‘Severe’ versus ‘air mass’ thunderstorms

After a start to May that made most people across the Prairies believe summer had arrived early, a deep upper low and a large arctic high combined to bring temperatures crashing back down. This reminded us just why our part of the world has some of the most interesting and variable weather. The rapid swing in temperatures leads nicely into this issue’s topic, severe thunderstorms.

Back in April we talked about what it takes to form severe thunderstorms: heat, humidity, lift, and some way to vent the air at the top of the storm. This week we’ll take a look at what it takes to create a severe thunderstorm and turn it into a thunderstorm to truly remember.

We have a hot humid air mass in place, the air a few thousand feet up is very cold, providing for good lift, and we have a strong jet stream overhead providing the venting at the top of the storm. Everything is in place for a severe thunderstorm, but what can Mother Nature add to the mix to make things even worse?

Probably the most important “extra” ingredient that can be added to the mix is to have the wind change direction with altitude. Remember, the atmosphere is three-dimensional — that is, air can flow horizontally, but this horizontal direction can change as you move upward. Why would this have an impact on our storm?

To put it in a nutshell, this change of direction can cause the developing storm to rotate. Picture what would happen if you take a rising parcel of air and push on it from the south when it is at the surface. Then, as it rises up a couple of thousand feet, the wind switches direction and now blows from the east. Then, a few thousand feet further up, it is blowing from the northwest. What would happen to our rising parcel of air? It would get twisted; it would start to rotate.

Remember, if we can get air to rotate counterclockwise, we have an area of low pressure. Air flows inward in a counterclockwise rotation and is then forced to move upward. One thing we get, if we can get our severe storm rotating, is a small-scale area of low pressure that helps the air to rise even more than it would without the rotation. The second thing a rotating thunderstorm can do is nicely separate the area of updrafts and downdrafts. This is important, since the downdrafts, even with a severe thunderstorm, will eventually cut the updraft off from its source of warm moist air. In a rotating thunderstorm, the source of warm moist air is maintained, giving these storms a long life and a lot of moisture to produce heavy rains.

Heavy rotation

Another aspect to the storm that a rotating column of air can provide is tornadoes. While we still do not understand how tornadoes are formed, we do know rotating thunderstorms can produce tornadoes. It is believed rotating columns of air can get squeezed into a narrower shape; as this happens, the wind speeds increase eventually producing the tornado.

Like most things in nature, thunderstorms rarely behave like a textbook example of a thunderstorm. Even when all the ingredients are there, no storms may form, or sometimes some key ingredient is missing, yet we get a really severe storm. This is what makes weather so interesting.

Now, not every thunderstorm that develops becomes severe; in fact, much of our summer rainfall comes from garden-variety thunderstorms, or what we call “air mass” thunderstorms. These storms, as the name indicates, develop in the middle of a typical warm summer air mass. Because they are in the middle of an air mass, some of the key ingredients for severe storms are missing.

Usually, in the middle of an air mass, temperature will not decrease that rapidly with height. The wind will usually remain constant with height, and there will probably not be a jet stream overhead. Nonetheless, we can still have enough heat and humidity for air to rise and thunderstorms will form. Since these storms don’t rotate or have any way to vent the rising air from the top of the storm, they rarely last long. The accumulating air at the top of the storm will eventually fall back down as a downdraft; this will wipe out the updraft, essentially killing the storm. The whole process, from the start of the storm to the downdraft killing it, can be anywhere from 30 minutes to an hour.

While these storms are short-lived, they can give brief periods of heavy rain and the odd good gust of wind, especially when the downdraft fist hits the ground. These storms often provide us with just the right amount of precipitation when we need it during the summer. I hope you now know just a little bit more about the nature of severe thunderstorms.

— Daniel Bezte is a teacher by profession with a B.A. (Hon.) in geography, specializing in climatology, from the University of Winnipeg. He operates a computerized weather station near Birds Hill Park, Man. Contact him by email with questions or comments. This article originally appeared in the May 19, 2016 edition of the Manitoba Co-operator (page 16).