Have you ever experienced a period of grey, wet weather that seems to drag on forever? While this can happen for a number of reasons, there is one phenomenon in particular that causes these weather conditions in mountainous areas. Residents of the southeastern US know this locally as “The Wedge”. The Wedge most recently affected areas of Northeast Georgia, near Atlanta and eastward towards South Carolina. It brought days of chilly, dreary, misty conditions. But what exactly causes the Wedge?
The Wedge is more technically known as a Cold Air Damming event. Cold Air Damming, or CAD, happens when cold air gets trapped by mountain ranges. Low-temperature air is denser than air of a higher temperature, which means that cold air is effectively heavier, and is thus unable to rise up over the mountains and dissipate – so it gets stuck there for a few days. However, this layer of cold air is very shallow, confined to the part of the atmosphere that is closest to the surface. Above this layer of cold and misty air, conditions are much warmer.
The arrows on the diagram above depict the direction of the winds coming towards the eastern side of the mountain, and the blue color depicts the cold temperature of the air that gets stuck near the surface.
Because the colder air is very dense and confined to the surface, warmer air can get pushed above the cold air, as shown by the red arrow in the diagram. Rising air, and the energy associated with it, is what causes thunderstorms. As warm air rises and cools, water vapor in the air condenses into clouds. When this warm air rises quickly, all of that energy of motion causes a thunderstorm to develop. In the case of CAD events, warm air is able to rise quickly because it is being pushed against the heavier cold air and forced rapidly upwards.
This is what was seen by residents of northeast Georgia in their most recent CAD event. There were many localized thunderstorms in the area caused by this phenomenon. These thunderstorms were brief, but brought torrential downpours and loud cracking thunder with them. The strength of these thunderstorms is a testament to just how strongly and quickly warm air is forced above the colder air when the Wedge/CAD is present.
While Cold Air Damming can bring about some dreary, dismal conditions, it’s also a pretty interesting weather event. Next time you experience a few days of cold and mist, check your local weather maps for an area of cold temperatures near a local mountain range. You just might be experiencing Cold Air Damming!
©2018 Meteorologist Margaret Orr
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