Major Tornadoes

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Abstract

Tornadoes are columns of air that spin at a high rate of speed. They are small in scale but can be very violent. The area affected by a tornado's passage is between about 40 and 400 metres in width and between 1.7 and 36 kilometres in length. During a tornado the damage is due to wind as well as an extremely sudden drop in pressure. Tornadoes vary in intensity, measured on the Fujita or F scale, graduated from 0 to 5 based on the level of damage. The main season for tornadoes is from April to October, and every province is subject to the risk of tornadoes.

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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).

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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.

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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

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).

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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.