12.1: Introduction
- Page ID
- 46270
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Satellite image of a mid-latitude cyclone over North America (Public Domain).
For well over a century, forecasters have been aware that areas of falling barometric pressures are often accompanied by precipitation and strong winds. However, it wasn’t until the early 1900’s that atmospheric scientists began piecing together a more complete picture of how low pressure systems develop, as well as the weather associated with them.
Recall that a cyclone is an area of low pressure, around which winds blow counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. This is due to the fact that winds blow from high to low pressure, but are deflected by the Coriolis force (perpendicular to the right of the motion vector in the Northern Hemisphere, left in the Southern Hemisphere). The focus of this chapter is cyclonic storm systems that form in the mid-to-high latitudes outside of the tropics. These storm systems are either called mid-latitude frontal cyclones, extratropical cyclones, wave cyclones, or simply frontal cyclones. Tropical cyclones will be the focus of a later chapter.
Shortly after World War I, Vilhelm Bjerknes, Jakob Bjerknes, Halvor Solberg, and Tor Bergeron published their Norwegian Cyclone model. This model proposed a life cycle for the development of mid-latitude cyclones, and was mostly based on surface observations. It became known as the Polar Front Theory of a developing wave cyclone. It was eventually modified and today provides a way to describe the structure, weather, and evolution of a moving cyclonic storm system in the mid-latitudes. First we will look at how a mid-latitude cyclone develops at the surface, and then we will look at how the surface evolution is affected by the winds aloft.
