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Thunderstorm season will soon be with us again

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Daniel Bezte has a special interest in farm weather, which he follows from a small farm near Winnipeg, where he has his own computerized weather station. He has been a regular contributor to other farm publications including the Farmers’ Independent Weekly and the Manitoba Co-operator. Daniel has a degree in geography, specializing in climatology, from the University of Winnipeg.

He welcomes questions and comments at[email protected]

In nature, one of the most awe-inspiring sights is that of a thunderstorm. Whether it is the towering cumulonimbus clouds stretching across the prairie sky, or the blinding rain and lightning that often accompany them, thunderstorms can either be things of beauty or destruction.

With the mild start to this year and a forecast for warmer than average conditions this spring and summer, I thought we should take a look at severe weather a little earlier than usual – even though for southern regions it has been seeming more like winter than spring lately.

Any thunderstorm has the potential to turn ugly, but certain atmospheric conditions have greater potential to produce severe weather. The first important condition is moisture. Without enough moisture you can have all the conditions in place for a thunderstorm and never get one. Moisture is the fuel of thunderstorms. As warm, moist air rises and starts to condense, heat is released, which helps to accelerate the rising air. So, the more moisture you have in the air, the more latent heat there is to be released. This increases the power of the updrafts, and the more powerful the thunderstorm can become.

A second important element is to have rising air. This can be accomplished by a couple of different methods. The first way you can get air to rise is to physically push it upwards. This is what happens when two different air masses meet. Cold air, being more dense than warm, will act like a wedge when it moves into warm air, pushing the warm air up and over itself.

Another way to get air to rise is in an unstable atmosphere. Our atmosphere normally cools as you move upwards. Air that is forced upwards in the atmosphere will also cool at a certain rate. If the atmosphere is stable, the rate at which a rising parcel of air will cool is faster than the rate at which the atmosphere cools. The rising parcel of air will quickly become cooler, and therefore heavier, than the air around it and will sink back down, unless something is physically pushing it upwards.

Having an unstable atmosphere means that our rising parcel of air is cooling slower than the rate at which the atmosphere cools. Under these conditions, the rising parcel of air remains warmer than the air around it. Being warmer, it is lighter, and will therefore continue to rise.

Having very cold air aloft or extremely warm temperatures at the surface can achieve this. This enhances the updrafts, allowing them to develop easily.

Capping the storm

While having an unstable atmosphere is important, severe thunderstorms often develop only when part of the atmosphere is unstable. Occasionally, the layer of air closest to the ground actually gets warmer as you go upwards. This creates a very stable situation. Above this layer the temperature of the atmosphere acts more normally and cools with height, creating an unstable layer.

When this situation occurs it is referred to as a “cap.” The stable layer near the surface acts like a cap or lid preventing thunderstorms from developing. This is kind of like a pot of water on the stove with a lid on it. If the conditions under the lid don’t get too warm the lid stays in place, but if it warms up too much the lid might fail, releasing all the energy stored up all at once.

This is what produces a severe thunderstorm – when warm air from below manages to push into the cool upper air, resulting in very strong updrafts.

It is strong updrafts that are the essential driving force behind severe storms. As they move upward in the storm they pull in huge amounts of water vapour that condenses and falls as rain, often heavy. These updrafts can be powerful enough to keep small pieces of ice, which formed in the storm, aloft, eventually building up in size and producing large hail.

The updrafts, while pulling in a lot of moisture, are also pulling in a lot of air into the storm. This air eventually begins to pile up at the top of the storm. At some point, the weight of this air becomes too great for the updraft to hold and this air falls back to the ground in the form of a downdraft.

When these downdrafts hit the ground they spread out in all directions, producing the violent winds often found in thunderstorms.

These same downdrafts will usually bring about the end of a thunderstorm, as they spread out along the ground and cut off the updrafts, but sometimes the right conditions exist for the development of a balance between updrafts and downdrafts. This leads to the development of supercell thunderstorms, or mesocyclones. This type of severe storm can last for several hours and will often produce the most severe form of weather, the tornado.

Next issue we will continue our look at severe summer weather by examining what is probably the most destructive weather phenomenon – the tornado.

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