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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

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.

(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 fatalities 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.

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