Cold Air Damming - Cause And Effects

Colder than expected temperatures, cloudy conditions, and persistent drizzle are all potential impacts from cold air damming. But what is cold air damming, where does it form, and why is it so hard to forecast?

Cold air damming is an atmospheric phenomena that requires a north-south oriented mountain range in order to develop. It is a common feature east of the Appalachian Mountains in the eastern U.S., but is also somewhat common east of the Rockies.

The feature itself involves a low-level layer of the atmosphere, below the level of the mountain range it forms up against. High pressure sliding eastward across the mountain range is a typical precursor to a cold air damming setup, as it generates winds from the north that helps pull cooler air down the mountain range.

Colder air is more dense and sinks, so when the northerly or northeasterly winds pull cooler air in toward the mountains, the air settles up against the mountain range and pours southward along it. Without a significant weather feature to push the air up over the mountains or away from the mountains, the cooler air will remain locked in near the surface.

In addition to northerly winds from high pressure systems, cold air damming along the Appalachian Mountains can be further aided by the cool moisture provided by the Atlantic Ocean, especially in the winter and spring months. The moist ocean air enhances the formation of low clouds and drizzle along the East Coast when pushed up against the mountains, which helps lock the cold air in place.

When the weather pattern finally changes and a low pressure system moves in, it can take a long time to erode the low-level cold air. The disturbance feeding warmer southwesterly winds into the region of cold air damming must first travel over the mountain range. As a result, this warmer air is lifted above the dam of cold air. With the less dense and warmer air rising above the more dense cold air dam, the process of removing the cold air dam can take many hours, if not days.

Because the layer of cold air is a relatively shallow atmospheric feature and is dependent on the size and shape of the neighboring mountain range, many weather models struggle to properly analyze and forecast the strength and duration of cold air damming. In order for the models to run in a reasonable amount of time, both the layers of the atmosphere and the geography of the mountains need to be simplified a bit. This simplification usually causes the weather models to erode and eliminate the cold air damming faster than it actually occurs.

In many situations, warm southerly winds pushing up against the cold air dam can produce sharp temperature changes on the order of 20 to 30 degrees over a short distance. Depending on where this sharp temperature change ends up, a local forecast can be considerably too cold or too warm if the sharp temperature change is off by just several miles.

The cold air at the surface with warmer air above it can also be the cause of freezing rain and sleet in the winter months. The warm air aloft could cause rain, or it could cause falling snow to melt. After that, the colder air near the surface could cause the rain to freeze on contact with cold surfaces, or even freeze in the air to become sleet.