Introductory Meteorology

Lesson 7. Mid-Latitude Weather Systems


a surface weather map
A surface weather map depicting a mid-latitude cyclone.

Have you ever seen the television program "How It's Made"? In each episode, they take you inside the factory and show you how various products are made. Some items have a fairly straightforward manufacturing process, while others are quite complex, requiring a myriad of components to be made first before the final product is assembled. What is most interesting is that even common, everyday objects can fall into this latter category of complexity. Think about some things that you take for granted which have incredibly complex internal workings. If you wanted to understand how such items worked, a good approach might first be to understand a little about how each of the component parts functioned. Our approach to understanding mid-latitude cyclones (a low pressure system with fronts) will follow this example.

Everyone has seen the local TV weathercaster say something like, "This low pressure system will continue to develop and move into our area...", but what does this actually mean? What are the vital components that make up a "low pressure system" and how do those components work together to make a low pressure system "develop"? These are the questions that we will address in this lesson.

So what components do we first need to have a grasp on before we can understand mid-latitude cyclones? First off, it might surprise you that "surface" low-pressure systems don't just reside at the surface, but are intimately connected with the middle and upper troposphere. Therefore, we will first examine upper-air patterns and how they evolve. Next, we will examine how these upper air patterns can influence surface weather features. Finally, we put it all together to see how a surface low pressure system develops, strengthens, and then decays. By the end of this lesson, you'll definitely have a better idea of "How It's Made"!