Originally published on May 21, 2024 on Manitoba Co-operator
By Daniel Bezte
It’s getting to be that time of the year again. As the sun’s intensity increases and temperatures really start to warm up, we begin to move into summer severe weather season.
I know I go over this every year or two, but I probably receive more questions about this topic than any other, with the possible exception of drought questions this spring.
When it comes to severe weather, there are three levels of alert. Knowing these levels and what they mean is the first step to keeping yourself informed and safe.
The first is a special weather statement. These are issued when unusual weather is expected to develop, but isn’t expected to meet or exceed severe weather levels. The weather will likely affect your day, but probably won’t cause a serious disruption to your routine.
The next is a weather watch. This means that severe weather conditions have not yet developed, but forecasters feel that there is a good chance that they will. A good analogy 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. The big question will be where that first bubble forms and breaks away from the bottom of the pot. In this metaphor, that’s our thunderstorm. You knew it was going to form if the heat was left on. Exactly where it will form is less clear.
The final level is a weather warning. This means that severe weather has developed and is occurring. When dealing with severe summer weather, warnings can sometimes be issued in advance, but lead time is often very short, sometimes only giving you a few minutes.
Watches and warnings are the most commonly mixed up of the weather alerts. When a watch is issued, you should pay attention to the weather around you and keep an eye on the sky. You should keep track of weather reports and other online weather sources. If you see what looks to be severe weather developing, there is a good chance it is and you should be prepared to take safety precautions and to act quickly should things upgrade to a warning. If you hear a warning for your area, it’s time to act on those precautions.
Baking up a thunderstorm
Now, let’s look at something we all love and hate at the same time: thunderstorms.
What are the ingredients for severe thunderstorms? First, you need rising air. To get that, you need heat. Or more accurately, you need a large difference in temperature between two areas. There are a couple of ways you can achieve this. One that most people are familiar with isto have a very hot day, but just having a 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.
We discussed this in our last round of Meteorology 101, when we talked about atmospheric stability. When hot air at the surface begins to rise and cool, if the air surrounding that rising parcel remains colder than the parcel itself, that upward motion will continue. The colder the air around the parcel, the faster it goes up; the faster it goes up, the stronger the storm typically is.
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. The first would be if there is very warm air a few thousand feet up from the ground. This warm air has cold air above it and, just like the case of a hot day on the ground, can rise, giving us elevated thunderstorms. We typically see these types of storms in the spring, but can experience them at 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. In other words, we have some type of front cutting through an area. On one side of the front, it is cold; on the other side, it is warm. The cold air acts like a wedge and forces the warm air up.
This sometimes occurs when a cold front is moving into an area. The day starts off warm and then the cold air pushes in, lifting the warm air in front of it and giving us thunderstorms. It can also happen when warm air is moving into a region. The day starts off cool and then storms develop as the incoming warm air rises over the cool air.
Simply having a big difference in temperatures will not give you a thunderstorm. At least, it will not give you a severe thunderstorm. There are a few more ingredients needed.
Water vapour, or humidity, is key. It takes energy to evaporate water. 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. 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 — a lot of heat. One kilogram of water vapour condensing releases 2.26 million joules of heat energy, enough to boil half a litre of water.
To put it together, 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, releasing 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 things in place for a severe storm, but not quite.
As we’ve previously discussed, 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 to take away all the rising air. We need a strong jet stream of air over top of the storm to “suck” away the accumulating air.
Now, we have the main ingredients for a severe thunderstorm to develop.
There are other factors that can come into play. These factors not only help develop heavy rains, but the other hallmarks of a severe thunderstorm like high winds, hail, severe lightning and tornadoes. We’ll dive more into these in coming issues.