MONSOON (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 Italian ocean which blow from the s.w. from April to October, and from the opposite direction, or me., 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, Hippolus was emboldened to sail across the open sea to Muzeris, the emporium of Malabar. The monsoons Jepend, in common wit:4* winds, whither regular or irregitlat on the ineiinaIity of 'teat at dif ferent 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-Iva:sus) would blow constantly from the n.e. in the n., and from the s.c. in the s. 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 disturb ances for which those parts of the earth are so remarkable. During the summer half of: the year, the n. of Africa and the s. 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 s. flows in to supply its: place, a general movement of the atmosphere of the Indian ocean sets in towards the n:,. 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 disuse tam will be communicated to the wind; and the combination of these two directions= results in the s.w. monsoon, which prevails there in summer. Since, during wintery 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 s. and west. As: this is in the same direction as the ordinary trade-wind, the effect in winter is no to• change the direction. but only to increase the velocity of the trade-wind. - Thus, while• s. of the equator, owing to the absence of sufficiently large tracts of land, the s.e. trade winds prevail throughout the year; on the n. of the equator we find the s.w. mO;a00m
in summer, and the n.e. in winter; it being only in summer and n. 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 Gtaineat in Africa, and Mexico in America. The e. and w. direction of the shore s ssA theses countries, or the large heated surfaCes to the u. of the seas which wash their coasts; produce, precisely as in the case of South Asia, a s.w. monsoon in summer. As, might have been expected, the monsoon off the coast of Mozambique is easterly, and thattoff thy coast of West Australia north-westerly. The trade-winds also suffer consider ablemhange• in their direction on the coasts of Brazil, Peru, Lower Guinea, etc These, H loue•some times considered monsoons, arc not truly such, for they do not change the it diretfons: periodically, so as to be opposite to each other, like true monsoons, but only yeertbronglit a few points of the compass. For a fuller account of these partial de flectiims;, see TRADE-Wu:Ds.
In April, the n.e. monsoon changes into the s.w.; and in October, the s.ws.futsistlie. n.e. These times depending on the course of the sun, and consequently varyihg with the latitude, arc' called the breaking up of the monsoons, and are generally accompanied. by variable winds, by intervals of calm, and by furious tempests and hurric angst.
Monsoons, when compared with the trade-winds, will be found to play a rtnesV fiene ficial and important part in the economy of the globe. Their greater velo city, and- slue periodical changes which take place. in their direction, secure increased fat ility if eont nimba] intercourse between different countries. But the full benefits follol vingsinsabeiir train are not seen unless they be considered in their relation to the rainfall of sautherm Asia. Indeed, the fertility of the greater part of this fine region is entirely du to th a MEM 80011$ ; for if the n.e. trade-wind had prevailed there throughout the year, central+ an& western India, and many other places, would only have been scorched ' aria barren. saharas. The rain-fall of depends entirely on the monsoons. The coast of has its rainy season during the s.w. monsoon, which brings thither the vanom off the ocean. On the Coromandel coast, on the other hand, it is the n.e. monsoon the rain from the bay of Bengal. The two coasts of Hindustan have therefai• thew seasons reversed, the dry season of the one corresponding with the wet set,sohhta£ Hie other.