The term Monsoon was first used in English in British India, which includes present day’s India, Bangladesh and Pakistan. The Southwest Monsoon winds are called 'Nairutya Maarut' in India.
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The reversal of winds start taking place over the Indian sub-continent during the summer season, when cold and dry continental northwesterly winds are replaced by warm and moist southwesterly winds. These winds originate in the Southern Hemisphere, cross the Equator (trade winds) and gets deflected to the right by the Coriolis force in the Northern Hemisphere.
The trade winds then approach the India sub-continent in two separate branches – the Bay of Bengal and the Arabian Sea.
The four-month long Monsoon season, which runs from June to September, brings 70% of the country's annual rainfall. It holds a Long Period Average (LPA) of 89 cm of rain but never behaves in a normal manner. In fact, rainfall distribution mostly remains uneven both in space and time. It always leaves one guessing and brings anxious moments, anticipatory worries and jitters.
Monsoon in India is controlled by two important features namely, the Monsoon trough and Monsoon systems.
What are Monsoon Troughs?
Monsoon trough is basically the meeting place of the Arabian Sea current and the deflected Bay of Bengal current. Wherever these two currents meet, a trough is created and oscillation of this trough brings rain.
The Monsoon trough is an imaginary line, spread across more than 2000 km, from Sri Ganganagar in Rajasthan to West Bengal. Width-wise the trough is just 100-150 km broad.
What are Monsoon Systems?
During the Monsoon season, low-pressure areas and depressions form on either side of the Indian Ocean – the Arabian Sea and the Bay of Bengal. However, these weather systems are more frequent in the Bay of Bengal. They are responsible for rainfall activity over the country during the Monsoon season. These systems lead to the commencement and further progress of Southwest Monsoon.
Commencement and Withdrawal of Monsoon in India
Southwest Monsoon in India is conceived as a complex phenomenon. A cautious approach is generally taken before announcing onset or withdrawal of Monsoon in India.
July and August are generally the active Monsoon months for the entire country. June is the onset month which witnesses an outburst of rain while, September is the withdrawal month, receiving more of sporadic rain.
Withdrawal is generally not attempted before September 1 and commences from extreme west Rajasthan. As Southwest Monsoon approaches the withdrawal phase from a particular region, certain conditions become prevalent. These include change of wind pattern, reduction in cloud cover, decrease in humidity, cessation of rain or sporadic rain, and temporary rising tendency of temperatures.
Before announcing the withdrawal of Monsoon from any particular place, a bigger picture is taken into consideration. For instance, commencement of withdrawal is not announced from the western parts of the state even if east Rajasthan receives some rain.
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