This time we’ll continue our look at severe thunderstorms, and specifically, the most deadly part: tornadoes. Before diving into this, there are a couple of weather stories I have to touch on.
First, tied into our discussion on thunderstorms, a stalled frontal system across southeastern Manitoba from June 6 to 8 allowed for several rounds of thunderstorms to develop and move across this region, bringing with it copious amounts of rain. The table here shows some of the rainfall totals in millimetres.
The second bit of weather news comes from the Weather Underground. For those of you who follow the weather closely, you know I often refer to a weather blog, Category 6, that is hosted by the Weather Underground as a source of some of the best weather information. Well, that blog is shutting down. I’m not going to go into the politics of it all; let’s just say the Weather Underground began as a university project which eventually was bought out by IBM, which runs The Weather Company. Since then I have been finding the Weather Underground less and less useful. Now this, enough said. Good news is that the two main contributors to this blog will be writing for Yale’s Climate Connections, so all is not lost.
What makes a tornado a tornado?
Now, on to tornadoes. The 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 this few hundred square km, 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 to 200 m. This means that, as far as weather phenomena are concerned, tornadoes are very small, which makes them very hard to study.
We’ll begin our look at tornadoes with one of the weakest members of the tornado family, and something we do see around Manitoba: the cold air funnel.
While Manitoba sees around 15 tornadoes on average each year, they are fairly rare. While we don’t see that many cold air funnel clouds, they are a little more common. 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. While a fair bit of research has been done on tornadoes and the storms that produce them, very little research has been done on cold air funnels, therefore, we know very little about them. In general, cold air funnels form in environments where we would not typically expect severe weather to develop — that is, in hot, muggy, unstable air. Usually, cold air funnels will form when there is a large pool of cold air aloft that is most often associated with an upper level low. These conditions provide two critical ingredients that are believed to be necessary for the development of cold air funnels: 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 very warm air at the surface, or very cold air in the upper atmosphere. Therefore, there needs to be a pool of cold air aloft for cold air funnels to form, because this provides the first ingredient: 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. Now, what scientists believe happens, is that one of these small eddies of spinning air gets caught in an updraft. 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.
These funnel clouds are generally very weak and short lived and will rarely become strong enough, or last long enough, to touch down. If they do touch down, they can then be referred to as tornadoes or land spouts, but even then, they rarely cause much damage. Since the potential exists for cold air funnels to touch down as tornadoes, Environment Canada will issue special weather statements to warn the public about them. These statements will usually urge the public to be watchful for these to occur and to take precautions if necessary.
In my next article we’ll continue our look at tornadoes.