From Out There Colorado (Spencer McKee):
Though Antero Reservoir might not hold the record for coldest temperature ever recorded in Colorado, it is known for a unique microclimate in the area that frequently results in the frigid temperatures.
Here’s a look into what’s at play:
Antero Reservoir is located in a high-elevation basin called South Park, which is a large valley at about 9,000 feet above sea level that’s surrounded by mountains on all sides, stretching for about 1,000 square miles. As a result, cold and heavy air slides down the sides of the surrounding mountains and into the open, flatter space below. This is a natural phenomenon known as ‘cold air draining.’
The ‘cold air draining’ isn’t the only way the unique geographic features around Antero Reservoir help keep the valley cold. Because Antero Reservoir is in the middle of a large and mostly flat plains area and is farther from the mountains that surround the South Park valley, air tends to stay relatively calm there. This lack of wind is something that can prevent warm air from being mixed in with cold air, with air likely to be more tumultuous when in close proximity to a slope.
Another factor in the cold temperatures that frequent Antero Reservoir and the greater South Park basin is something called ‘radiational cooling.’ This process takes place on calm and clear nights, when longwave radiation (and heat) is able to escape back into the atmosphere amid a lack of clouds, dropping temperatures in the absence of the sun.
It’s also worth noting that something called an ‘inversion’ is known to take place frequently in the area when clouds are present. Basically, the cold air in the valley settles beneath a cloud, while the hot air settles above the cloud, keeping the ground level frigid for as long as that cold air stays trapped. This cloud also tends to block the sun from sending more warmth to the earth’s surface. It’s sometimes easy to tell when this phenomenon is taking place, as the valley can appear to be filled with a thick fog.