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| Moonson rain |
No single element plays a more decisive role in the life of the far-flung realm of tropical Asia than the rain. On the plains of India, in parts of Burma, Thailand and Indonesia, where it is dry and dusty for half the year, men, plants and animals build their lives around the expectation of the seasonal rains— and when they come, they come on walls of towering clouds and change the landscape from seared brown to verdant green, from parched river bed to raging torrent. In other parts of the region, from the Malay Península southward and southeastward through the great are of the islands, steady rain throughout most of the year has shaped flora and fauna up to the highest mountain ridges. The ultimate determinants of this type of climate which is so characteristic of the Oriental region are winds—the monsoon winds, whose name derives from the Arabic word
mausim, or "season." There is a regularity to these winds which has long fascinated meteorologists, and even today the challenging question of just why they blow at the times and in the manner that they do has not been answered to the entire satisfaction of the scientific mind. Basically, they arise from low pressure areas created by the heat of the sun in tropical regions. Where pressures are low, air from surrounding areas moves in—and these movements are felt tangibly as winds. In the heated, low pressure areas, the air rises, flowing outward at the apex of its rise, north and south in the respective hemispheres, toward the poles.
In Southeast Asia, this "weather machine" is peculiarly affected by the differential heating between land mass and ocean. Asia is the largest of the world's land masses, and in summer the sun, blazing down on its arid interior, warms the earth to such torrid extremes that the equatorial low pressure system moves northward toward the Tropic of Cancer. This now becomes the dominant low for the season, drawing in increasing air masses from surrounding regions— and the air that pours in from the south is saturated with moisture drawn up from the southern seas. All along the island are and over the drought-parched Indian plains this moisture-laden air, moving toward the heart of Asia, raleases its burden as rain.
