_Heat the Cause of Winds._—As already stated, the primary cause of all winds is heat, substantially on the principle according to which draught is created in our domestic fires. The hot air rising creates an indraught from all directions, especially from that which it can most readily come; viz., from the ocean,[101] or from level lands, rather than across mountain chains. Hence the sea-breeze in the after part of the day, when the land has become heated; while the sea, requiring a much larger amount of heat to change its temperature to a similar extent, remains relatively cool. But at night the earth cools more rapidly than the sea, by radiation; hence toward evening the sea-breeze dies down, and toward and after sunset the land-breeze takes its place. [101] A striking case in point is the regular wind which in summer blows through the “Golden Gate,” a gap in the Coast Range connecting the Pacific Ocean directly with the great interior valley of California, along the bays of San Francisco, San Pablo, and Suisun. The great interior valley and adjacent mountain slopes becoming intensely heated during the rainless summer, the ascending air is replaced by a steady indraught from the sea, which is bordered by a belt of cold water causing fogs along the coast. The fogs are quickly dispelled on reaching the edge of the valley near the middle of its length; whence steady breezes blow northward and southward, up the valleys of the Sacramento and San Joaquin respectively. These winds, popularly often, but erroneously, called trade-winds, are really “monsoons” similar in their origin to those of India, which, when coming from the sea cause rains, but when from the heated land itself are hot and dry; as in the case of the sirocco of Italy and North Africa, the terral of Spain and the northers of California.