An IIT Madras-led team says the monsoon’s real arrival over peninsular India this year came on 22 June, not with the first showers.
The India Meteorological Department declared the southwest monsoon over Kerala on 4 June 2026, three days after the normal date of 1 June. That same day IMD placed the monsoon’s northern limit through Udhagamandalam, Kodaikanal and Thoothukudi, and expected it to cover the rest of Tamil Nadu within two or three days.
Researchers at IIT Madras and Gauhati University count differently. By their method, the large-scale onset over the Indian peninsula this year came on 22 June, followed by heavy rain in Mumbai by 24 June and in Delhi by 1 July. The study, published in the Journal of Climate of the American Meteorological Society, was announced in Chennai on Tuesday.
Rain at one place is not the monsoon
The method treats the country as a network. Each location is a point, and points are linked when rain sets in at them; the monsoon is taken to have arrived at scale only when local rainfall joins the largest connected cluster, marked by a sudden jump in that cluster’s size and then its spread northward. Prof R I Sujith of IIT Madras said the first rainfall at a place does not necessarily mark the monsoon’s arrival, since pre-monsoon thunderstorms can come well before the sustained system, and that the approach could help avoid false signals.
The team used 84 years of rainfall records. Tamil Nadu sits inside that peninsula.
On average, they found large-scale onset over Northeast India around 15 May and over the peninsula around 9 June. An earlier preprint by the same five authors argued that the monsoon begins in Northeast India and spreads westward and northward, against the conventional view that it starts in the south. Prof B N Goswami of Gauhati University said the method is meant to complement, not replace, the operational definitions forecasters use.
The IIT Madras authors, Yogenraj Patil, Gaurav Chopra, Shruti Tandon and Sujith, work at the institute’s Centre of Excellence for Studying Critical Transitions in Complex Systems.
Checked against official sources on 11 November 2026.
Sources
- Patil, Chopra, Tandon, Goswami and Sujith, ‘Climatic Phase Transitions Unravel the Onset and Withdrawal of Indian Monsoon’, arXiv:2503.15148
- India Meteorological Department, Onset of Southwest Monsoon over Kerala, 4 June 2026
- Statement of IIT Madras and Gauhati University on monsoon onset study, 10 November 2026
