism in a tornado is
the dip in barometric pressure across the wall of the
funnel. It is not how low the pressure gets in the eye