298. The rainy side of mountains.—Why there is more rain on one side of a mountain than on the other.—We may now, from what has been said, see why the Andes and all other mountains which lie athwart the course of the winds have a dry and a rainy side, and how the prevailing winds of the latitude determine which is the rainy and which the dry side. Thus, let us take the southern coast of Chili for illustration. In our summer-time, when the sun comes north, and drags after him the belts of perpetual winds and calms, that coast is left within the regions of the north-west winds—the winds that are counter to the south-east trades—which, cooled by the winter temperature of the highlands of Chili, deposit their moisture copiously. During the rest of the year, the most of Chili is in the region of the south-east trades, and the same causes which operate in California to prevent rain there, operate in Chili; only the dry season in one place is the rainy season of the other. Hence we see that the weather side of all such mountains as the Andes is the wet side, and the lee side the dry. The same phenomenon, from a like cause, is repeated in intertropical India, only in that country each side of the mountain is made alternately the wet and the dry side by a change in the prevailing direction of the wind. Plate VIII. shows India to be in one of the monsoon regions: it is the most famous of them all. From October to April the north-east trades prevail. They evaporate from the Bay of Bengal water enough to feed with rains, during this season, the western shores of this bay and the Ghauts range of mountains. This range holds the relation to these winds that the Andes of Peru (§ 297) hold to the south-east trades; it first cools and then relieves them of their moisture, and they tumble down on the western slopes of the (Ghauts, Peruvian-like, cool, rainless, and dry; wherefore that narrow strip of country between the Ghauts and the Arabian Sea would, like that in Peru between the Andes and the Pacific, remain without rain for ever, were it not for other agents which are at work about India and not about Peru. The work of the agents to which I allude is felt in the monsoons, and these prevail in India and not in Peru. After the north-east trades have blown out their season, which in India ends in April, the great arid plains of Central Asia, of Tartary, Tibet, and Mongolia become heated up; they rarefy the air of the north-east trades, and cause it to ascend. This rarefaction and ascent, by their demand for an indraught,
