80 



THE DESEET 



Clear air. 



Dust 

 particles. 



Hazes, 



the quantities of dust and vapor in the air are 

 comparatively small, the distances that one can 

 see are enormous. A mountain seventy miles 

 away often appears sharp-cut against the sky, 

 and at sunset the lights and shadows upon its 

 sides look only ten miles distant. 



But desert air is not quite like the plateau 

 air of Wyoming, though one can see through it 

 for many leagues. It is not thickened fey moist- 

 ure particles, for its humidity is almost noth- 

 ing ; but the dust particles, carried upward by 

 radiation and the winds, answer a similar pur- 

 pose. They parry the snnshaft, break and color 

 the light, increase the density of the envelope. 

 Dust is always present in the desert air in some 

 degree, and when it is at its maximum with the 

 heat and winds of July, we see the air as a blue, 

 yellow, or pink haze. This haze is not seen so 

 well at noonday as at evening when the sun\ 

 rays are streaming through canyons, or at dawn 

 when it lies in the mountain shadows and re- 

 flects the blue sky. Nor does it muffle or ob- 

 scure so much as the moisture-laden mists of 

 Holland, but it thickens the air perceptibly and 

 decreases in measure the intensity of the light. 



Yet despite the fact that desert air is dust- 

 laden and must be thickened somewhat, there 



