As the temperatures rise, and the sun’s intensity increases, we’re beginning to move into severe summer weather season.
This is a topic I always get many questions about, and it’s also the type of weather than arguably has the biggest impact on our daily lives.
When it comes to severe summer weather — or any 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.
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The first level 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. This type of alert means that the weather will likely affect your day but probably won’t cause a serious disruption to your daily routine.
The next level is a “weather watch,” which indicates severe weather conditions have not yet developed, but forecasters feel that there is a good chance that they will develop. When a watch is issued it means that you should pay attention to the weather around you and keep an “eye on the sky” as they say. You should keep track of weather reports and other online weather sources. If you see what looks to be severe weather developing, you need to be prepared to take safety precautions and to act quickly should severe weather develop, and a warning is issued.
Finally — you guessed it — is the “weather warning” which means that severe weather has developed and is occurring. When dealing with severe summer weather, warnings can sometimes be issued in advance, but due to the nature of summer severe weather the lead time is often very short, sometimes only giving you a few minutes.
This is why you need to be paying attention when there is a weather watch issued. If you hear of a warning for your area, act immediately.
One of the most common forms of severe summer weather is that phenomenon that’s both loved and hated — the thunderstorm.
The ingredients for a severe thunderstorm are clear.
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 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).
What we are talking about is atmospheric stability. When hot air at the surface begins to rise, it cools; if the air surrounding the rising parcel of air remains colder than the rising parcel, that rising parcel will continue to rise. The colder the air around the parcel, the faster it goes up. The faster it goes up, the stronger the storm typically becomes.
Sometimes we can get severe thunderstorms even when we don’t have particularly warm air at the surface.
The first scenario is when 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. 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 is cold 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 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 warm air rises over the cool air as it moves into the region.
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 most important 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. A lot of heat. One kilogram of water vapour condensing releases 2.26 million joules of heat energy or enough energy to boil half a litre of water.
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.
If you have air continually rising, 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 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.
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, but also the other severe thunderstorm threats, namely high winds, hail, severe lightning and tornadoes.
More about these in the coming issues.
