The air over the tropical regions becomes heated and ascends; it then diverges in two high currents, one towards the north, and the other towards the south pole, where, being cooled, it again descends, and returns towards the equator to replace the air as it ascends therefrom. There is, therefore, a constant revolution of vast currents of air between the tropics and the poles, producing north and south winds.
670. Why do the trade winds blow from east to west, though, in their origin, their direction is from north to south and from south to north?
Because, as the north and south winds blow towards the equator, they are affected by the revolution of the earth from west to east. As the two winds from the poles approach the equator, they are gradually diverted from their northerly and southerly course, to an easterly direction, by the revolution of the earth.
671. Why is there a prevalence of calms at the equator?
Because, as the north and the south winds move towards the equator, they drive before them volumes of atmosphere, which, meeting in opposite directions, resist and counterpoise each other, and abide in a state of stillness between the north and south-easterly winds, one on the north and the other on the south of the equator.
Monsoons are periodical winds which blow at a given period of the year from one quarter of the compass, and in another period of the year from the opposite quarter of the compass.
673. What is the cause of monsoons?
Monsoons are caused by changes in the position of the sun. When the sun is in the southern hemisphere, it produces a north-east wind, and when it is in the northern hemisphere, a north-west wind. The north-east monsoon blows from November to March, and the south-west monsoon from the end of April to the middle of October. The region of monsoons lies a little to the north of the northern border of the trade wind, and they blow with the greatest force, and with most regularity, between the eastern coast of Africa and Hindustan.