In this article, we continue our look at severe thunderstorms, and specifically the most deadly part: tornadoes.
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 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 area, the really severe weather may only occur in a smaller space 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 m. This means that, as far as weather phenomena are concerned, tornadoes are very small, which makes them very hard to study firsthand.
To begin our look at tornadoes, let’s first take a look at one of the weakest members of the tornado family, and something we do see around Manitoba: the cold air funnel.
While Manitoba does see around 15 tornadoes on average each year, in reality, they are fairly rare. While we don’t see that many cold-air funnel clouds, they are a little more common. Personally, I have never seen a tornado, but I have seen a cold air funnel. All tornadoes develop out of what we refer to as funnel clouds. 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 as 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. 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.
Rising and spinning air
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. This is why 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, containing lots 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. This updraft then pulls on and elongates the eddy, causing it to contract in width, and, just like a figure skater pulling his or her arms in 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, but even then they rarely cause much damage, often comparable to that of a very strong dust devil. In fact, when these cold air funnels do touch down, they are sometimes referred to as land spouts.
Since the potential exists for cold air funnels to touch down as tornadoes, Environment Canada has to issue special weather statements to warn the public about them. Since they rarely touch down, and even when they do they rarely cause damage, such statements will usually urge the public to be watchful for these to occur and to take precautions if necessary — that is, you don’t have to go diving for the nearest storm shelter if you do see one of them forming.
In the next issue, I think it’s time to take a break from tornadoes and move onto a new severe summer weather topic: heat waves and drought. While it might not seem that these are severe weather topics, they probably do the most damage out of all of the different severe summer weather events combined.