Various questions are raised about the efficacy of southwest monsoon as the originator of precipitation. These questions do not support the classical theory of the origin of Indian monsoon.
There is no doubt that the southwesterly monsoon currents which enter the sub-continent after travelling thousands of kilometers over warm tropical oceans are capable of yielding much precipitation.
But the fact remains that the monsoon current themselves are not originators of precipitation. If it were not so, there was every possibility of continuous rainfall during the rainy season. As a matter of fact, both the time and amount of monsoonal precipitation are characterized by a higher degree of uncertainty.
Besides this, the annual total differs widely from year to year and from place to place. In addition, other rainfall characteristics, such as its seasonal and diurnal distribution, intensity, duration and frequency of rainy days also exhibit spatial as well as temporal variations.
Thus, rainfall which is of great importance for tropical agriculture is a highly variable climatic element within the tropics in general and the Indian sub-continent in particular.
During the rainy season, there are occasional breaks in the monsoon, when for several days together not a single drop of rain falls on the ground. There are periods of prolonged draughts leading to famine conditions over a large part of the sub- continent.
On the other hand, there are spells of heavy downpours that bring about unprecedented floods in various rivers. These floods are equally harmful to our national economy. Therefore scarcity of rainfall or its excess is the two salient features of monsoonal precipitation in India.
Even the arrival of monsoon is not certain. Sometimes the onset of southwest monsoon is delayed by a few weeks, whereas at times the summer monsoon sets in much before its expected time. Both of these elements are equally damaging.
From the foregoing discussion it will be clear that even though the southwest monsoon is full of immense potentiality for heavy precipitation, some other mechanisms of precipitation are required for its occurrence.
Flohn has also expressed the view that the monsoon itself does not yield precipitation; rather it needs certain other favourable atmospheric conditions for the rains to occur. This is borne out by the fact that when the southwest monsoon currents are most vigorous except on the wind-ward slope of mountain barriers lying in their paths, precipitation is every where scarce.
Thus, it is clear that the most southwesterly winds over the Indian subcontinent are necessary but not a sufficient condition for summer rainfall. The major contribution of summer monsoon is that it fills the lower atmosphere over India with great quantities of humidity.
For precipitation, monsoon depressions play the most important role. These depressions increase convergence in the monsoon current and its speed. They enhance rainfall through dynamic and mechanical lifting.
In addition to monsoon depressions, there are other atmospheric disturbances in the form of waves or cyclones that bring about spells of clouds and showers during the southwest monsoon period.