MONSOON, originally the name given by the Arabs to those seasonal winds of the Arabian sea which blow for approximately six months from the north-east and six months from the south west. The name (Arabic Mausim or Mawsim, season), has been less correctly extended to seasonal winds in other parts of the world, e.g., lower Mississippi States of North America; northern Australia ; eastern central Africa. In India the term "monsoon" is specially used for the rain which falls during June to September with the onset of the south-west winds, hence a good monsoon means sufficient rainfall as contrasted with a deficiency for a bad monsoon. The total amount of rainfall bears little definite relation to the strength of the winds. The exact causes of the Asiatic mon soons cannot be conveniently summarized. The usual explanations are that in summer there is a high temperature and consequent low barometric pressure over the land with an indraft of sea air towards the interior, where it forms, or is formed by, great ascend ing air currents. G. C. Simpson points out that certainly as far
as India is concerned the land is much hotter in May before the monsoon sets in than in July when it is at its height ; again, the hottest part of India—the north-west—receives no rain during the wet monsoon and statistics prove that the average temperature is much greater in the years of bad monsoons than in the years of good monsoons.
The primary cause is certainly the difference of temperature over land and sea, and the south-west wind is a diverted south east wind from the south of the Indian ocean which after some 4,000 miles journey is highly charged with water vapour and on reaching the north of India is caught in the box-like arrangement of the mountains. It is forced to rise and heavy rain results. For complete details of the Indian monsoon see G. C. Simpson "The South-West Monsoon." Q.J.R. Met. Soc. (1921).