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 When the fog belt hangs high over the hills it is the season of dryness. When the fog belt thickens and extends from sea level to the summit of the hills rain may fall. The fog bank is thickest from June to September, and in that period the sun may be hidden for weeks at a time except for occasional glimpses through the fog or at sunset when it peeps from beneath the cloud cover before disappearing below the horizon.

There is a somewhat sympathctic relation between the dry- ness of a place and the duration of fog. The rainfall of the coast of southern Peru diminishes southward in a general way, and the driest part of the coast of Chile is from Arica to Cal- dera. Thence southward there is a slight but distinct increase in the rainfall. The southern end of the desert to a marked degree has rains which follow upon great atmospheric dis- turbances in the cordillera. South of Coquimbo this is par- ticularly the case, and in that direction the fog bank on the coast diminishes in thickness, being of consequence only in the winter. The coast is here hidden by mist rather than the characteristic and pronounced fog of more northerly situa- tions. From southern Peru 900 miles southward to the end of the Desert of Atacama in 32° S. the fog bank of the coast has little effect upon vegetation in spite of the greater height of the Coast Range. This condition of coastal dryness corre- sponds with the extreme aridity of the desert that lies between the coast ranges and the cordillera and is explained not by the relations of coastal scarp to cold sea, as on pages 51 and 52, but rather by the height and breadth of the mountain zone east of the nitrate desert and the general system of winds and rains that affect all places in this latitude. It must not be thought that this terrestrial wind system has anything more than a general expression at any given point on the earth's surface. ‘The weather from day to day is the effect of local causes or agencies—a mountain, a regional wind from this or that quarter, a fog bank or the absence of it, a cloud belt. The variations in these things affect the hourly and daily changes of weather in a given place, but their range in turn is determined by their situation with respect to the great belts of wind and rain that in a general way control the weather