Tornadoes are columns of air which spin at a high rate of speed,
which are small in scale but can be very violent. They strike quickly,
randomly and often without warning. The winds they generate can
reach speeds of 100 metres per second (360 kilometres per hour).
[D] Click for larger version, 7 KB Tornado, July 5, 1973, Kelvington, Saskatchewan
A tornado is recognized visually by the presence of a cloud in
the form of a funnel coming out of thunderstorm clouds. Tornadoes
are often preceded by thunderstorms and are associated with a dark
sky, strong winds, lightning, thunder and heavy rain or hail.
[D] Click for larger version, 8 KB Tornado, July 8, 1983, Blackfoot, Alberta
As tornadoes whirl around, they move laterally over the ground
at speeds of up to 70 kilometres per hour. They can also remain
stationary for short periods. In general, tornadoes do not follow
a straight line. They usually travel eastward or northeastward.
Depending on the strength of a tornado, it is estimated that the
area affected by its passage is between about 40 and 400 metres
in width and between 1.7 and 36 kilometres in length. The area
of destruction is clearly demarcated next to the undamaged area.
Also, tornadoes do not always remain on the ground, so the amount
of damage is sometimes irregular along their paths.
During a tornado, damage is not only due to the wind, but also
to the sharp, extremely sudden drop in pressure, which causes, among
other things, the explosion of windows in buildings. The pressure
inside the funnel can be as much as 90% lower than normal atmospheric
pressure. This creates a suction effect within a tornado which can
lift heavy objects into the air.
Wind Strength of Tornadoes
Tornadoes vary in intensity. They are measured on the Fujita or
F scale. This scale is graduated from 0 to 5 and is based on the
level of damage caused by a tornado's passage. The various categories
are as follows (Table 1):
Table 1. Fujita Scale
F-scale Wind-speed (kilometres per hour) Damage
F0 |
64
to 116 |
little |
F1 |
117 to 180 |
moderate |
F2 |
181
to 253 |
considerable |
F3 |
254
to 331 |
severe |
F4 |
332
to 418 |
site
devastated |
F5 |
419
to 512 |
unbelievable |
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Source: Emergency Preparedness Canada
It should be noted that the damage caused by a tornado is not necessarily
related to wind speed. One critical factor is the route followed
by the tornado. In 1987, for example, in Edmonton, the bulk of the
damage occurred when the tornado was weakest (in its F1 to F2 phase).
However, that was when it struck a mobile home park. At its strongest,
this tornado had a strength of F4.
Canada has yet to experience an F5 tornado, and only 1% of our
tornadoes are of F4 strength.
Tornado Occurences in Canada: When do they occur?
Only the United States receives more tornadoes than Canada. Prior
to the 1970s, tornadoes were thought to be rare events in Canada,
but various studies have since revealed that they are much more
common than previously thought. Tornadoes can strike anytime during
the year, but the main season is from April to October, particularly
in June and July.
Where do tornadoes strike?
In Canada, more than 70 tornadoes a year strike the populated regions.
Fortunately, most are too weak to cause damage. In Canada, every
province is subject to the risk of tornadoes (Figure 1).
[D] Click for larger version, 24 KB Figure 1. Map of the Annual Number of Tornadoes in Canada
The risk of tornadoes is highest in southern Ontario. The extreme
southern part of the Prairies receives the second largest number.
The tornado risk is high in south central Alberta, southern Saskatchewan
and Manitoba, northeastern Ontario, and western Quebec.
Overall, a third of the tornadoes occur in Ontario, and most of
these are in the extreme southern part of the province. This region
is also more likely to receive tornadoes of high intensity. In fact,
of nine tornadoes of F4 strength recorded in Canada, seven were
in southern Ontario and two were in western Canada. (However, these
last two tornadoes were the worst Canada has ever known).
In Manitoba, at least one tornado a year causes property damage.
However, because of the low population density, tornadoes rarely
cause loss of life. In southern Saskatchewan, another region frequently
affected, tornadoes are not very destructive but strike repeatedly.
Finally, in the rest of Canada - British Columbia, northern regions,
northwestern Ontario, the St. Lawrence valley in Quebec, and the
Atlantic provinces - tornadoes are rarely experienced.
Descriptions for each of the major tornadoes shown on the map are
found in the document Major
Tornadoes.
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