The official start of winter is still weeks away, but Charlotte is already experiencing cold, gray days thanks to a weather phenomenon known as the wedge. Even with temperatures dropping slowly, people must stay prepared for the possible cold threat.

The wedge, or cold air damming, happens when cold air flows up the eastern slopes of the Appalachian Mountains and becomes trapped, or "wedged," at the surface by warm air rising above it.

The mountains act as a barrier to the wind, similar to a water dam. Because cold air is less dense, it sinks to the surface, pools at the base of the mountains and drains into Western North Carolina, upstate South Carolina and northern Georgia.

The Appalachian Mountains provide the textbook example of elevation trapping air in place between the mountains and the coast, preventing the air from moving out.

The result is that the wedge of cold air is below freezing. It can bring snow and ice to these locations. When it is not below freezing, the wedge brings cool, foggy, cloudy and drizzly days. Temperatures can vary widely from one part of the state to the other.

Gloomy Charlotte skyline

Photo of an overcast Charlotte skyline.

Wedge events happen year-round and can last up to a few days. They can occur two to three times a month. In the colder seasons, they can happen four or five times a month with more pronounced effects, according to the Washington Post.

Their impact depends on how intense the cold air moves into the area and how much moisture is in the atmosphere. Clouds and precipitation can also strengthen cold air damming through evaporation as they cool the air.

Depending on the temperatures and depth of the cold layer at the ground, precipitation types can change drastically. The wedge can be the difference between receiving rain, a wintry mix like freezing rain or sleet and snow. 

An above or near-freezing temperature reading combined with a shallow cold bubble would be more likely to produce rain or freezing rain. If temperatures were below freezing, then there would be a better chance of sleet or snow. 

The type of precipitation is necessary to know, not just for deciding what to wear but also for how to prepare for the conditions going forward. For places in the South that are ill-equipped to deal with snow, wintry precipitation can cause hazardous road conditions, infrastructure damage and power outages.

The National Weather Service suggests stocking up on non-perishable food, water and supplies such as warm clothes, blankets and firewood before a dangerous cold event. The NWS has alerts to inform people of potentially threatening winter weather. Wedges have been responsible for several crippling ice storms in the Carolinas and Georgia.

Cold air damming provides a challenge for weather prediction models and forecasters. According to Suzanna Lindeman, who wrote a thesis at Virginia Tech, cold air damming greatly impacts the Appalachian Mountains. The tricky bit lies in how the weather models interpret the shallow cold bubble of air and how long it might last, particularly regarding swift changes in temperature and precipitation designations.

Wedges clear out when cold fronts move through the area, taking the clouds and the cold, damp conditions. We all know it is getting colder outside, but it is essential to take preparedness more seriously if cold air damming becomes prevalent.