As we slowly move toward summer and all of its associated weather, I figured we need to look at the topic of severe summer weather. So far this year we have seen a few thunderstorms, but without any significant hot weather, most have been on the weak side. That’s not to say there hasn’t been any severe weather — just that most of the severe weather has been very short lived and we have yet to see any wide outbreaks of severe summer weather.
Thunderstorms, whether simple garden-variety ones or the powerful supercells, are not the only type of severe summer weather that we see across the Prairies. Over the next few articles we’ll look at the various types of severe summer weather and attempt to explain what conditions need to come together to bring us these types of weather phenomena. That said, I figured the best topic to begin our look at severe summer weather would be to take a detailed look at our good ol’ thunderstorms.
For thunderstorms to form, the atmosphere needs certain ingredients in place. First of all, you need rising air, and to get that you need heat — or rather, you need a large difference in temperature between two areas. There are a couple of ways you can achieve this difference in temperature. One way with which most people are familiar is to have a very hot day — but just having a very hot day does not mean there is a large difference in temperature. To get thunderstorms on a hot day you need to have cool air aloft (up above the ground). We need to remember that the atmosphere is 3D, that there can be differences not just from place to place on the ground, but also as we move upward. This seems like a simple idea, but most people forget about the upward part of the atmosphere.
When we have warm or hot air at the surface and cold air aloft, the hot air at the surface begins to rise and encounters cold air as it continues to rise up. This means our rising air will remain warmer than the air around it and will continue to rise up. The cooler the air around it, the faster it goes up; the faster it goes up, the stronger the storm (typically).
Now, sometimes we can get severe thunderstorms when we don’t have particularly warm air at the surface. Two different scenarios can play out when this happens that can still lead to severe thunderstorms. The first scenario would be that there is very warm air a few thousand feet up from the ground. This warm air then has cold air above it, and just like the hot day on the ground, this warm air in the upper atmosphere can rise up, giving us elevated thunderstorms.
The second scenario is when there is a strong contrast of warm and cool air at the surface, or in other words, we have some type of front cutting through an area. On one side of the front it is cool and on the other side it is warm. The cold air acts like a wedge and forces the warm air up. Sometimes this occurs when a cold front is moving into an area, so the day begins warm and then the cold air pushes in, lifting the warm air up in front of it, giving us thunderstorms. The other way is when warm air is moving into a region. The day begins cool, then storms develop as the warm air rises up over the cool air as it moves into the region.
Now, simply having a big difference in temperatures will not give you a thunderstorm, or at least, will not give you a severe thunderstorm. There are still a couple more ingredients needed.
The next key ingredient is water vapour, or humidity. It takes energy to evaporate water, so the more water vapour there is in the air, the more potential energy there is. To get at this energy the water vapour needs to be changed back into a liquid form — that is, it needs to condense. As our warm air rises, it cools, and as it cools, water vapour will begin to condense. When it condenses it releases the energy it absorbed when it evaporated. This energy is released in the form of heat.
Our rising air is cooling as it rises, but not cooling as fast as the air around it, so it continues to rise. Then condensation starts taking place, which releases heat into the air. This makes our rising air even warmer than the air around it, so it rises even faster. Now it’s starting to sound like we have everything in place for a severe storm… but not quite.
If you have air continually rising up, eventually the amount of air accumulating at the top of the storm will become so great that it just has to fall back down again, wiping out the storm in the process. To get around this problem we need some kind of vent at the top of the storm that takes away all the rising air that is accumulating there. We need a strong jet stream of air over top of the storm, which will help to “suck” away the accumulating air.
There, we now have the key ingredients for a severe storm, but like any good chef, Mother Nature has additional ingredients she can use to make some storms truly awesome. We will look at those ingredients and other types of severe summer weather over the next few articles.
Daniel Bezte is the weather columnist for the Manitoba Co-operator. His article appeared in the May 16, 2019 issue.