Keep watch: How to spot the warning signs of a tornado

A dark area between the storm and the ground, a green tint to the sky, and a line of swirling clouds are all danger signs

Keep watch: How to spot the warning signs of a tornado
Reading Time: 3 minutes

This next topic in our look at different types of severe summer weather is severe thunderstorms, and in particular, high winds. When we think of severe thunderstorms one word that should make you worried and nervous is tornado.

While tornadoes are the most destructive feature that can occur within a thunderstorm, they are fairly rare, with the vast majority of areas never experiencing a direct hit because of their small size. Nonetheless, if they do hit they can be, and often are, totally devastating.

For that reason, we’ll begin our look at severe thunderstorms and winds by looking at tornadoes and then we’ll look at straight-line winds, which can sometimes be almost as destructive and cover much larger areas.

The first question is, how can you know if a storm is capable of producing severe weather?

The answer is actually not that easy. The first thing is to check in with Environment Canada for watches and warnings. A watch issued by Environment Canada means the potential exists for severe thunderstorms, but they have not yet developed in your area. When you hear there is a watch, you should scan the sky for any development, knowing any storms that develop 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 developed, and has been confirmed by an eyewitness or radar. This means you should take precautions immediately.

The trouble is that we are often out somewhere and may not have access to a radio or Internet, so what should you watch for then?

First of all, recognize the conditions: How warm and humid is the air? Remember, a moist atmosphere means there is a lot of energy available. I’m often asked if it can be too hot for thunderstorms to develop and the answer is yes — kind of. While it can be very hot and humid at the surface with lots of potential energy for thunderstorms to develop, the conditions in the atmosphere above the storm also have to be favourable. If the upper levels of the atmosphere are also very warm they can act like a cap preventing the air at the surface from rising. (I will write more about this in a later article.)

What else should you look for in trying to determine if a storm is becoming severe?

Look for a dark or threatening sky — especially the area between the storm and the ground. If you can see through it, the storm is likely not severe yet.

Lots of lightning or nearly continuous thunder is a good indication of a severe storm.

Keep an eye out for things like a green sky, roll clouds, and mammatus clouds (clouds with bag-like sacks hanging below them). These conditions usually indicate the storm contains huge amounts of water and has very strong updrafts and downdrafts. As a severe storm approaches you will often see a uniform line of black clouds that is preceded by a thin line of swirling- or rolling-looking clouds. These clouds are formed by the wind blowing out from the storm and will announce the arrival of the severe storm.

Finally, watch out for any kind of rotation within the storm. This means the storm has become very strong and has the capability of producing a tornado.

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 occurring each year, with an average of about 70 reported. Southern Ontario experiences the highest number of tornadoes, followed by southern Manitoba, Saskatchewan, and finally 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 typically 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.

About the author

AF Contributor

Daniel Bezte

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



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