92 



Sierra Club Bulletin. 



there. Is there, then, really an updraft wherever the 

 sun shines and a downdraf t in every shadow patch ? Most 

 assuredly there is. That is one of the peculiarities of the 

 valley, the immediate outcome of its exceptionally bold 

 cliff topography. Every cliff that casts a shadow thereby 

 creates a downward breeze. And thus, there are in spots 

 throughout the valley local breezes that recur daily at 

 certain hours as the shadows come and go. One may 

 readily test this to his satisfaction on a place like Glacier 

 Point. In the morning, when the great cliff is still in 

 shadow, a bit of paper tossed over the brink at once dis- 

 appears, sucked down by a descending current, but at 

 noon when the sun beats on the cliff, the very opposite 

 will happen; instead of saiHng down, the paper shoots 

 upward, and continuing upward, disappears like a tiny 

 white speck in the blue. 



But let it not be thought that there are none but local 

 air currents in the valley. There is also a great general 

 movement, itself the resultant of all the lesser ones. How 

 it is brought about is not difficult to explain. As the 

 afternoon wears on and the lengthening shadows advance 

 over the landscape, the downward breezes progressively 

 gain in force, extinguishing one after another the upward 

 currents, until at last with the lowering of the sun they 

 become general over the entire surface of the cooling 

 land. Sliding down from every slope and cliff, they join 

 in the bottom of the valley, there to form a broad air- 

 stream or river that flows on toward the plains below. 

 Every side valley or cafion, moreover, sends its reinforce- 

 ments, for in every one of them the same thing is hap- 

 pening; and thus, with nightfall there is organized a 

 great system of confluent air-streams corresponding 

 closely to the valley system of the land. 



All night long this down-valley movement continues, 

 until at length the morning brings the warming sun again. 

 Then, as summit after summit, and slope after slope is 

 heated — insolated is the technical term — the warm up- 

 drafts are revived again. At first feeble and in spots only, 



