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Bezte: How do tornadoes form?

In our peak tornado months, cold air remains close by for severe storm development

For this article I thought it was time that we continue our look at severe thunderstorms, and specifically the deadliest part: tornadoes.

The word tornado for most people brings about a feeling of awe and even a little fear. Unless you have already witnessed a tornado firsthand, many who are interested in weather secretly wish they could safely experience the awesome beauty and power of a tornado.

Worldwide, Canada is second only to the U.S. 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, they 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. has 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.

What are tornadoes and how do they form? A classic definition of a tornado is a violently rotating column of air that extends from a thunderstorm to the ground, which may or may not be visible as a funnel cloud. For this rotating column of air to be classified as a tornado it must touch the ground.

As to how tornadoes form, the real answer is, we just don’t know. Tornadoes usually develop from super cell thunderstorms, which are difficult to predict. Even if we were able to accurately predict where and when these thunderstorms would develop, the intense part of the thunderstorm usually only covers an area of a few hundred square kilometres. Within these few hundred square kilometres, the really severe weather may only occur in a small area of maybe 10 to 20 square km. Now, if we look at the size of a tornado, we would find that they range from as small as about 40 metres to as large as two km across, with the average width being around 100-200 metres. This means that, as far as weather phenomena are concerned, tornadoes are very small, which makes them very hard to study first hand.

Funnel clouds

All tornadoes develop out of what we refer to as a funnel cloud. In strong thunderstorms, these funnels elongate and may eventually touch the ground to become a tornado, but a funnel cloud all by itself is not considered a tornado. For a tornado to form in a thunderstorm we need two key ingredients: instability and vorticity.

If you think back to when we talked about instability in the atmosphere, you’ll remember that warm air will rise and cold air will sink. If the atmosphere is unstable you need either really warm air at the surface, or very cold air in the upper atmosphere. In the summer we often have the very warm surface temperatures, but we do not often have the cold air aloft. When we do, this combination provides the first ingredient needed to form a tornado: instability, or rising air.

The second ingredient is vorticity. This simply means spinning air. Areas of low pressure are large areas of spinning air, too large to form into a funnel cloud or tornado. But within this large area of spinning air smaller regions get “spun up,” creating what meteorologists call a vorticity-rich environment that contains a lot of little eddies of spinning air. What scientists believe happens is that one of these small eddies of spinning air gets caught in an updraft caused by the unstable air. This updraft then pulls on and elongates the eddy, causing it to contract in width and, just like a figure skater pulling in his or her arms during a spin, this causes the rotation to speed up, creating a funnel cloud. If the rotating column of air continues and intensifies then it can elongate, eventually meeting the ground and becoming a tornado.

Luckily for us, across the Prairies, most tornadoes that do form are relatively weak, coming in at F0 or F1 on the Enhanced Fujita intensity scale shown in the table here:

Table: Tornado intensity

Scale.    .
Potential Wind speed (km/h)
F0 Minor damage 105-137
F1 Moderate damage 138-178
F2 Considerable damage.   . 179-218
F3 Severe damage 219-266
F4 Extreme damage 267-322
F5 Incredible damage Above 322

We have seen some really strong tornadoes on the Prairies, with the devastating Edmonton tornado of July 31, 1987 coming in as an F4. The Elie, Man. tornado on June 22, 2007 was a small but extremely strong tornado, that ended up coming in as an F5 and is still Canada’s only official F5 tornado.

While tornadoes can be awe inspiring, I hope we don’t see any of these potentially devastating weather events any time soon.

About the author

Co-operator contributor

Daniel Bezte is a teacher by profession with a BA (Hon.) in geography, specializing in climatology, from the U of W. He operates a computerized weather station near Birds Hill Park.

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