Originally the term ‘monsoon’ was used to signify the surface winds of Southern Asia with a complete reversal of wind direction between winter and summer. Now, the term is applied to a whole series of different phenomena, e.g., the stratospheric monsoon, the European monsoon, the Malayan monsoon, etc.
Chang-Chia-Ch’eng gives the following definition: “monsoon is a flow pattern of the general atmospheric circulation over a wide geographical area, in which there is a clearly dominant wind in one direction in every part of the region concerned, but in which this prevailing direction of wind is reversed (or almost reversed) from winter to summer and from summer to winter”.
According to Pierre Pedelaborde, the term ‘monsoon’ in singular form is sometimes applied to planetary winds which affect upper layers of the atmosphere over the whole globe. Sometimes it is used for regional winds blowing in the lower layers of the atmosphere over a well defined territory.
The word ‘monsoon’ may also be applied to a regional surface wind system which reverses its direction with the changing season. Indian monsoon is the typical example. There are some authors according to whom the word ‘monsoon’ should be used for south-west winds of the summer which blow in the Indian sub-continent or West Africa.
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Some authors prefer the term ‘Indian monsoons’ which include both ‘winter monsoon’ as well as ‘summer monsoon’. However, so far as the system of winds over the whole of Monsoon Asia is concerned, it consists of three distinct regional systems: the Indian Monsoon, the Japanese monsoon and the Malayan monsoon.
According to Nieuwolt, “the word monsoon is used only for wind systems where the seasonal reversal is pronounced and exceeds a minimum number of degrees.”
He applies the term to systems of wind which show at least 120 degrees of change of wind direction with a change of the season, and which are characterized by constancy higher than 40 percent and a mean resultant speed of more than 3 meters/second. Such monsoons occupy a very large area situated mainly in the tropics.