— Radiation inversion



During the day time the earth warms up (on the side facing the sun!)  To what temperature it warms depends on factors like the degree of cloud cover, the morning temperature of the site, and the composition of the site on the earth.  But the earth doesn’t just absorb energy that falls on it while it is facing the sun.  It also radiates energy away from all its surfaces 24 hours a day.  The energy it radiates is called infrared radiation.  And it is this energy flow that cools the earth each night after the sun goes down.  The cooling of the earth has implications for the air above it and for lights seen on the Marfa plain. 

 

The air near the surface of the earth is always in some degree of motion.  This is especially true on the Marfa plain, where in 2013, for example, the wind blew 3 mph or faster 84% of the time during the entire 24 hours of the 365 days of the year. 

 

The winds blow against the soil, rocks, and vegetation.  In doing so, the air transfers some of its thermal energy to the cooler objects on the ground, thus lowering its temperature.  In turn, the parcels of air that get cooled by the ground, cool parcels of air above them.  And the cooling process continues upward.  The coolest air is that next to the ground.  Higher up the air is warmer.  This variation in the temperature of the air is a temperature inversion.  And it is caused by infrared radiation from the earth.

 

Radiation inversions are also called ground inversions, perhaps because the inversions start right at the ground.  These inversions may not extend upward more than about 100 meters.  But that is enough to affect the appearance of lights on the Marfa plain. 

 

There are two reasons for the impact on lights.  First, the temperature extremes in the inversions can be large, perhaps 10°C.  And second, the inversions are right where the lights travel – between a ground-level light source and a ground-level observer. 

 

The whimsical illustration in Figure 6 suggests the interaction of lights and air when the inversion occurs in the same range of heights and dimensions as observer and luminous object.  Air temperature is indicated by the blueness of background in the figure.  Coldest air (bluest) is at the bottom.

 

We chose the ghost of Alsate, the great chief of the Chisos Apaches, and his horse to challenge the modern observer on the left.  This selection was inspired by the stories of Alsate’s ghost being seen in the mountains south of Marfa after Alsate’s execution in 1882.  (See History for Alsate's story.)

In our book,[3] radiation inversions, are treated quantitatively.  The shapes of possible temperature profiles and dimensions are illustrated. 

Radiation inversions can affect light propagation in a way that contributes to mysterious light behavior.  More than just illustrating that radiation inversions can affect light propagation, we show exactly how that occurs, what factors enable it, and what an observer should look for to interpret the effect.  Check out Chapter 9, “Larger Than Life,” to discover how radiation inversions add mystery to the lights on the Marfa plain.