Little Studies in the Yosemite Valley. 93 



they soon wax stronger and more general, and, as the 

 shadows retreat and dwindle before the oncoming light 

 invasion, they finally gain the upper hand. The nocturnal 

 air-streams cease to flow and a general movement is in- 

 augurated in the opposite direction, up toward the high- 

 lands at the valley head. It is not usually so noticeable 

 as the night wind, for its tendency is naturally to spread 

 and diffuse upward, while the nocturnal movement is one 

 of condensation and concentration, especially vigorous 

 along the valley floor. But it is none the less a well-de- 

 fined, characteristic movement that continues throughout 

 the day. Late in the afternoon, with the growing of the 

 shadows it gradually comes to a stop and the tide turns 

 back again. Thus the air of the Yosemite Valley goes 

 through a daily ebb and flood, reversing early every morn- 

 ing and again late in the afternoon. 



Most mountain valleys have similar alternating night 

 and day winds, but those of the Yosemite Valley are ex- 

 ceptionally pronounced. All conditions in its case favor 

 the orderly consummation of the process and conspire to 

 accentuate each phase. No general winds sweep over the 

 country to interfere with the local up- or downdrafts, 

 except at intervals of many weeks; and so exceedingly 

 dry and pure is the atmosphere of the Sierra, so few 

 particles of dust or moisture does it hold, that the sun's 

 rays plunge through it almost without let or hindrance. 

 Insolation, consequently, is particularly intense and begins 

 almost immediately with the rising of the sun, while 

 radiation is equally rapid and sets in promptly the moment 

 the sun disappears. And thus it comes that the reversals 

 in the Yosemite Valley take place with clock-like regu- 

 larity, and the entire movement assumes the rhythmic 

 swing of a pendulum. Nothing was better calculated to 

 make this visible to the eye than the smoke column from 

 the forest fires that raged persistently at the lower end 

 of the valley during the summer of 1905. Every morning 

 the valley was clear, having been swept out, so to speak, 

 by the nocturnal down-valley current, and the smoke pall 



