Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/756

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690
[climate
ASIA

velocity of about 1002 miles per hour, while the air arriving from the equator (supposing it not to have been affected by friction) would be moving from west to east 35 miles per hour faster than the surface, and would there fore be felt as a wind having that velocity from the west. In fact, however, the motion from the south would be combined with that from the west, and the air would blow as a south-west wind ; while the friction against the earth's surface would gradually check the excess of velocity toward the east, and no such great westerly velocity as that named would be developed. In a corresponding manner, air impelled from places situated on a higher latitude towards those on a lower, will be felt as wind having an easterly component. The south-westerly winds, which prevail north of the equator during the hot half of the year, to which navigators have given the name of the S.W. monsoon (the latter word being a corruption of the Indian name for season), arise, in the manner just explained, from the great diminution of atmospheric pressure over Asia, which begins to be strongly marked with the great rise of temperature in April and May, and the simultaneous relative higher pressure over the equator and the regions south of it. This diminution of pressure, which continues as the heat increases till it reaches its maximum in July soon after the solstice, is followed by the corresponding develop ment of the S.W. monsoon; and as the barometric pressure is gradually restored, and becomes equalised within the tropics soon after the equinox in October, with the general fall of temperature north of the equator, the south-west winds fall off, and are succeeded by a N.E. monsoon, Avhich is developed during the winter months by the relative greater atmospheric pressure which then occurs over Asia,

as compared to the equatorial region.

58. Although the succession of the periodical winds follows the progress of the seasons as just described, the changes in the wind s direction everywhere take place under the operation of special local influences which often dis guise the more general law, and make it difficult to trace. Thus the S.W. monsoon begins in the Arabian Sea with west and north-westerly winds, which draw round as the year advances to south-west, and fall back again in the autumn by north-west to north. In the Bay of Bengal the strength of the S.W. monsoon is rather from the south and south-east, being succeeded by north-east winds after October, which give place to northerly and north westerly winds as the year advances. Among the islands of the Malay Archipelago the force of the monsoons is much interrupted, and the position of this region on the equator otherwise modifies the directions of the prevailing winds. The southerly summer winds of the Asiatic seas between the equator and the tropic do not extend to the coasts of Java, and the south-easterly trade winds are there developed in the usual manner. The China Sea is fully exposed to both monsoons, the normal directions of which nearly coincide with the centre of the channel between the continent of Asia and the eastern islands.

59. The south-west monsoon does not generally extend, in its character of a south-west wind, over the land. The current of air flowing in from over the sea is gradually diverted towards the area of least pressure, and at the same time is dissipated and loses much of its original force. The winds which pass northward over India blow as south-easterly and easterly winds over the north eastern part of the Gangetic plain, and as south winds up the Indus. They seem almost entirely to have exhausted their northward velocity by the time they have reached the northern extremity of the great Indian plain; they are not felt on the table-lands of Afghanistan, and hardly penetrate into the ranges of the Himalaya, by which mountains, and those which branch off from them into the Malay peninsula, they are prevented from continuing their progress in the direction originally imparted to them.

60. Among the more remarkable phenomena of the hotter seas of Asia must be noticed the revolving storms or cyclones, which are of frequent occurrence in the hot months in the Indian Ocean and China Sea, in which last they are known under the name of typhoon. The cyclones of the Bay of Bengal appear to originate over the Andaman and Nicobar islands, and are commonly propagated in a north-westward direction, striking the east coast of the Indian peninsula at various points, and then often advancing with an easterly tendency over the land, and passing with extreme violence across the delta of the Ganges. They occur in all the hot months, from June to October, and more rarely in November, and appear to be originated by adverse currents from the north meeting those of the S.W. monsoon. The cyclones of the China Sea also occur in the hot months China, of the year, but they advance from N.E. to S.W., though occasionally from E. to W. ; they originate near the island of Formosa, and extend to about the 1 Oth degree of N. lat. They are thus developed in nearly the same latitudes and in the same months as those of the Indian Sea, though their progress is in a different direction. In both cases, however, the storms appear to advance towards the area of greatest heat. In these storms the wind invariably circu lates from N. by W. through S. to E.

61. In the cyclones observed in the Southern Indian Ocean off the coast of Madagascar the wind circulates in the opposite direction. These storms advance from N.E. to S.W., with a tendency to turn off to the S.E. as they die out. They occur between the months of December and April, and between the 10th degree of S. lat. and the southern tropic.

62. In all these cases the cyclones occur during the hot months of the year, when strong winds are developed by the proximity of large heated areas of land and relatively cool areas of sea, and when the air, being highly charged with vapour, is liable to great disturbances of temperature on any considerable condensation being set up. More over, they most frequently happen at the times when the direction of the dominant winds is changing, and when important variations of atmospheric pressure are certainly taking place. Actual barometric observations have not yet been obtained in sufficient number or continuity to establish the precise conditions under which these storms arise, but there is no reason to doubt the correctness of the general views held regarding them, or that with the progress of knowledge much may be done to enable mariners to avoid their worst consequences.

63. The heated body of air carried from the Indian Ocean over Southern Asia by the S.W. monsoon comes up highly charged with watery vapour, and hence in a condition to release a large body of water as rain upon the land, whenever it is brought into circumstances which reduce its temperature in a notable degree. Such a reduction of temperature is brought about along the greater part of the coasts of India and of the Burmo-Siamese peninsula by the interruption of the progress of the wind current by continuous ranges of mountains, which force the mass of air to rise over them, whereby the air being rarefied, its specific capacity for heat is increased and its temperature falls, with a corresponding condensation of the vapour originally held in suspension.

64. This explanation of the principal efficient cause of the summer rains of South Asia is immediately based on an analysis of the complicated phenomena actually observed,

and it serves to account for many apparent anomalies. The heaviest falls of rain occur along lines of mountain of some extent directly facing the vapour-bearing winds, as

on the Western Ghats of India and the west coast of the