Geoscience Reference
In-Depth Information
scale of tornado intensity has been developed; called
the Fujita Tornado Intensity Scale, it is based on wind
speeds (Table 4.1). The most intense tornado ranks as
an F5 event on this scale. Such tornadoes tend to
completely destroy houses, leaving nothing but the
concrete slab, and badly damage steel-reinforced
concrete structures. Wind speeds during such mael-
stroms range from 419-510 km hr
-1
. Note that winds
during Cyclone Tracy (mentioned in the previous
chapter) not only twisted steel girders but also
transported them through the air. These winds
must have exceeded F5 on the Fujita scale. Only
0.4 per cent of tornadoes in the United States reach
category F5; however with 1200 tornadoes observed
each year, this statistic equates to five extreme events
annually. Over 50 per cent of tornadoes produce only
minor damage to homes.
Mini-vortices are responsible for the haphazard and
indiscriminate damage caused by tornadoes. People
driving a horse and cart have witnessed a tornado
taking their horse and leaving the cart untouched. One
tornado that swept through a barn took a cow, and left
the girl milking it and her milk bucket untouched.
Numerous photographs exist that show houses and
large buildings cleanly cut in half with the remaining
contents undisturbed.
subpolar regions where convective instability is sup-
pressed by stable, cold air masses. About 80 per cent of
all tornadoes occur in the United States, particularly on
the Great Plains. Figure 4.12 illustrates the density of
tornadoes and the preferred path of travel in this
region. The area with five or more tornadoes per year,
which is referred to as 'tornado alley', takes in Kansas,
Oklahoma, Texas, Missouri, and Arkansas. Tornadoes
occur commonly in this region, because here cold
Arctic air meets warm air from the Gulf of Mexico,
with temperature differences of 20-30°C. The flat
nature of the Plains enhances the rapid movement of
cold air, while the high humidity of Gulf air creates
optimum conditions of atmospheric instability. Torna-
does in the United States tend to occur in late spring,
when this temperature difference between air masses
is at its maximum. At this time of year, the polar jet
stream still can loop far south over the Great Plains.
Between 1950 and 1999 there were 40 522 tornadoes
recorded in the United States, an average of 810 events
per year. Of these, approximately 43 per cent were
F2-F5 events. Since 1950, the frequency of tornadoes
has been increasing from 600 tornadoes per year to a
current average of 1200 per year (Figure 4.13). While
Texas has the greatest number of tornadoes, the greatest
density of tornadoes occurs in Florida. Kansas reports
the most tornadoes in North America by population:
3.4 per 10 000 people. Surprisingly, the greatest number
of deaths per unit area occurs in Massachusetts. The
greatest number of tornadoes reported in a single
week occurred 4-10 May 2003 (384 reports), while the
Occu
rrence
(Miller, 1971; Nalivkin, 1983; Grazulis, 1993)
Tornadoes do not form near the equator because
Coriolis force is lacking. Nor do they form in polar or
Fujita Tornado Intensity Scale and frequency of tornadoes on the Fujita Scale in the USA (from Fujita, 1981; Grazulis, 1993).
Table 4.1
F-scale
Wind Speed
Type of Damage
Frequency
F0
64-116 km/h
MINIMAL DAMAGE: Some damage to chimneys, TV antennas,
82.0%
roof shingles, trees, and windows.
F1
117-180 km/h
MODERATE DAMAGE: Automobiles overturned, carports destroyed,
11.0%
trees uprooted.
F2
181-253 km/h
MAJOR DAMAGE: Roofs blown off homes, sheds and outbuildings
4.0%
demolished, mobile homes overturned.
F3
254-332 km/h
SEVERE DAMAGE: Exterior walls and roofs blown off homes.
1.8%
Metal supported buildings severely damaged. Trees uprooted.
F4
333-418 km/h
DEVASTATING DAMAGE: Well-built homes levelled. Large steel
0.9%
and concrete missiles thrown far distances.
F5
419-512 km/h
INCREDIBLE DAMAGE: Homes leveled with all debris removed.
0.4%
Large buildings have considerable damage with exterior walls
and roofs gone. Trees debarked.
