MONSOON
MONSOO’N (Malayan, Musim) is derived from the Arabic word Mausim, a set time or season of the year, and is applied to those winds prevailing in the Indian Ocean which blow from the southwest from April to October, and from the opposite direction, or north-east, from October to April. The existence of these winds was made known to the Greeks during the Indian expeditions of Alexander, and by this knowledge, Hippalus was emboldened to sail across the open sea to Muzeris, the emporium of Malabar. The monsoons depend, in common with all winds whether regular or irregular, on the inequality of heat at different places and the earth’s rotation on its axis; but more particularly they are occasioned by the same circumstances which produce the trade-winds and the land and sea breezes, being, in fact, the combined effect of these two sets of causes.
If the equatorial regions of the earth were entirely covered with water, the trade-winds (see TRADE-WINDS)would blow constantly from the north-east in the north, and from the south-east in the south of the torrid zone, with a belt of variable winds and calms interposed; the whole system, following the sun’s course, moving northward from December to June, and southward from June to December. But, especially in the eastern hemisphere, large tracts of land stretch into the tropics, and give rise to the extensive atmospheric disturbances for which those parts of the earth are so remarkable. During the summer half of the year, the north of Africa and the south of Asia are heated to a higher degree than the Indian Ocean, while Australia and South Africa are much colder. As the heated air of Southern Asia expands and rises, and the colder air from the south flows in to supply its
place, a general movement of the atmosphere of the Indian Ocean; sets in towards the north, thus giving a southerly direction to the wind; but as the air comes from those parts of the globe which revolve quicker to those which revolve more slowly, an easterly direction will be communicated to the wind; and the combination of these two directions results in the south-west monsoon, which prevails there in summer.
Since, during winter, South Asia is colder than the Indian Ocean, which, again, in its turn, is colder than South Africa, a general motion of the atmosphere sets in towards the south and west. As this is in the same direction as the ordinary trade-wind,. the effect in winter is not to change the direction, but only to increase the velocity of the trade-wind. Thus, while south of the equator, owing to the absence of sufficiently large tracts of land, the south-east trade-winds prevail throughout the year; on the-north of the equator we find the south-vest monsoon in summer; and the north-east in winter; it being only in summer and north of the equator that great changes are effected in the direction of the trade-wind.
Similar, though less strongly-marked monsoons prevail off the-coasts of Upper Guinea in Africa, and Mexico in America. The east and west direction of the shores of these countries, or the large heated surfaces to the north of the seas which wash their coasts, produce, precisely as in the case of South Asia, a southwest monsoon in summer. As might have been expected, the monsoon off the coast of Mozambique is easterly, and that off the coast of West Australia north-westerly. The trade-winds also suffer considerable change in their direction on the coasts of Brazil, Peru, Lower Guinea, &c. These, though sometimes considered monsoons, are not truly such, for they do not change their directions periodically, so as to be opposite to each other, like true monsoons, but only veer through a few points of the compass. For a fuller account of these partial deflections, see TRADE-WINDS.
In April, the north-east monsoon changes into the south-west; and in October, the south-west into the north-east. These times depending on the course of the sun, and consequently varying" with the latitude, are called the breaking up of the monsoons, "and are generally accompanied by variable winds, by intervals of calm, and by furious tempests and hurricanes.
Monsoons, when compared with the trade-winds, will be found to play a most beneficial and important part in the economy of the-globe. Their greater velocity, and the periodical changes which take place in their direction, secure increased facility of commercial intercourse between different countries. But the full benefits following in their train are not seen unless they be considered in their relation to the rainfall of Southern Asia. Indeed, the fertility of the greater part of this fine region is entirely due to the monsoons; for if the north-east trade-wind had prevailed there throughout the year, Central and Western India, and many other places, would only have been scorched and barren saharas. The rainfall of India depends entirely on the monsoons. The coast of Malabar has its rainy season during the south-west monsoon,. which brings thither the vapors of the ocean. On the Coromandel coast, on the other hand, it is the north-east monsoon which-brings the rain from the Bay of Bengal. The two coasts of Hindustan have therefore their seasons reversed, the dry season of the one corresponding with the wet season of the other.