Effect of winds.
Sand-storms.
There is a more positive tinting of the air produced sometimes by high winds. The lighter particles of sand are always being drifted here and there through the aërial regions, and even on still days the whirlwinds are eddying and circling, lifting long columns of dust skyward and then allowing the dust to settle back to earth through the atmosphere. The stronger the wind, and the more of dust and sand, the brighter the coloring. The climax is reached in the dramatic sand-storm—a veritable sand-fog which often turns half the heavens into a luminous red, and makes the sun look like a round ball of fire.
Reflections upon sky.
Blue, yellow, and pink hazes.
The dust-particle in itself is sufficient to account for the warmth of coloring in the desert air—sufficient in itself to produce the pink, yellow, and lilac hazes. And yet I am tempted to suggest some other causes. It is not easy to prove that a reflection may be thrown upward upon the air by the yellow face of the desert beneath it—a reflection similar to that produced by a fire upon a night sky—yet I believe there is something of the desert’s air-coloring derived from that source. Nor is it easy to prove that a reflection is cast by blue, pink, and yellow skies, upon the lower air-strata, yet certain effects shown in the mirage (the water illusion, for instance, which seems only the reflection of the sky from heated air) seem to suggest it. And if we put together other casual observations they will make argument toward the same goal. For instance, the common blue haze that we may see any day in the mountains, is always deepest in the early morning when the blue sky over it is deepest. At noon when the sky turns gray-blue the haze turns gray-blue also. The yellow haze of the desert is seen at its best when there is a yellow sunset, and the pink haze when there is a red sunset, indicating that at least the sky has some part in coloring by reflection the lower layers of desert air.
The dust-veil.
Summer coloring.
Whatever the cause, there can be no doubt about the effect. The desert air is practically colored air. Several times from high mountains I have seen it lying below me like an enormous tinted cloud or veil. A similar veiling of pink, lilac, or pale yellow is to be seen in the gorges of the Grand Canyon; it stretches across the Providence Mountains at noonday and is to be seen about the peaks and packed in the valleys at sunset; it is dense down in the Coahuila Basin; it is denser from range to range across the hollow of Death Valley; and it tinges the whole face of the Painted Desert in Arizona. In its milder manifestations it is always present, and during the summer months its appearance is often startling. By that I do not mean that one looks through it as through a highly colored glass. The impression should not be gained that this air is so rose-colored or saffron-hued that one has to rub his eyes and wonder if he is awake. The average unobservant traveller looks through it and thinks it not different from any other air. But it is different. In itself, and in its effect upon the landscape, it is perhaps responsible for the greater part of what everyone calls “the wonderful color” of the desert.
Local hues.