Temperature Sinks - Antero Reservoir, Colorado Hits -50°F

 

 

On the evening of December 29- 30, 2020, Antero Reservoir, deep in the Colorado Rockies, recorded a low temperature of -50°F, the lowest temperature in the U.S. so far this winter.  In fact, it was the coldest temperature in the U.S. since February of 2019!  How in the world can the temperature get so cold?  In this blog entry I hope to share the conditions that led up to this frigid night, and also highlight some other locations across he U.S. and worldwide where specific topography and weather conditions can produce wild temperature extremes like this one.  

I love to look around the US each day and see where the coldest weather occurred.  I regularly check the  National Weather Service link for the “National Daily High and Low Temperature” reported across the US.  What I have found so interesting is that the same locations tend to show up from time to time and the minimum temperatures amaze me!  By the way, they are not always International Falls MN, the reputed “icebox” of the nation.  So, here’s a question for you.  What do Antero Reservoir, Canaan Valley WV, “The Barrens” outside State College PA and Peter Sinks, UT have in common?  Well, they all are what are referred to as “temperature sinks” and they get darn cold compared to locations in close proximity. 

What Is A Temperature Sink

A temperature sink, as the name implies, is a location in a natural sink, sinkhole or bowl, typically found in the mountains, surrounded by higher ridges.  Without an outlet down a mountain valley, cold air does not drain from these “sinks” and they can continue to get colder and colder as the night progresses.

Antero Reservoir is a classic temperature sink.  Its topography, along with some fascinating properties of cold air, and select meteorological conditions, led up to this frigid temperature.  As you may know, cold air is more dense than warm air.  As the sun begins to set on a clear, calm, dry night, the ground begins to radiate its heat into the atmosphere and the temperature right near the surface cools. As the air temperature drops, the cold, dense air acts like molasses, sliding down mountain sides and pooling in the low-lying areas between slopes as shown below. You can even feel this effect locally if you live in an area where there are dips in a road or depression along a trail you might be walking along.  As the sun sets, that colder air will pool in those areas and as you walk down the slope you will feel like you may have even walked into a fridge or freezer. 

Simple schematic showing what happens around sunset on a clear, calm night.  As the ground begins to radiate heat into the atmosphere, the air temperature cools and the colder, more dense air slides down the mountain slopes into the valley below.

From a meteorological standpoint, the downslope movement of air that occurs in an otherwise calm evening with no breeze is known as a katabatic wind, or drainage wind and it leads to cold air pooling within mountain valleys.  As the cold air pools in the valleys, the local atmosphere develops a temperature inversion, where the coldest temperatures are at the surface and there are warmer temperatures aloft.

 Antero Reservoir and Perfect Frigid Weather Conditions

Cross-section of Antero Reservoir in the Colorado Rockies shows the bowl that it is located in with mountain ridges surrounding it in all directions. 

That’s exactly what happened at Antero Reservoir on the night of December 29^th-30^th, 2020.  The entire region was under a fair-weather High Pressure system accompanied by clear skies and calm winds.  In addition, this region had a deep, fresh snowpack in place. On a calm, clear night, fresh snow-covered surfaces cool much more quickly than a bare surface at the same temperature. Snow “radiates” heat very efficiently, and that increases the rate of heat loss at the surface. Snow is also a great insulator and that prevents heat from rising through the snowpack, allowing the surface to cool quickly. In contrast, bare soil conducts much more heat upward from below, which helps slow down cooling at the surface.

High Pressure was strecthed across the Colorado Rockies and wind conditions were calm in the area around Antero Reservoir.

This satellite animation taken during the following daytime on December 30, 2020 shows the fresh snowcover across Colorado and the clear skies.  Along with the calm winds, the atmopshere set up for a perfect night to chill down to extremes. 

As a result of all of these factors coming together, cold air then pooled down at the base of the bowl in which Antero Reservoir is located and the temperature plummeted to the reading of -50°F.

Other Locations That Are Temperature Sinks

As I noted above, this is not the only location in the U.S. where there are weather instruments available to monitor these extremes.  One of the most famous is Peter Sinks, Utah. According to the Utah Climate Center “On Feb. 1, 1985, the temperature at Peter Sinks location plummeted to -69.3°F, the second coldest ever recorded in the lower 48 states. The lowest was -69.7°F at Roger's Pass, Montana in January 1954.” Peter Sinks has been studied a lot by the meteorological community. In fact, there is weather equipment set up on the rim of this sink as well as the base of the bowl, only about a 300 ft. difference in elevation but wow can the temperature differences be extreme. This is the essence of micro-meteorology. 

Comparison of daily minimum temperature at the base and rim of Peter Sinks, Utah on several consecutive nights from December 25th through 30th, 2020.  The rim of this 1km. wide sinkhole in the Wasatch Mts of northeastern Utah is less than 300 ft. above the base, but temperature differences at night can be 50°F or more.

The conditions that were so favorable for Antero Reservoir on the evening of December 29-30 were similar the night before in Utah. Check out the temperature profile I plotted from December 25 through December 30.  Notice on the might of December 29, the temperature at the base of the bowl, or sink, dropped to -38°F, while less than 300 ft. higher on the rim of this 1km. wide bowl the temperature was 50 °F higher at 12°F!  No, this did not take place on the planet Mars, rather a remote site in the Wasatch Mountains of northeast Utah.

What’s fascinating about some of these locations where temperatures get so cold at night is that they share other characteristics of the micro-climate including the soil type as well as the vegetation or lack thereof.  Why are those factors important?  Well, to amswer that question, let's visit a place called “The Barrens”, located in Central Pennsylvania, about 4 miles west of the Penn State campus.  According to The Pennsylvania State Climatologist this location has similar daytime maximum temperatures each day to the Penn State Campus, but at night, under conditions described above there can be as much as a 30°F difference in the minimum temperature at the 2 locations!  The table below shows the distinct differences between daytime high temperature and overnight lows at The Barrens vs. its neighbor at State College.  Note, that during this study State College only experienced 1 night with a temperature below 0°F while The Barrens saw temperatures below 0°F 31 times!

 

A Frequency Distribution of Temperatures at the Barrens and State College (1200Z to 1200Z from December 1977 to November 1978).  (courtesy: The Pennsylvania State Climatologist)

Once again, topography plays a major role in the micro-climate of The Barrens, but there are also other characteristics of the site, shared by the other locations around the U.S. mentioned above, that allow it to cool so rapidly at night.  The Barrens has a surface of sandy soil which allows water to be absorbed deep into the ground, keeping the surface layer dry.  That results in a rapid loss of heat from the ground after the sun drops below the ridges. The other feature of the Barrens that contributes to rapid temperature loss is very little vegetation other than some scrub trees.  In fact, in many of these “sinks” or depressions, there may even be a “vegetation inversion”, with only s few scrub trees and grasses where the temperatures undergo drastic daily changes, while up on the slopes of the mountain there are more mature trees and more lush vegetation. 

Next, let’s stay in the East, and visit a beautiful location in The Appalachian Mountains at Canaan Valley, WV.   It’s another great place to study micro-meteorology, because at this site, maintained by Virginia Tech, there are two weather sensors that measure temperature, one at the base and one at the  ridge of the bowl.  Below is a cross-section I derived off Google Earth to show Canaan Valley.  As you can see, the location sits in a depression or bowl surrounded by higher ridges.  The elevation difference between the weather sensor at Canaan Valley (elevation 3150 ft.) and the Cabin 2 site on the right-hand ridge (elevation 4035ft.) is just under 1,000 ft.  and a distance of a little over a mile.   The sensor itself is located on a barren piece of ground, with a sandy soil and not much vegetation.

 

Cross-section of Canaan Valley WV, showing the location of the sensor in relation to the elevation changes from one side of the bowl to the other.

Back on Christmas Day of 2019, under clear skies and calm winds, Canaan Valley dropped to 10°F while the ridge sensor was reporting a temperature of 43°F.  That equates to a 33°F difference in temperature over a distance of a little over a mile! In fact, under a fair-weather High Pressure system, with a very dry atmosphere, clear skies and calm winds, this area saw 3 straight days of extreme temperature differences.

Temperature trace for Canaan Valley WV and Cabin Mtn2 on 3 consecutive days from December 24th through December 26, 2019 showing the extreme difference in readings at night, while daytime highs showed very little difference.

There are many, many other locations across the U.S. and of course worldwide, where the phenomenon of “cold air sinks” occurs.  In fact, in Central Europe, one of the most well-known is Gruenloch Sinkhole in Austria, where conditions that created the sinkhole, including a collapsed limestone rock base, are perfect for the development of extremely cold temperatures.  I believe the site still holds the record for the coldest minimum temperature in Central Europe, at -62.7°F (-52.6°C).   There has been much research in other parts of the world including Scandinavia and Japan on these features as well. 

Summary

In closing, there are several factors that combine to produce such cold temperatures. In addition to clear skies and calm winds they include:

-        Topography - A valley setting with slopes on all sides that form a bowl, allowing for cold air drainage and with no valleys in the bowl, the cold air becomes trapped to continue to chill throughout the night.

-        Soil type – Sandy soils or limestone lose heat to the atmosphere more quickly than other soils, namely because they allow water to permeate through them and dry soils lose heat more quickly.  If there isn’t much ground cover in the form of trees or other vegetation, that adds to heat loss more quickly.

-        Snow cover – These mountain valleys will stay colder, and hold snow longer in winter, and snow cover is a great emitter of long wave radiation, or heat loss to the atmosphere.

-        The development of a temperature inversion over the valley will prevent turbulent mixing and it effectively “cuts off” the bubble of cold air from the atmosphere above it, allowing these areas to continue to cool through the night.

As someone who hikes quite often, I remind myself how important it is to remember that when you decide to make camp for the night on a clear, calm evening, especially during the colder times of the year, a decision to pitch your tent in a little depression or valley could not only result in a really cold night, it could be life-threatening.  Setting up a tent or hammock in a valley bottom under cold, clear conditions, will expose you to the coldest temperatures in the area as that cold, dense air slides down the slopes and envelopes your campsite. In these cases, if at all possible, it’s often better to set your camp up a little way up the mountainside.

Keep an eye on the “National Daily High and Low Temperature” each day if you get a chance.  Check out the locations and see if they share some of the factors that I have discussed in this article. Don’t get caught with that “sinking” feeling.