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Monsoon onset on June 1 has occurred six times in 125 years
| Photo Credit:
Thulasi Kakkat
There is a notion that Southwest monsoon usually sets in over Kerala on June 1. However, a look at monsoon onset dates over 125 years shows that the actual onset has happened either a day earlier or a day later. In fact, June 2 has been the most frequent onset date, occurring 10 times, followed by May 29, May 31 and June 3 with nine occurrences each. Monsoon onset over Kerala on June 1 has occurred six times in 125 years.
India welcomed the monsoon to Kerala on May 24 this year. But this is not the earliest onset. Monsoon onset over Kerala has occurred between May 11 and May 20 on eight occasions in this century-and-a-quarter under review. The earliest monsoon onset occurred on May 11 in 1918.
What about delayed onset? Monsoon onset has occurred between June 10 and June 17 on 18 occasions during the 125-year period under review. In 1972, monsoon onset happened as late as June 17.
In 125 years, the monsoon onset over Kerala has happened 51 times in May and 74 times in June.
Source data
Data for this story were sourced from a 2017 article that appeared in the Journal of Earth System Science. The article titled ‘Variability of the date of monsoon onset over Kerala (India) of the period 1870–2014 and its relation to sea surface temperature’ was authored by PN Preenu, PV Joseph and PK Dineshkumar. The article cited a research paper by Ananthakrishnan R and Soman MK for data from 1901-1970 period.
The objective dates of the monsoon onset over Kerala for 1971-2005 were taken from a paper by Pai DS and Rajeevan M, and for the later years till 2014 from the annual monsoon reports published by India Meteorological Department (IMD) in its journal Mausam. businessline used data available with IMD for the years between 2015 and 2025.
20th century pattern
An analysis of the onset dates over more than 12 decades shows a gradual shift in patterns from predominantly June onsets in the early part of 20th century to more frequent May onsets in the latter part of the century. However, monsoon onset returned to June in most years during the last two decades of the century.
The early part of the 20th century (1901-1940) saw Kerala receiving its first spell of monsoon rainfall in June. In fact, all onset dates in the first decade (1901-1910) were in June. In the next three decades, a majority of monsoon onsets over Kerala were in June.
A subtle transition to May onsets was observed during the period between 1941 and 1950, occurring five times during that decade.
Monsoon arrived in May in 14 out of 20 years between 1951 and 1970. These May onsets were scattered across the period.
A transition to June onsets was observed during the period between 1971 and 1980, occurring five times during that decade. The last two decades of the 20th century saw the monsoon arriving in June in 14 different years.
Pattern in new millenium
Most monsoon onsets over Kerala happened in May (six times) in the first decade of this century, and June (seven times) became the more frequent onset month in the second decade.
May monsoon onsets have happened in three out of five years during the period between 2021 and 2025.
Published on June 2, 2025
This article first appeared on The Hindu Business Line
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