188 WILLIAM S. COOPER 



water; and the ocean fogs, which cover parts of the mountains 

 on a large majority of summer days, do this effectively, as has 

 been pointed out by several writers. Fogs also add a certain 

 amount to the soil water supply, as Cannon has noted. ^ The 

 ground under a redwood tree during a fog is often decidedly wet, 

 but whether this water is abundant enough or penetrates deeply 

 enough to be of much consequence is doubtful. At any rate 

 the decrease of evaporation is of very great importance. 



Light might be thrown upon the problem if data could be 

 obtained concerning the extent of the region which is ordinarily 

 covered by the fog blanket. I can give here some general 

 observations which are helpful. From Palo Alto the east face 

 of the Santa Cruz Mountains, extending from a point about 

 west of Redwood City southeastward to the mass of Black Moun- 

 tain, is in full view. The northwestern half is clothed with red- 

 woods; the Black Mountain mass has none. On a majority of 

 days during the summer a fog blanket may be seen covering the 

 northwestern end of the range, sometimes of greater, sometimes 

 of less extent, but centering in the region of King's Mountain, 

 where the redwood forest is heaviest. Frequently it pours over 

 the edge and descends the eastern slope. Redwoods are fre- 

 quent upon this slope and are not entirely confined to the immedi- 

 ate vicinity of streams. Less frequently the fog is seen to spread 

 southward along the summit ridge, but very rarely indeed does 

 it reach Black Mountain. The explanation probably is that 

 in the region of King's Mountain the range is relatively narrow 

 and the fog attains the eastern slopes; west of Black Mountain 

 the mountain region is much broader, and the fog blanket, if 

 of the same dimensions as farther north, stops short of the 

 eastern edge of the mountains, possibly near Butano Ridge. 

 The fog-frequented area around King's Mountain includes the 

 four stations which I established in the redwood forest region; 

 the fogless drea of Black Mountain and its environs includes 

 all of my four stations located in country devoid of redwood - 

 forest; and the latter region has slightly the higher rainfall of 

 the two. It is therefore apparent that the first condition neces- 



1 Cannon, W. A. On the relation of redwoods and fog to the general precipita- 

 tion in the redwood belt of California. Torreya 1 : 137-139. 1901. 



