Refracted rays.
It has been said that our atmosphere breaks, checks, and diffuses the falling sunlight like the globe of a lamp. It does something more. It acts as a prism and breaks the beam of sunlight into the colors of the spectrum. Some of these colors it deals with more harshly than others because of their shortness and their weakness. The blue rays, for instance, are the greatest in number; but they are the shortest in length, the weakest in travelling power of any of them. Because of their weakness, and because of their affinity (as regards size) with the small dust particles of the higher air region, great quantities of these rays are caught, refracted, and practically held in check in the upper strata of the atmosphere. We see them massed together overhead and call them the “blue sky.” After many millions of these blue rays have been eliminated from the sunlight the remaining rays come down to earth as a white or yellow or at times reddish light, dependent upon the density of the lower atmosphere.
Cold colors, how produced.
Warm colors.
Now it seems that an atmosphere laden with moisture particles obstructs the passage earthward of the blue rays, less perhaps than an atmosphere laden with dust. In consequence, when they are thus allowed to come down into the lower atmosphere in company with the other rays, their vast number serves to dominate the others, and to produce a cool tone of color over all. So it is that in moist countries like Scotland you will find the sky cold-blue and the air tinged gray, pale-blue, or at twilight in the mountain valleys, a chilly purple. A dust-laden atmosphere seems to act just the reverse of this. It obstructs all the rays in proportion to its density, but it stops the blue rays first, holds them in the upper air, while the stronger rays of red and yellow are only checked in the lower and thicker air-strata near the earth. The result of this is to produce a warm tone of color over all. So it is that in dry countries like Spain and Morocco or on the deserts of Africa and America, you will find the sky rose-hued or yellow, and the air lilac, pink, red, or yellow.
Sky colors.
I mean now that the air itself is colored. Of course countless quantities of light-beams and dispersed rays break through the aërial envelope and reach the earth, else we should not see color in the trees or grasses or flowers about us; but I am not now speaking of the color of objects on the earth, but of the color of the air. A thing too intangible for color you think? But what of the sky overhead? It is only tinted atmosphere. And what of the bright-hued horizon skies at sunrise and sunset, the rosy-yellow skies of Indian summer! They are only tinted atmospheres again. Banked up in great masses, and seen at long distances, the air-color becomes palpably apparent. Why then should it not be present in shorter distances, in mountain canyons, across mesas and lomas, and over the stretches of the desert plains?
Color produced by dust.
Effect of heat.
The truth is all air is colored, and that of the desert is deeper dyed and warmer hued than any other for the reasons just given. It takes on many tints at different times, dependent upon the thickening of the envelope by heat and dust-diffusing winds. I do not know if it is possible for fine dust to radiate with heat alone; but certain it is that, without the aid of the wind, there is more dust in the air on hot days than at any other time. When the thermometer rises above 100° F., the atmosphere is heavy with it, and the lower strata are dancing and trembling with phantoms of the mirage at every point of the compass. It would seem as though the rising heat took up with it countless small dust-particles and that these were responsible for the rosy or golden quality of the air-coloring.