With the development of meteorological observation the term monsoon is applied to a wind regime above a level of 20 km. where there is a regular reversal of wind about the time of the Equinox. At these levels the winds are westerly in winter and easterly in summer and they appear to have the same consistency as the Indian monsoon.
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Following aspects of monsoon rainfall will help in understanding the monsoons in a better way:
(i) The Onset and Advance of the Monsoon:
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It is believed by many that main mechanism of monsoon is the differential heating of land and sea in the summer season. The landmass in the north-west India gets intensely heated during summer causing a very low pressure area there. The high pressure develops on Indian Ocean due to comparative coolness.
This causes a northward shift in the position of the Intertropical convergence zone (ITCZ) and the South-West trade winds cross the equator and after deflection these winds enter India as South-West monsoon winds. The shift in the position of the ITCZ is also related to the withdrawal of the westerly jet stream from the south of the Himalayan range.
(ii) Mechanism of Monsoon:
The easterly jet stream which helps in the onset of monsoon is caused by the summer heating of the Himalayan and the Tibetan highlands. As these highlands are heated in summer due to northward shift of the sun a clockwise circulation of the wind takes place in the middle of the troposphere resulting in the birth of two main streams of air from this landmass. One of these streams goes towards the equator. The equator ward flow enters India as easterly jet stream helping in the onset of the monsoon.
(iii) Rain Bearing ‘System of Monsoon’:
The rainfall from the monsoon never takes place in continuous form. Monsoon rain comes as wet spells followed by dry spells. The northern plain of India gets rainfall from tropical depression formed in the Bay of Bengal, while Arabian Sea current causes rainfall on the west coast of India. The frequency and the track of Bay of Bengal depressions depend upon the position of ITCZ which is known as monsoon trough.
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A change in the position of ITCZ causes a change in the axis of the tropical depression which brings about a change in the intensity and distribution of rainfall. There is the decrease in the amount of rainfall on the west coast of India from west to east-north east and over the Indian plain and peninsula from east-south-east towards north-west.
(iv) Break in the Monsoon:
This refers to the failure of monsoon in bringing rainfall for two or more weeks and resulting in a dry-spell in the rainy season. Such a kind of break is caused either by failure of the tropical depression or by dislocation of the monsoon trough (ITCZ) over north- India. A dry spell is experienced on the west coast due to the parallel movement of the monsoon winds. The inversion of temperature prevents rain-bearing winds from rising and resulting in rainfall in Rajasthan.
(v) The Retreat of the Monsoons:
By about the second or third week of September, the southwest monsoon dies out due to the southward migration of the sun. By 15th September it retreats from most parts of Punjab, Haryana, Rajasthan and Gujarat. It retreats from most parts of India by mid-October except the southern peninsula. The rainy area is therefore at this time limited to the east coast of Tamil Nadu and the south of the peninsula where the rainiest period is between October and November.
By the beginning of December the low pressure area moves further south and by its end it passes out of the Bay limit into the equatorial belt, thus the change that begins in October is usually completed by mid-December. The retreating monsoon picks up moisture from the Bay of Bengal and establishes itself over Tamil Nadu coast in the month of December and causes rainfall in Tamil Nadu.
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The time of advance and retreat of monsoon wind is different in different parts of India. It reaches north-western part of the India last of all but retreats from this part first of all. In Delhi, Punjab, Haryana the monsoon reaches on 1 July and retreats from here by mid-September giving them just two and half months of rainy season.
In opposite to it the monsoon reaches the Coromandel coast in the beginning of June and retreat in mid December giving them the duration of monsoon rain for six and half months.
(vi) Significance of Monsoon Rainfall:
Life in India is primarily based on agriculture which is dependent for the very existence on the south-west monsoon. The impact of monsoons on Indian economy is illustrated in the sentence that “the budget is largely a gamble on rain”.
The unifying influence of the amount of rainfall strongly determines the cropping pattern. Areas with over 200 cm of rainfall produce rice, sugarcane, coffee, rubber etc. The areas securing 100 cm to 200 cm rainfall rice predominates. In areas between 50 cm to 100 cm rainfall dry zone crops maize, barley, millets, rice and wheat can be grown with the help of irrigation. In areas without rainfall below 50 cm agriculture is almost impossible without irrigation.