Looking at severe summer weather

Usually around this time of the year my mind turns to summer and summer weather. In particular, I start to think about thunderstorms and severe summer weather. This year, with the summer-in-March weather pattern, we saw some really early thunderstorms, but with the cooler April weather pattern things have settled down. Now, I don’t want anyone to get nervous, but the last time my records show us experiencing March thunderstorms was back in 2004 — which, for those of you who have blocked it from your memory, was the year without a summer. Let’s hope that’s not the case this year!

How can you know if a storm is producing severe weather? The answer is not that easy. The first thing is to listen to Environment Canada for watches and warnings. If a watch is issued, it means that the potential exists for severe thunderstorms, but they have not yet developed in your area. When you hear there is a watch, you should watch the sky for any development, and if any storms do develop, they have the potential to become severe. If Environment Canada issues a warning, this means a thunderstorm with some or all of the characteristics of a severe storm has already developed and has been confirmed by eyewitnesses or radar. This means you should take precautions immediately.

So what are the characteristics that define a severe thunderstorm? A severe thunderstorm is defined as a thunderstorm that has at least one of the following conditions:

  •  Wind gusts of 90 kilometres per hour or greater
  •  Hail of at least two centimetres in diameter
  •  Heavy rainfall

When dealing with thunderstorms that are producing heavy rainfall, across the Prairies a thunderstorm is considered to be severe if rainfall amounts of 50 millimetres or more occur within a one-hour period.

Along with these three conditions, thunderstorms can also produce 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 first hand, 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 United States in the number of tornadoes that occur 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 in warm conditions; however, we still have cold air fairly close by to our north.

If a tornado watch has been issued that means that the conditions are favourable for tornadoes to form. Needless to say, if there is a tornado watch you will also be under a severe thunderstorm watch or warning already. If there is a tornado warning, this means a tornado has been spotted by a reliable spotter or that there is evidence on radar that a tornado has formed.

Now just 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, and 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 this 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. This makes them very hard to study first hand. Unfortunately, this is all the room I have, so more on this in our next article.

About the author

Co-operator contributor

Daniel Bezte

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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