Well, it seems like it’s finally thunderstorm season, so I think we should take advantage of this fact and take a look at the topic of thunderstorms. Each year I touch on this topic, because, well, it’s one of the most important weather topics of which to have an understanding during the summer on the Prairies.
First I need to talk about one of my weather pet peeves: when people mix up weather watches and weather warnings. When we talk about thunderstorms, a severe thunderstorm watch is when the potential exists for severe thunderstorms to occur. This means severe thunderstorms haven’t yet formed. There may be some thunderstorms around, and you need to be wary of them, but so far none are severe. A severe thunderstorm warning means severe thunderstorms have developed and conditions which meet 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 aren’t 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 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 is the hard part.
So just what are the ingredients for severe thunderstorms? First 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 doesn’t mean a large difference in temperature. To get thunderstorms on a hot day you need to have cool air aloft (up above the ground).
When this occurs, the hot air at the surface begins to rise and encounters cool air as it continues to rise up. Our rising air will now 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’s 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 moves 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, giving us thunderstorms. The other way is when warm air moves into a region. The day starts off 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. To find out what these other ingredients are, you’re going to have to wait until the next article, as I’m running out of space! Hopefully we will have a couple of examples of thunderstorms to talk about next time. Not that I’m hoping for a severe outbreak of thunderstorms; it’s just that if we do see some thunderstorms, it will hopefully mean summer has finally arrived!