Last issue we began our look into severe weather and ended our discussion by introducing tornadoes. Over the last week there have been several outbreaks of severe weather over central and eastern parts of the U.S., with dozens of tornadoes reported and nearly 50 people killed. While this is still well to our south, it won’t be long until the severe weather season begins here. Before we continue our look at tornadoes in more detail, let’s first take a look at one of the weakest members of the tornado family, one we see around Manitoba especially early in the season: the cold air funnel.
All tornadoes develop out of what we refer to as afunnel 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; rather, they develop under conditions of relatively cold 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 believed necessary for the development of cold air funnels: instability and vorticity.
RISING AND SPINNING
If you can remember back to some of the articles I wrote on instability in the atmosphere, you might remember that warm air will rise and cold air will sink. For the atmosphere to be 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, as 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. Within this large area of spinning air, smaller regions
get “spun up,” creating what meteorologists call a vorticityrich environment, containing lots of little eddies of spinning air. Picture in your mind’s eye a turbulent flow of water in a ditch. If the conditions are right (such as those usually found around a culvert) you will see lots of little whirlpools or eddies form in the flow of water. This is similar to what happens in the atmosphere when you have a vorticity-rich environment. Now, scientists believe, one of these small eddies of spinning air gets caught in an updraft, which 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, 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 asland spouts.
Since the potential exists for cold air funnels to touch down as tornadoes, Environment Canada will usually issue special weather statements to warn the public about them. Also, not everyone is aware of just what cold air funnels are, and because of this lack of understanding they will mistakenly think tornadoes are forming. This can cause unnecessary stress and can lead to panic among the public. Since these cold core funnels rarely touch down, and even if 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. In other words, you don’t have to go diving for the nearest storm shelter if you see one forming.
We’ll continue our discussion on more severe types of tornadoes and other forms of severe summer weather in upcoming issues.
Funnelcloudsaregenerallyveryweakand shortlivedandwillrarelybecomestrong enoughorlastlongenoughtotouchdown.