Southwest monsoon

“ During the northern summer, southern Asia, being under the rays of the vertical sun, becomes intensely heated; and the cooler and denser air of the adjacent ocean, and of southern Africa, flows towards it, producing the southwest monsoon, which lasts from April or May to September or October. ”

“ In India the prediction of great droughts has long been held to be one of the most important questions that can be attacked by the weather bureau of that country, and eminent men have worked upon it for twenty years past. The progress of their studies has gradually led us to see that the moist air of the southwest monsoon from which the rain falls has come from an unexpectedly great distance, namely, from the southern Indian Ocean. As you see on this globe before you, during the Asiatic winter season we have northeast trade winds here, and southeast trade winds there, south of the equator ”

“ These winds are most apt to occur during the months of August and September. There is, therefore, this remarkable difference between these gales and those of the East Indies: the latter occur about the changing of the monsoons, the former during their height. In August and September, the southwest monsoons of Africa (§ 479) and the southeast monsoons of the West Indies (§ 474) are at their height; the agent of one drawing the northeast trade winds from the Atlantic into the interior of New Mexico and Texas, the agent of the other drawing them into the interior of Africa. ”

“ There are narrow monsoon belts in the Atlantic, along{36} the coast of Africa and of Brazil, also on the Pacific coasts of North and South America; but the phenomena they exhibit are of a much less striking character. On the African coast, in general, the winds blow from sea to land in summer, from land to sea in winter; on the Brazilian, the wind is from the northeast in summer, while in winter the southeast trade resumes its sway. The monsoons of the Pacific coast of America blow from the northwest and north during the southern summer; from the southwest and south during the northern. ”

“ While, as stated, the northeast winds blow during the periods mentioned, they have the southwest monsoons, which blow from April to October. As these violent winds are the most tempestuous during the period when the sun crosses the equator, it has been argued that it is due to the action of the sun being in such a position that its rays strike the earth in the center of its rotation, thus heating up the air and causing it to rise rapidly along the middle belt. ”

“ The north-east and south-east monsoons, the former on the north and the latter on the south side of the equator, may be considered as trade winds, explicable upon the same principles, but counteracted for a certain time by causes which produce winds from a different quarter, the south-west and north-west monsoons. It has been observed that the south-west monsoon, which prevails to the north of the equator, is coincident with the sun being vertical to that region, when Hindûstan, Siam, and the adjacent countries receive their maximum of heat. ”

“ The south-east trade-winds, arriving at the equator during this period, and finding no north-east trades there to contest their crossing the line, continue their course, and blow home as a south-west monsoon, where they deposit their moisture and ascend. These southwardly monsoons bring the rains which divide the seasons in these parts of the African coast. The region of the ocean embraced by these monsoons is cuneiform in its shape, having its base resting upon Africa, and its apex stretching over till within 10° or 15° of the mouth of the Amazon. ”

“ It is not till the S.W. monsoons have been extended far out to sea that they commence to blow strongly, or that the rainy season begins in India. By this time the mean daily barometric pressure in this place of ascending air, which is also a calm place, has become less than it is in the equatorial calm belt; and the air which the S.E. trade-winds then bring to the equator, instead of rising up there in the calm belt, pass over without stopping, and flows onward to the calms of Central Asia as the S.W. monsoon. It is drawn over to supply the place of rarefaction over the interior of India. ”

“ In winter, the north-east monsoon, the normal cold-season outflow from Asia combined with the north-east trade, and generally cool and dry, covers the same district, extending as far north as latitude 30°. Crossing the equator, these winds reach northern Australia and the western islands of the South Pacific as a north-west rainy monsoon, while this region in the opposite season has the normal south-east trade. Other monsoons are found in the Gulf of Guinea and in equatorial Africa. Wherever they occur, they control the seasonal changes. ”

“ Here the deserts often reach the sea, as on the western coasts of South America, Africa and Australia. Cold ocean currents, with prevailing winds along-shore rather than on-shore, are here hostile to rainfall, although the lower air is often damp, and fog and cloud are not uncommon.
Monsoon Climate.—Exceptions to the general rule of rainier eastern coasts in trade-wind latitudes are found in the monsoon regions, as in India, for example, where the western coast of the peninsula is abundantly watered by the wet south-west monsoon. ”

“ The winter monsoon is off-shore and the summer monsoon on-shore under typical conditions, as in India. But exceptional cases are found where the opposite is true. In higher latitudes the seasonal changes of the winds, although not truly monsoonal, involve differences in temperature and in other climatic elements. The only well-developed monsoons on the coast of the continents of higher latitudes are those of eastern Asia. ”

“ But, again, the belt of rains is so far from being a belt of calms strictly, that its monsoons in the Indian, Atlantic, and Pacific Oceans, at times, extend hundreds of miles out over the ocean. That of the Atlantic, triangular, with its base resting on Africa, according to Lieutenant Maury, extends sometimes almost to the coast of South America, a distance of one thousand miles, and thus under the supposed ascending vortex. Where is the great uprising suction during the prevalence of this extensive surface horizontal monsoon beneath it? Manifestly it does not exist. ”

“ Then the props and stays are taken away from the air, and the wind seems ready to rush violently hither and thither, and whenever there is from any cause a momentary disturbance of the equilibrium. In such an atmosphere, the latent heat that is liberated by every heavy rain-shower has power to brew a storm. Throughout the monsoon region, the people know beforehand, almost to a day, the coming of this interregnum, which they call the changing of the monsoons, for the annual changing at the same place is very regular. ”

“ Some of the scientific men say that the north-east monsoon is not a monsoon at all, but only the north-east trade-wind taking its regular course, which has been disturbed by the more powerful wind from the south-west. ”

“ For six months at a time, that is from April to October, scarcely any rain ever falls, and in a country such as Java, notorious for its unhealthy climate and damp, unwholesome heat, the commencement of the wet monsoon is always a welcome period. ”

“ The climate is the pleasantest during the north-east monsoon, which lasts from October to April. It is during the south-west monsoon that the heavier rains, accompanied by electrical disturbances, occur. ”

“ Where, on the other hand, the land slopes upwards gradually to a central massif or ridge the effect of the differences of temperature is, as it were, cumulative, and the monsoons may extend over large areas, affecting regions distant from those in which the causes producing them are directly operative, and the monsoon winds may develop great strength. ”

“ Small wonder then that the advent of the creative monsoon is more heartily welcomed in India than is spring in England. No sound is more pleasing to the human ear than the drumming of the first monsoon rain.
But alas! the physical relief brought by the monsoon is only temporary. The temperature rises the moment the rain ceases to fall, and the prolonged breaks in the rains that occur every year render the last state of the climate worse than the first. ”

“ It is thought to be a general rule, that a monsoon blowing directly in from the sea, produces rain, and from off the land, fine weather, with sea and land breezes; this I found exemplified on the west side of the Gulph of Carpentaria, where the rainy north-west monsoon, which then came off the land, brought fine weather: the rain came with eastern winds, which set in occasionally and blew strong for two or three days together. It seems even possible; that what may be the dry season on the North Coast in general, may be the most rainy on the west side of the Gulph ”