MONSOON' (from Fr. monson, moncon, mous son, it. monsoe, Sp. momidn, l'ort. ?not/cif°, from :\lalay r 1 8 season, year, monsoon, from Hind. mausitn, from Ar. inausim, monsoon, from ma sama, to mark). In general. any wind or sys tem of winds that ehatnges regularly with the months or seasons. This term was brought to England from the East Indies by the Portu guese, Spanish, and Italian navigators. Its use in English first occurs in llakluyt's Voyages. In India. Siam, and the East Indies there is a very regular change of the winds with the seasons. They blow from southwest or south front April to October, and from northeast or north from Oc tober to April. The existence of these winds in India was first made known to the Europeans by the expeditions of Alexander the Great. :Modern knowledge of the corresponding winds in Siam and the Philippines dates from the mid dle of the sixteenth century: similar regular seasonal changes in the wind direction charac terize many portions of the globe, so that Aus tralia, Texas, Brazil, Africa. and Europe have prevailing winds at each season of the year: yet the contrast is nowhere so strongly marked as on the south and southeast coasts of Asia and the neighboring islands. During the winter sea son the cool air from the interior of the conti nent, flowing outward and keeping near the ground, becomes a north or northeast wind as it flows southward over the China Sea into the bay of Ibmgal and into the Arabian Sea. This wind even passes beyond the equator to latitude 10° 5., by which time it has been deflected into a .vest wind and flows eastward over the north western coast of Australia. :south of this zone of north winds are the southeast trade winds of the Southern Hemisphere. By reason of the change front winter to summer the Asiatic con tinent becomes heated; consequently the north east monsotm eelptPs and a strong indraught takes place. and eventually. in July and August, the greater part of the air over the south Indian Ocean 1.1,1 IS to t his indraught, so that from latitude 25' s. to latitude 5° 5. ;t strong south east trade wind prevails. latitudes 5° south and 5° north this southeast trade crosses the equator as a southerly wind. and turning toward the right beeomes the southwest 111011410H wind of India. Siam. and the adjneent seas. was for a long time considered 410111,1 fill the of the Southern Hemisphere could thus cross the emintor and enter the Northern llend sphvre, hut the monsoon (harts 1 t h,hished Ilaily for many N.ears by the tiovernment of India leave ui no room to doubt this remarkable .1118112O. in • 1,1 of the southeast of t he southern °Veil ucenr violent typhoons, which move from the neighborhood of Java and Northern Australia westward and away the equator, turning in their course before they reach Madaga-ear and move south eastward until they are lost. These typhoons
generally develop in the South Indian Ocean when the northeast monsoon is at its maximum in India.
The southwest monsoon is usually accompanied by rain in port inn, of India and in the adjacent East Indies. The northeast monsoon bring- rain to the west coast of the Bay of Bengal. In gen eral, the locations of the rain areas vary with the direction of the wind in accordance with the rule that a wind that is forced to ascend over a hilly coast brings rain to it. Through this inter change between the of northeast and southwest monsoons, all of India has an oppor tunity of being well watered and of raising an nual crops of grain. Nevertheless it occasionally happens that the southwest monsoon fails to bring much rain; this may indeed happen for several successive years (as in 1595 and 1596. and again in 1599). lw reason of which distressing famines and great hiss of life are caused. Since ISSO the Government meteorologists of India, II. F. Blanford and John Eliot, have devoted a great deal of attention to methods of predicting the probable character of the monsoon rains. These predictions are generally issued in the month of April and relate to the coming mouths of July and August. A remarkably large percentage of these predieti011A have been successful, but the failure of the forecasts for 1599 indicated that abnormal conditions prevailed in some distant region, and has greatly stimulated the study of the relation between the Indian monsoon awl the condition of the atmosphere over the whole globe.
The general statement of the conditions that bring about monsoons is discussed at length by Prof. William Ferrel in Treatise on the Winds (New York, 1589). Ile has emphasized the im portant rule that monsoons are stronger in pro portion as the heated interior land surface is elevated above sea-level. on the coast of Ja maica, West Indies, the diurnal sea breeze ie., remarkably strong, owing to the steep gradient of the land as it aseends from sea-level to the tops of mountains. In India the southwest mon soon develops on a grand scale because of the average elevation of the Ilintalayass which stretch east and west for 1300 mile- at an average alti rude of 18.1100 feet. and also because of the mountains and plateaus behind the Himalayas in the interior of Asia. The monsoon, like the daily bind and sea breeze. depends fur its inten sity' ultimately on the heat produced by solar radiation. Any change in the radiation will pro dtme corresponding etteets on the monsoon. E. D. Archibald. in Au/tiro (.lone 22, 1S93. London), has maintained that there are systematic mon soon variations parallel to the variations of the spots on the sun. But these changes are barely appreciable. and further investigatimi may molli fy hi.: result-.