— Advection inversion-2


Cold
Air Advected Under Warm Air


One type of advection that does occur in Marfa, and can have an effect on lights, is the common weather cold front.  As we said above, these events happen when cold air slides under warmer air already in place at a location.  The warm air is lifted up, and a temperature inversion results where warm rides above a cold-air layer.  The temperature change across the inversion can occur in a relatively short distance, and the temperature difference can be large. 

 

This inversion has implications for pollution, haze, and dust.  These particulates and aerosols can be trapped below the inversion because the air carrying them is colder than the air aloft, and it does not rise and disperse the haze.  To the contrary, the air sinks and concentrates the particles and aerosols.

 

Marfa usually has hazy skies.  The reasons for the haze are discussed in Chapter 10, “Lights on the One O One Flat,” of our book.[3]  We also discussed what the haze does to lights that shine through it.  Generally, the haze makes lights appear to be redder/yellower than they actually are.  If the haze is slight, or well dispersed up through the air, the effect on lights is small and perhaps not noticeable.  But if the haze is concentrated, the effect is large.

 

Figure 4 shows the kind of haze that is normally present at Marfa.  The photo was taken from the top of Mount Locke (6778 asl) [see MAP] and shows Twin Mountains in the foreground with Cathedral Mountain in the distance 40 miles away.  Cathedral Mountain is located on the eastern side on the Marfa plain.  Notice that there is a moderate haze, seemingly everywhere, extending high up above the ground, and creating a filter-like appearance to the picture.

Being wide spread and dispersed, this density of pollutants probably would not have much of an impact on lights propagating through the haze.  But look at Figure 10-5 in our book.[3]  That picture was taken from the same location on Mount Locke as the photograph in Figure 4 above. 

 

What makes the difference between the two photographs is that the photo in the book was taken just nine hours after a cold front went through Marfa.  The book photo shows that the pollution has been capped at a height about two-thirds of the way up the mountains.  The parts of the mountains that stick up above the inversion are clear.  But the bottom two-thirds of the mountains, and the valley, are difficult to see.  Lights seen through this high density of haze will be reddened considerably.