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Where do thunderstorms come from?

What causes a thunderstorm to form? The most common answer I get is that a thunderstorm forms when cold air meets warm air. Now… to some degree that is true. In fact… you have probably heard me mention that on my nightly weather casts. But… that is a very simplistic answer. In fact… I just don’t have enough time to explain the nitty gritty details about how thunderstorms form during a weather cast. But… since I now have my own blog, I now have the space (insert evil laugh here!)

My hope is that once you finish reading this blog, you’ll have a better understanding of how one of the most common summertime phenomenon occurs. And don’t worry, there will be no quiz.

First of all, the atmosphere must contain three basic ingredients. They are 1) moisture, 2) instability, and 3) lift.

Moisture. We all know what that is. It is the available water vapor in the air. You want to think of moisture as the fuel a storm needs to eat in order to develop and mature.

Instability. You hear us use this term on air a lot during the summer. Simply put, instability refers to the overall buoyancy of the air.  The more unstable the air is, the easier it is for a column of warm and moist air to rise. An unstable air mass is usually one where there is a layer of very warm and moist air near the surface and  progressively cooler and drier air as you rise into the atmosphere. 

Lift. Or should I say a lifting mechanism. You can have all of the moisture in the world and the atmosphere can be tremendously unstable. But… if you don’t have a way to start the air rising, chances are you will not see that thunderstorm. There are several ways air can get lifted. One way is when it travels up the side of a mountain, or large hill. This is called orographic lift. This is very common along the front range of the Rocky Mountains… not so common here in Wisconsin. Another form of lift occurs because of differential heating. The sun does not heat the surface of the earth evenly. As a result, some areas may get warmer quicker than others. This results in convective currents, where the air begins to bubble much like the way water in a pot comes to a boil. If there is enough instability and moisture present, a thunderstorm could develop.  Finally, another very common way for lift to occur is through  mechanical means like a front. For example… a cool front that pushes into warm and moist air will begin to lift that air into the cooler and drier air above. The result a thunderstorm.

So now that we know the basic ingredients, let’s talk about the life cycle of a thunderstorm. Just like people, thunderstorms have a birth and a death. Kind of weird, huh. Unlike humans, they have three stages 1) the cumulus stage 2) the mature stage and 3) the dissipating stage. Here’s how it works…

The storm starts when a chunk of warm and moist air is lifted into cooler and drier air aloft. The moisture in that rising air cools and forms a puffy cumulus cloud. Cumulus clouds look like cottonballs or giant pieces of cauliflower. This chunk of rising air is called an updraft.

As the updraft strengthens, more warm and moist air gets drawn upward and the cloud grows.

Eventually, the now huge cloud can no longer support the condensed water vapor in it and the water in the cloud begins to fall as rain. As the rain falls, it creates drag and begins to pull some of the cooler air aloft down toward the ground. This is called a downdraft. As this rain cooled air tumbles out of the cloud, it strikes the ground. This is the blast of cool air you feel as the thunderstorm rolls into your area. This also marks the change to the mature stage. This is when the storm contains both an updraft and a corresponding downdraft. This is also the time a thunderstorm is most dangerous, producing strong winds, lightning and thunder. the strength of the updraft will, in part, determine how strong the storm gets.

The rain cooled air spilling from the bottom of the storm eventually cuts of the supply of warm and moist air that the storm needs to survive. Once this happens, the storm begins to collapse in on itself and evetually rains itself out. This is the dissipation stage. 

An average thunderstorm only lasts about 20 to 30 minutes. The reason why it seems longer, is in most cases, there is more than one storm. The storms can be clustered together and consist of individual cells that are in different stages of the thunderstorm life cycle. Plus, as one storm dies, it’s rain cooled air may fire another thunderstorm adjacent to it.

What I have described above is a simplified process for your average run of the mill thunderstorm.  There are other factors such as wind shear and other variables which will determine the type of thunderstorm (multi-cellular cluster, multi-cellular lines, super cells, etc). These variables will also help determine if a thunderstorm will become severe. But… that is for another blog.

Talk to you later!

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