A glance at a map will show that the northern half of the great Indian Ocean is enclosed by land masses as no other large body of water is. Consequently, while in the southern section the southeast trade is present, the northeast trade of the northern part is so modified by the surrounding land areas as to almost entirely lose its distinctive character. Hence, most tropical regions have, properly speaking, but two seasons: the rainy, and the dry. As the clouds swept in meet with an intensely heated region, the trade never chills them sufficiently to produce snow, except in extremely elevated regions.
This is the direct cause of the monsoons: 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. The time of its beginning and its close varies in different latitudes, according to the time at which the sun is vertical in each.
During the southern summer, southern Africa being under the vertical sun and intensely heated, the cooler air of the surrounding seas, and of southern Asia, flows towards it. This produces the northeast monsoon, which lasts from October or November to April. This monsoon is, in fact, only the regular northeast trade-wind somewhat intensified.
A similar exchange takes place between Asia and Australia, but it is less marked, owing, perhaps, to the great islands lying between these continents.
The period of transition of the monsoons, in spring and autumn, is marked by sudden and violent gales, and terrific thunder storms. Destructive hurricanes, also, are of frequent occurrence. This corresponds with the period of equinoctial storms in higher latitudes.
There are narrow monsoon belts in the Atlantic, along 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. The regular trade-wind makes itself so strongly felt in northern Brazil, which is unusually level, that a boat can sail almost as rapidly up the swift current of the Amazon as it can row down: and Humboldt records that he found it of great strength at the foot of the eastern slope of the Andes.