The Carolina Wedge (Cold Air Damming)
As a person living in the Carolinas I am sure you heard of the weather event known as the "Carolina Wedge". It occurs many times during the year and normally brings about cloudy, cool, misty, and dull weather. This article will go into detail about what this "wedge" is and why it happens and how it can cause havoc in the winter.
So to start off, what causes this weather to occur?
Well more formally known as Cold Air Damming, this occurs when cool or cold air is pushed up against mountains. Due to the fact that cold air is more dense then warm air, it does not go over the mountains and will begin to pool up behind them. The event is often called 'the wedge" due to the resemblance of a wedge (triangle) in shape. This can be shown in the diagram below.
In the case of the Carolinas, "the wedge" is normally caused when a strong high pressure centers over the New England region. Notice the "bumps" in surface pressure from Virginia to Georgia, this is what we call ridging.
As this happens, the air moves in from the north east and is pushed into the mountains. From here, air begins to move southward since it has no place to go. This can be seen clearly below as the colder temperatures begin to progress southward in the model. Also, if you look carefully the temperatures follow the pattern of ridging shown in the model above. Do notice the warmer temperatures in the mountains themselves.
FUN FACT: Towns in the mountains often have higher temperatures than cities in the Piedmont due to the fact that they are too high up to be in the layer of cold air.
So why all the cloudy, foggy, and misty weather during Cold Air Damming?
Well to answer this, we have to look at a sounding during the event. Located on the diagram below the green line represents dewpoints while the red represents temperature. The more you move up the graph, the higher you are in the atmosphere.
Looking at the sounding we see the layer of cold air at the bottom. This is indicated by the red line moving to the left as it goes up. After this the line moves to the right meaning it gets warmer with height. This is known as an inversion.
So now to address the question. The reason why we see rain, clouds, mist, and fog during Cold Air Damming is due to the fact that the cold air gets trapped at the surface where it approaches or equals the dewpoints (the green line). When both of these lines meet it usually indicates there are clouds or rain at this part of the atmosphere since the air is saturated.
Now during the summer, Cold Air Damming can lead to cloudy, 70° days. But in the winter (given the right conditions) it can cause a mess. Recall the first diagram in the article, with this shallow cold air and warm air aloft (where the inversion ocurrs) precipitation can fall in many forms. Going off of the diagram below you can see that anything from rain to sleet can fall dependent on the amount of warm and cold air. The largest threat that does occur though is freezing rain. And many times when we do see a freezing rain events here in the Carolinas, Cold Air Damming is usually the culprit.
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