The above title is the title of an interesting report sent out by Russ Schumacher, the Colorado state climatologist. What he is referring to is not so much that Gunnison is cold, we already know that. The recent month of December was exceptionally warm throughout Colorado, and we still stayed “stubbornly,” record-breakingly cold.
As Schumacher points out, for the majority of December, most of the state was 3-9 degrees warmer than average. For the week of December 15-21 “it’s even more stark: almost the entire state in a deep red of warmth, with Gunnison again in a cold purple.”
According to the official Gunnison County Electric Association weather station for the first 20 days of December we were 13 degrees colder than average. We didn’t get a day above freezing until Dec. 26. We’ve never had a December without a high above freezing before, and we set record lows on Nov. 30 and Dec. 1.
The long-term average high December temperature is 29.7 degrees, but this year it was 20.3 degrees. The long-term average temperature was 13.5 degrees, compared to this year’s 6.5 degrees. The coldest December we ever had was in 1928 with an average monthly temperature of 1.1 degrees. We had three days in a row of 45-below temperatures that year at Christmas.
Okay, why is Gunnison so cold, and why this year? First, this year is relatively easy to explain. We had this big dump of snow on the day before Thanksgiving. Reliable witnesses in town tell me it started snowing at 2 a.m. Then it snowed hard all day, accumulating to about 10 inches in most of town. Furthermore, there was enough snow to cover all the grass and low-lying vegetation throughout the area.
Albedo is the fraction of incoming solar energy that a surface reflects to the atmosphere and dissipates into space. Snow, of course, reflects a large part of that energy back to the atmosphere, whereas black surfaces ,like asphalt streets, absorb most of the energy and reflect very little. Grass reflects less than 20% of solar radiation, and hence warms the ground.
Worse, because we didn’t get melting, all we had was sublimation. This is the direct passage of snow from a solid state to the vapor state (which is very slow and inefficient), so we kept all this snow on the ground. It’s still mostly there and you get the idea.
But that isn’t the whole story. Our topography is such that Gunnison lies in an almost closed basin with four or five drainages coming in from the surrounding high mountains. This is sometimes called a “frost basin.” Taylor Park is a good example.
Since cold air is heavier than warm air, we get the gravity drainage of cold air, which then pools in the Gunnison Basin. For many years, a few other weather watchers and geeks, and I have speculated that because we also had a bottleneck just west of town in the narrow Gunnison Canyon, this is a major cause of the cold pool forming. But I’m not so sure anymore.
There are other places in Colorado and the Mountain West where we have a similar situation and they’re not as cold as Gunnison. Our neighbor to the east, Salida, has a very similar topographic setup at nearly the same elevation. It is practically in the tropics compared to Gunnison.
The impressive Arkansas canyon starts just east of Salida. Granted, only two major drainages come into town from the high mountains, but that should be sufficient to cool the place down.
Perhaps the difference lies in the fact that the Arkansas Valley is highly subject to strong winds from the Sawatch Range, also found along the northern Front Range from Boulder to Ft. Collins. These winds stir things up and literally blow the cold air away. In conclusion, I’m not sure!
(Bruce Bartleson is a retired emeritus professor of geology at Western Colorado University. He spends most of his time now watching the weather.)
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