Geoscience Reference
In-Depth Information
Figure 9.33
The synoptic conditions favouring severe storms
and tornadoes over the Great Plains.
Figure 9.34
Schematic diagram of a
complex tornado with multiple suction
vortices.
Source
: After Fujita (pp. 1, 251, fig. 15),
by
permission
of
the
American
Meteorological Society.
cycloidal path. The whole tornado system gives a
complex pattern of destruction, with maximum wind
speeds on the right-side boundary (in the northern
hemisphere), where the translational and rotational
speeds are combined. Destruction results not only from
the high winds, because buildings near the path of the
vortex may explode outwards owing to the pressure
reduction outside. Intense tornadoes present problems
as to their energy supply, and it has been suggested
recently that the release of heat energy by lightning and
other electrical discharges may be an additional energy
source.
Tornadoes commonly occur in families and move
along rather straight paths (typically between 10 and
100 km long and 100 m to 2 km wide) at velocities
dictated by the low-level jet. Thirty-year averages
indicate some 750 tornadoes per year in the United
States, with 60 per cent of these during April to June
(see Figure 9.32B). The largest outbreak in the United
States occurred on 3 to 4 April 1974, extending from
Alabama and Georgia in the south to Michigan in the
north and from Illinois in the west to Virginia in the east.
This 'Super Outbreak' spawned 148 tornadoes in twenty
hours with a total path length of over 3200 km.
Tornadoes in the United States cause about 100 fatal-
ities and 1800 injuries each year on average, although
most of the deaths and destruction result from a few
long-lived tornadoes, making up only 1.5 per cent of the
total reported. For example, the most severe recorded
tornado travelled 200 km in three hours across Missouri,
Illinois and Indiana on 18 March 1925, killing 689
people.
Tornadoes also occur in Canada, Europe, Australia,
South Africa, India and East Asia. They are not
unknown in the British Isles. During 1960-1982 there
were fourteen days per year with tornado occurrences.
