The phrase “If you don’t like the weather, wait a minute” is never so true than during the spring across the Prairies and so far, this spring is living up to the saying. As we get ready for May, thoughts begin to switch from snowstorms and cold snaps to heatwaves and thunderstorms. So for this article I thought we might get an early start on thunderstorm season. I am still working on the winter summary; it is just taking longer than expected to sift through all the data.
Before we begin our look at severe summer weather, the global temperatures for March have been crunched and both NASA and NOAA ranked March as the eighth-warmest on record, with the University of Alabama in Huntsville ranking it as the 11th-warmest in its 43-year period of satellite observation. This “cooldown” is thanks to the lingering effects of La Nina, which produces cooler-than-average water temperatures across a large portion of the Pacific Ocean. That said, NOAA states that 2021 has a 99 per cent chance of coming in as one of the 10 warmest years on record.
OK, now onto our first look at severe summer weather — and, in particular, thunderstorms. To start us off, what are the ingredients needed for severe thunderstorms? First, we need rising air, and to get that we need heat, or more specifically, a large difference in temperature between two areas. There are a couple of ways you can achieve this. One way most people are familiar with is to have a very hot day. But just having a very hot day does not mean that 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).
When this occurs, the hot air at the surface begins to rise 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, 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 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 is moving into a region. The day starts off cool, then storms develop as the warm air rises over the cool air as it moves into the region.
Simply having a big difference in temperatures will not give us a thunderstorm, or at least will not give us a severe thunderstorm. There are still a couple of 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. Condensation then 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.
If air is continually rising, eventually the amount of air accumulating at the top of the storm will become so great 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 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, and the gentle garden-variety thunderstorm, a couple of issues from now. Next up will be our monthly look back and look ahead to see what, if any, changes there are to the long-range weather forecasts for May and June — along with our first look at July.