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The Southwest Monsoon made its onset over Kerala on Saturday (May 24), eight days ahead of its typical onset date of June 1, according to the Indian Meteorological Department (IMD).
This marks its earliest ever onset since 2009, when it made its entry on May 23, 2009. The state has witnessed a significant increase in rainfall over the past week.
WHAT IS THE MONSOON?
While colloquially understood as the rainy season in the subcontinent, the IMD defines the monsoon as the seasonal reversal of winds and associated rainfall.
The term is derived from the Arabic word ‘mausim’, meaning season.
WHAT CAUSES THE MONSOON?
According to the IMD, the seasonal reversal of winds is a function of the annual fluctuations in the Sun’s apparent position between the Tropics of Cancer and Capricorn.
This causes the thermal equator, the region which receives the maximum heat from the Sun and is distinct from the geographical equator, to oscillate.
Given that different regions of the Earth, as well as its land and seas, receive varying degrees of the Sun’s heat, the intensity of the oscillation of the thermal equator may vary, affecting other variables like temperature, pressure, wind, rain etc. Thus, different regions experience a single monsoon differently.
SO, WHAT IS THE SOUTHWEST MONSOON?
The southwest monsoon refers to the southwesterly wind that brings rainfall to the subcontinent, typically from June to September. The onset of the 2025 monsoon on Saturday marks the commencement of the season, which brings about 70% of the country’s annual rainfall.
HOW IS THIS DIFFERENT FROM THE NORTHEAST MONSOON?
The northeast monsoon, which typically occurs between October and December, begins after the southwest monsoon has withdrawn from northern and central India, as well as the northern parts of the peninsular region. The wind pattern changes from the southwesterly to the northeasterly, hence its name.
According to the IMD, the Northeast Monsoon is the major period of rainfall in the south peninsula, and is the chief rainy season in Tamil Nadu, accounting for nearly 60 % of annual rainfall in the coastal districts.
DOES AN EARLY ONSET IMPLY A BETTER RAIN FORECAST?
No, there is no connection between the arrival date of a monsoon and its progress.
A delay of a few days or an early arrival of the monsoon has no bearing on the quality or amount of rainfall, or its regional distribution across the country, during the four-month monsoon season.
In a recent year, the onset of the monsoon occurred two days in advance of the normal date, and it rained heavily for about 10 days after that — however, the season as a whole ended with 14% less rain than normal.
