When most people hear the word tornado it usually brings about a feeling of fear and maybe even a little awe. I think the fear of tornadoes comes from the fact that they are kind of mysterious. We know they occur during thunderstorms — but which thunderstorms will produce tornadoes, where will they occur in a thunderstorm, and will you see it coming? All these unknowns, combined with the power behind even a weak tornado, make them something to be rightfully feared.
Worldwide, Canada is second only to the United States in the number of tornadoes occurring each year, with an average of about 70 reported. Southern Ontario experiences the highest number of tornadoes, followed by southern Manitoba, Saskatchewan and central Alberta. While these areas report most of Canada’s tornadoes, tornadoes have occurred in nearly all regions of Canada.
Tornadoes can strike at any time of the year, but in Canada, tornado season runs from April to October, with the peak months being June, July and August. This differs from the U.S., where tornadoes peak in April and May. This is due to the amount of cold air that is available for severe storm development. In the spring, the southern and central U.S. have become quite hot, but cold air is still closely available to help develop thunderstorms. By midsummer, most of the cold air has retreated well into Canada, putting our region into warm conditions; however, we still have cold air fairly close by to our north.
Hard to measure
Before I try to attempt an explanation of just what tornadoes are and how they form, I figured I would discuss how strong tornadoes can be. Direct measurements of wind speed inside a tornado do not really exist. So how can we know how strong these winds are?
The basic answer is by indirect methods. Observations can be made of a tornado, and by watching the movement and kind of debris that occur around the tornado you can get a fairly good idea of just how strong the tornado is. For most tornadoes this is usually impractical, since if you are close enough to be observing the debris field surrounding the tornado, then you should be taking cover!
So wind speed for most tornadoes is determined by the evidence left after the tornado has moved through.
The scale developed by American meteorologist Tetsuya (Ted) Fujita, known as the F scale (the Americans are now using a new enhanced F scale or EF scale, but it is still basically the same), breaks down tornado wind speed or tornado strength into five categories as follows, according to Environment Canada:
- F0 light (winds of 64-116 km/h; some damage to chimneys, TV antennas, roof shingles, trees, signs, and windows), accounts for about 28 per cent of all tornadoes in Canada;
- F1 moderate (winds of 117-180 km/h; automobiles overturned, carports destroyed, trees uprooted), accounts for about 39 per cent of all tornadoes in Canada;
- F2 considerable (winds of 181-252 km/h; roofs blown off homes, sheds and outbuildings demolished, mobile homes overturned), accounts for about 24 per cent of all tornadoes in Canada;
- F3 severe (winds of 253-330 km/h; exterior walls and roofs blown off homes, metal buildings collapsed or severely damaged, forests and farmland flattened), accounts for about six per cent of all tornadoes in Canada;
- F4 devastating (winds of 331-417 km/h; few walls, if any, left standing in well-built homes; large steel and concrete objects thrown great distances), accounts for about two per cent of all tornadoes in Canada; and
- F5 incredible (winds of 418-509 km/h; strong frame houses lifted off foundations and carried considerable distances; automobile-sized objects fly through the air in excess of 100 metres; trees debarked; steel-reinforced concrete structures badly damaged), accounts for about 0.1 per cent of all tornadoes in Canada.
Until a June 2007 tornado at Elie, Manitoba, just west of Winnipeg, there had been no F5 tornadoes officially recorded in Canada. From talking with a few people involved with some of the video of the Elie tornado, the F5 rating came from part of a video showing a tanker truck being thrown through the air.
One important point to note is that the size of a tornado does not necessarily relate to its strength. A small tornado can be very strong, while a large tornado can be weak.
Next issue we’ll continue our look at tornadoes by trying to understand just how they form.