Imagine a scenario where southern New England's temperature is near or above 50 degrees, while the foothills aligning the Appalachian Mountains of Maine, New Hampshire, and Vermont are stuck at or slightly above 32 degrees. Despite a southerly airflow, the air never warms. Known as the damming effect, the scenario creates many problems such as icing and extremely low visibility that can last days.
The damming effect, also known as cold air damming, is a common phenomenon that takes place over the coastal plain and the Appalachian Mountain Range of the United States. This atmospheric contributor occurs during winter and early spring. It has produced some of the fiercest ice storms in history, as well as days of fog with low visibility. In fact, the North Conway region has recently lived with this phenomenon. Remember the moderate ice storm on February 10th and 11th? That event was due in part to the damming effect. More recently, on February 21st, the area witnessed moderate rainfall with unusually dense fog all day long. The event created automobile accidents and was also due in part to the damming effect.
The damming effect occurs when cold air is banked against the Appalachian Mountain Range along the eastern coastal plain. As the warm air streams northeast over the cold air and the mountain range, it sets a stage for dangerous conditions. To be a little more in depth, as a cold high-pressure system settles toward the northeast, the clockwise circulation around its center brings northeasterly winds into our area. The northeasterly winds then store cold air against the eastern slope of the Appalachian Mountains. The counterclockwise circulation of a low pressure moving towards our vicinity in a northeasterly fashion tends to push warm air from the Midwest over the Appalachians. Since warm air aloft is lighter and less dense then cold air near the surface, the cold air remains stuck in the area as the warm air continues to overrun the cold air. With this in mind, what precipitation falls would do so in the form of freezing rain or very cold rain. Depending upon the strength of the effect, dense fog develops.
The next time the area sees what many call dreary conditions, remember the force that produces the unwanted scenario. Though many may dread fog or an ice storm, it takes much more then a simple task to produce the weather conditions. One may look up and view the thick overcast, but there is much more going on then what is viewed through the naked eye.