90 Sierra Club Bulletin. 



















FIG. I. 



the valley but will remember toiling up some never-ending 

 zigzags on a hot and breathless day, with the sun on their 

 backs and their own dust floating upward with them in 

 an exasperating, choking cloud. Perhaps they thought it 

 was simply their misfortune that the dust should happen 

 to rise on that particular day. It always does on a sun- 

 warmed slope. 



But again, memories may arise of another occasion when, 

 on coming down a certain trail the dust ever descended 

 with the travelers, wafting down upon them from zigzag 

 to zigzag as if with malicious pleasure. That, however, 

 undoubtedly happened on the shady side of the valley. 

 For there the conditions are exactly reversed. When the 

 sun leaves a slope the latter begins at once to lose its 

 heat by radiation, and in a short time is colder than the 

 air. The layer next to the ground then gradually chills 

 by contact, and, becoming heavier as it condenses, begins 

 to creep down along the slope (see Fig. 2). There is, 

 thus, normally a warm updraft on a sunlit slope and a 

 cold downdraft on a shaded slope — and that rule one may 

 depend on almost any day in a windless region like the 

 Yosemite. Indeed, one might readily take advantage of it 

 and plan his trips so as to always have a dust-free journey. 



