I’d said we were going to examine which weather conditions bring long-duration rainfall warnings, and also dig into some of the rainfall records from across southern and central Manitoba in this issue, but that topic will have to wait until next time. An early deadline for this issue due to the long weekend, combined with a three-day backpacking trip with some kids from my school, and I simply didn’t have the time to properly research the rainfall records. Instead, we’ll jump ahead a little bit and take our first look at the most obvious of severe summer weather: thunderstorms.
I think when it comes to talking about weather, one of the things that upsets me the most is when people mix up weather watches and weather warnings. Basically, when we are talking about thunderstorms, a severe thunderstorm watch is when the potential exists for severe thunderstorms to occur. This means that severe thunderstorms have not yet formed. There may be some thunderstorms around, and you need to be wary of them, but so far none of them has become severe. A severe thunderstorm warning means that severe thunderstorms have developed and conditions meeting the “severe” criteria have been recorded either directly by observers or by radar. When you hear a warning, it means you need to take immediate precautions.
Severe thunderstorm watches are typically issued when all the ingredients for severe storms are in place, but forecasters are not sure where, or sometimes even if, thunderstorms will develop. An analogy you can use is a pot of water on the stove. If you turn on an element and put on a pot of water, eventually it will boil, but the big question is: where will that first bubble form and break away from the bottom of the pot? That would be our thunderstorm. You knew it was going to form; exactly where it will form is the hard part.
So, just what are the ingredients for severe thunderstorms? 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 most people are familiar with 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).
As we said last issue, when this occurs, the hot air at the surface rises, and encounters cool air as it continues to rise. This means our rising air will remain warmer than the air around it and will continue to rise. 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. We typically see these types of storms in the spring, but we can experience them any time during the summer and fall.
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’s cold, and on the other side it’s 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 starts off warm and then the cold air pushes in, lifting the warm air up in front of it and giving us thunderstorms. The other way is when warm air is moving into a region. The day starts off cool and 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; 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 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 is starting to sound like we have everything in place for a severe storm — but not quite.
Again, as we discussed in the last issue, if you have air continually rising up, eventually the amount of air accumulating up 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 to take away all the rising air accumulating there. We need a strong jet stream of air overtop of the storm, which will help to “suck” away the accumulating air. We now have the main ingredients in place for a severe thunderstorm to develop, but there are still other factors that can come into play. These factors not only help develop the heavy rains we have been discussing, but also the other severe thunderstorm threats — namely, high winds, hail, severe lightning and tornadoes. More about these in future articles.