Figure 1. Dutchman's Canyon, New Mexico
Inversions are another interesting phenomenon coming from the physics of air. On a clear day the sun shines through the clear atmosphere and hits the ground. The ground absorbs a large fraction of the solar radiation (and reflects the rest, the reflected portion is called the albedo). The ground heats up, air near the ground heats up and, because it is now less dense, it rises creating a thermal and perhaps an afternoon thunderstorm.
At night the situation is reversed. No, wait . . . there’s some sort of problem here. On a clear, relatively calm night, the earth emits more radiation than it receives. As the air cools it increases in density and sinks. But, since we’re already at the ground surface the air can’t really sink! The daytime situation results in what we call an unstable atmosphere with thermals rising and a high degree of mixing. The night time situation is very stable, the more the air cools, the more it sticks to the land surface – it resists mixing. As the land and near surface air cool below the temperature of the atmosphere just above a temperature inversion forms. Usually, because of the adiabatic lapse rate, the air gets cooler as we move upwards (e.g., to the mountain top). At night the situation is sometimes reversed (temperature inversion) and temperatures are lower at low elevations.
Temperature inversions have been measured and are strongest near the earth’s surface, because the air is primarily cooled by the even colder ground. Temperatures even one meter above the ground’s surface can be significantly warmer. When wind speeds are high temperature inversions that set up in protected valleys are “blown out”. Temperature inversions form preferentially on clear, calm nights where the radiation imbalance is present and not counterbalanced by convection.
In mountainous terrain the sinking of cold air leads to drainage flows as cold air flows down the topography just like water. The coldest location to camp is at the bottom of the valley or in a little depression. Even trees around a meadow can serve to trap cold air. Camping underneath a forest canopy is usually warmer at night because the canopy reflects and emits infrared radiation throughout the night. Very small changes in camp elevation (< 1 meter) can lead to large temperature differences.
Figure 2. Frozen pool at the bottom of Dutchman's Canyon, New Mexico after days and nights of above freezing temperatures on the bench above.
At night and early morning the flow is down the valley. In the afternoon winds are more typically up valley. Up valley afternoon winds are the bane of river rafters. Cold air in hollows has more to do with radiation than just sinking. The cold air can form here and then is stable. When cold air comes down the mountain it warms at the adiabatic lapse rate and thus may no longer be cold when it reaches the bottom.
A January backpacking trip in the Pelincillo Mountains of southern New Mexico provided an excellent introduction to some of the important concepts. Wesley and I chose to sleep on the bench above the bottom of Dutchman's Canyon (an upper portion of Skeleton Canyon where Geronimo surrendered) in order to enjoy the views and to stay warmer. The minimum nighttime temperature was ~40°F on the bench near the ridge top. The next morning I took a walk along the bottom of Dutchman's Canyon. The canyon is steep walled at the bottom and trends East/West meaning the sun never reaches the canyon bottom in the winter. At the bottom were nice erosional features (Figure 1) as well as frozen pools and snow (Figure 2). Why was the canyon bottom so much colder than the bench above? It couldn't be drainage flows of cold air because the air above was warmer and would have warmed further (adiabatic lapse rate) when descending into the canyon.
Direct sun is blocked at this location in the winter because of the shape of the valley, but infrared radiation can still escape from the bottom. The cooling below ambient temperatures occurs because the infrared radiation flux is unbalanced. More goes out than comes in. Heat comes up from the deeper ground and from the air to the soil surface to balance this radiation loss. The cool situation is also stable since cold air sinks. This makes it difficult for warmer air to reach the canyon bottom in winter. Notice that the water in the rock chute at the canyon bottom (Figure 1) was not frozen even though it is close to the frozen pool (Figure 2). This is because the high thermal conductivity of the rock brings heat up from the ground very efficiently and the partial tree coverage of the rocks traps some of the infrared radiation.
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Copyright 2014 John Walton