<img class="styles__noscript__2rw2y" src="https://s.w-x.co/util/image/w/niz-sinks-fig1.jpg?v=at&w=485&h=273" srcset="https://s.w-x.co/util/image/w/niz-sinks-fig1.jpg?v=at&w=485&h=273 400w, https://s.w-x.co/util/image/w/niz-sinks-fig1.jpg?v=ap&w=980&h=551 800w" > Comparison of daily minimum temperature at the base (orange) and rim (blue) of Peter Sinks, Utah, Oct. 21-31, 2019. The rim of this kilometer-wide sinkhole in the Wasatch Mountains of northeastern Utah is less than 300 feet above the base, but temperature differences at night can be 30 degrees or more.

At a Glance Some of the most extreme cold temperatures occur in areas known as sinks.

These are small-scale valleys in mountain terrain where cold air accumulates.

Winter lows can plummet into the minus 40s or colder.

It can be 30 degrees warmer just a few miles away or a few hundred feet higher in elevation.

On Oct. 30, 2019, a sensor registered a temperature of minus 46 degrees Fahrenheit at the bottom of a little depression known as a sink in mountainous terrain.

Just 300 feet higher, on the rim of this kilometer-wide sink, another sensor recorded a temperature of minus 7, nearly 40 degrees warmer.

This did not take place on Mars, but rather a remote site in the Wasatch Mountains of northeastern Utah called Peter Sinks – historically one of the coldest places in the continental United States.

According to the Utah Climate Center , Peter Sinks plunged to minus 69.3 on Feb. 1, 1985, the second-coldest temperature ever recorded in the Lower 48. The coldest was minus 69.7 at Roger's Pass, Montana, in January 1954.

This is the essence of micrometeorology. I love to look around the U.S. each day and see where the coldest weather occurred. I regularly go to the National Weather Service link for the national daily high and low temperature reported across the country.

What I found so interesting is that the same locations tend to show up from time to time and the minimum temperatures amaze me.

The nation’s cold spot is not always International Falls, Minnesota – the reputed “icebox of the nation.”

Have you ever heard of Canaan Valley, West Virginia, the “Barrens” outside State College, Pennsylvania, or Antero Reservoir, Colorado?

They all have something in common; they get downright frigid compared to locations in close proximity.

How It Gets Cold: The Science of Inversions

So, let’s get back to why these locations get so cold.

It all has to do with topography and one of the properties of cold air. As you may know, cold air has less energy than warm air. Air molecules don’t move around as much in cold air. They are packed more tightly together. Therefore, 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 mountains and pooling in the low-lying areas between slopes, as shown in the diagram below.

<img class="styles__noscript__2rw2y" src="https://s.w-x.co/util/image/w/niz-inversion-fig2.jpg?v=at&w=485&h=273" srcset="https://s.w-x.co/util/image/w/niz-inversion-fig2.jpg?v=at&w=485&h=273 400w, https://s.w-x.co/util/image/w/niz-inversion-fig2.jpg?v=ap&w=980&h=551 800w" > 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.

You can feel this effect if you live in an area where there are dips in a road or depressions on 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, it might feel like you walked into a fridge or freezer.

The downslope movement of air that occurs in an otherwise calm evening with no breeze is known as a drainage wind. 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.

Other Sinks

The weather instruments I will refer to at these sites are also in mountain valleys, some located in a particular geologic formation called a sink or sinkhole located within the mountains. Lacking an outlet down a mountain valley, cold air does not drain from these sinks, so they can continue to get colder as the night progresses.

What’s fascinating about some of these locations where temperatures get so cold at night is that they share other characteristics of the microclimate, including the soil type and the vegetation, or lack thereof.

Why are those factors important? Look to the Barrens, located in central Pennsylvania about 4 miles west of the Penn State University campus, for the answer.

According to The Pennsylvania State Climatologist , this location has similar daytime high temperatures each day to the Penn State campus. But at night, under conditions described above, there can be as much as a 30-degree difference in the low temperature at the two locations.

The table below shows the distinct differences between daytime high and overnight low temperatures at the Barrens compared to nearby State College. In a 12-month period ending in November 1978, State College only experienced one night with a temperature below zero while the Barrens saw subzero temperatures 31 times.

<img class="styles__noscript__2rw2y" src="https://s.w-x.co/util/image/w/niz-table-fig3.jpg?v=at&w=485&h=273" srcset="https://s.w-x.co/util/image/w/niz-table-fig3.jpg?v=at&w=485&h=273 400w, https://s.w-x.co/util/image/w/niz-table-fig3.jpg?v=ap&w=980&h=551 800w" > A frequency distribution of temperatures at the Barrens and State College, Pennsylvania, from December 1977 to November 1978. (Data: The Pennsylvania State Climatologist)

Once again, topography plays a major role in the microclimate of the Barrens, but there are also other characteristics of this and other sites that allow for so much rapid cooling at night.

The Barrens has 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 at its base.

In many of these sinks or depressions, there may even be a vegetation inversion. The base has more sparse vegetation, including scrub trees and grasses, where the temperature undergoes drastic daily changes. Up on the slopes of the mountain, the vegetation is more lush with mature trees and dense brush.

Next, let’s visit a beautiful location in the Appalachian Mountains: Canaan Valley, West Virginia. It’s another great place to study micrometeorology because there are weather sensors at the base and the ridge of the valley.

The location sits in a depression, or bowl, between two ridges. The elevation difference between the weather sensor at Canaan Valley (elevation 3150 feet) and the Cabin 2 site on the right-hand ridge (elevation 4035 feet) is just under 900 feet. The weather sensors are only about a mile apart. The site itself sits on a barren piece of ground, with a sandy soil and not much vegetation.

<img class="styles__noscript__2rw2y" src="https://s.w-x.co/util/image/w/niz-canaan-wv-fig4.jpg?v=at&w=485&h=273" srcset="https://s.w-x.co/util/image/w/niz-canaan-wv-fig4.jpg?v=at&w=485&h=273 400w, https://s.w-x.co/util/image/w/niz-canaan-wv-fig4.jpg?v=ap&w=980&h=551 800w" > Cross-section of Canaan Valley, West Virginia, showing the location of the sensor in relation to the elevation changes from one side of the valley to the other. (Map: Google Earth)

On Christmas Day 2019, under clear skies and calm winds, Canaan Valley dropped to 10 degrees while the ridge sensor was reporting a temperature of 43 degrees.

That’s a 33-degree difference in temperature over a distance of a little over a mile. Under a high-pressure system, with a dry atmosphere, clear skies and calm winds, this area experienced three straight days of extreme temperature differences around Christmas.

<img class="styles__noscript__2rw2y" src="https://s.w-x.co/util/image/w/niz-canaan-graph-fig5.jpg?v=at&w=485&h=273" srcset="https://s.w-x.co/util/image/w/niz-canaan-graph-fig5.jpg?v=at&w=485&h=273 400w, https://s.w-x.co/util/image/w/niz-canaan-graph-fig5.jpg?v=ap&w=980&h=551 800w" > Temperature trace for Canaan Valley and Cabin Mountain, West Virginia, Dec. 24-26, 2019, illustrating the extreme difference in readings at night, while daytime highs showed very little difference.

Finally, let’s head west to the beautiful Colorado Rockies, no stranger to cold weather.

In December 2019, for two days in a row , Antero Reservoir, west of Colorado Springs, recorded the lowest temperature in the U.S. at minus 48 degrees.

By now, you might be able to guess the characteristics of this location. Once again, Antero Reservoir is a high-elevation depression or valley, surrounded by mountains.

<img class="styles__noscript__2rw2y" src="https://s.w-x.co/util/image/w/niz-antero-fig6.jpg?v=at&w=485&h=273" srcset="https://s.w-x.co/util/image/w/niz-antero-fig6.jpg?v=at&w=485&h=273 400w, https://s.w-x.co/util/image/w/niz-antero-fig6.jpg?v=ap&w=980&h=551 800w" > Cross-section of Antero Reservoir, Colorado, showing the location of the sensor in relation to the elevation changes from one side of the valley to the other. (Map: Google Earth)

Jim Kalina, a forecaster at the National Weather Service in Boulder, Colorado, said they typically get that cold a couple times a year.

Kalina added that another feature of these high-elevation locations that aids in getting these cold extremes in winter was fresh snow cover, which is an excellent emitter of long-wave radiation, or heat release to the atmosphere. During winter, many of these sites will retain their snow cover, which aids in the rapid temperature drop overnight.

There are many other locations around the world where the phenomenon of cold air sinks occurs.

In central Europe, one of the most well-known is Austria’s Grunloch Sinkhole, where conditions that created the sinkhole, including a collapsed limestone rock base, are perfect for the development of extremely cold temperatures.

This site holds the unofficial record for the coldest temperature in central Europe – minus 62.7 degrees Fahrenheit.

There has been much research in other parts of the world, including Scandinavia and Japan, on these features.

As someone who hikes 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 as that cold, dense air slides down the slopes and envelopes your campsite. In these cases, it’s often better to set your camp up a little way up the mountainside, if possible.

Don’t get caught with that sinking feeling.

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.