False monsoon onset can mislead farmers: IIT Madras develops new method to track rainfall

Researchers at the Indian Institute of Technology (IIT) Madras and Gauhati University, Assam, have developed a new method to identify the actual arrival of the Indian monsoon by examining how rainfall patterns connect and spread across the country.

The approach aims to prevent isolated rain spells from being mistaken for the onset of the monsoon.

The climate-network-based method examines the relationship between rainfall activity at different geographical locations. It identifies when scattered episodes of local rainfall become part of a larger, interconnected system, indicating the establishment of the large-scale monsoon.

According to IIT Madras, the research offers an objective way to determine the large-scale onset of the monsoon at individual locations. Instead of considering local rainfall alone, the methodology follows the formation, expansion and merging of rainfall clusters across India.

The researchers said the approach could work alongside existing operational definitions of monsoon onset. More reliable identification of the monsoon's arrival could help farmers determine sowing schedules and assist authorities with water resource management, food security planning and flood preparedness.

How the climate network tracks monsoon arrival

The study uses a network-based framework in which geographical locations are represented as nodes. Links between these nodes indicate spatial connections between places experiencing local rainfall onset.

Researchers examined how these connections develop over time, tracking the formation and movement of rainfall clusters. The central idea is to identify when smaller, geographically separated clusters combine to form a much larger system associated with the arrival of the monsoon.

Prof RI Sujith, Head, Critical Transitions in Complex Systems (CoE-CTCS), IIT Madras, highlighted the importance of distinguishing genuine monsoon onset from temporary rainfall activity.

He said that "by avoiding false onsets, our new perspective of defining large-scale onset of monsoon at every location will allow farmers time for sowing and help the policymakers to plan for food security and water resource management."

Prof BN Goswami of the ST Radar Centre, Gauhati University, Assam, clarified that the proposed framework is not intended to discard existing methods of determining monsoon onset.

He added that "this method is intended as a complement to conventional operational definitions, rather than as a replacement for them. Its distinctive feature is that it connects a local observation to the larger-scale organization of the monsoon, allowing the same framework to identify large-scale onset at different locations."

How rainfall clusters develop over Northeast India and Kerala

The researchers used the development of rainfall clusters over Northeast India to explain how their methodology works. During April, smaller clusters of rainfall onset begin appearing in the region, including those triggered by thunderstorm activity.

These clusters expand as the monsoon season advances. The rainfall cluster over Northeast India eventually connects with another cluster developing around the Bay of Bengal.

The resulting merger produces a sudden increase in the size of the largest connected cluster. The researchers use this abrupt expansion to identify the large-scale monsoon onset over Northeast India.

A similar pattern helps determine the monsoon's arrival over the Indian peninsula, although the process involves an additional distinction between local rainfall and large-scale monsoon activity.

A smaller rainfall cluster initially forms over Kerala because of local precipitation. However, the occurrence of rain alone does not establish that the larger monsoon system has reached the region.

The researchers identify the large-scale onset only when the Kerala cluster connects with the biggest rainfall-onset cluster advancing westward from Northeast India.

This connection indicates that rainfall over Kerala has become integrated with the wider monsoon system. The approach can therefore distinguish an isolated rain spell in Kerala from the arrival of the larger monsoon system.

What 84 years of rainfall data reveal about monsoon onset

To establish the typical timing of large-scale monsoon arrival, the researchers examined 84 years of rainfall data.

Their analysis indicates that the large-scale onset generally takes place around May 15 over Northeast India and June 9 over the Indian peninsula.

The transition is characterized by a sudden expansion of the largest connected rainfall cluster. Its size continues to increase as the system progresses spatially and moves northward across the country.

The study also identified a relationship between the large-scale onset over the Indian peninsula and the subsequent movement of monsoon rainfall.

Researchers found that the onset is followed by a strong northward progression of rainfall linked to the first pulse of the monsoon intraseasonal oscillation (MISO).

This progression provides another physical indicator that the identified transition represents the establishment of the larger monsoon system, rather than a temporary episode of rainfall.

IIT Madras researchers track 2026 monsoon using new method

The research team also tested its definition by monitoring the 2026 monsoon season. Its analysis placed the large-scale onset over the Indian peninsula on June 22, 2026.

Rainfall subsequently progressed northward, with heavy rainfall episodes reported in Mumbai by June 24 and in Delhi by July 1.

The researchers said the ability to distinguish local rainfall from the establishment of the larger monsoon system could make onset identification more useful for practical planning.

For farmers, the distinction is particularly relevant because an early spell of rain does not necessarily indicate that conditions are suitable for sowing. A more reliable assessment of monsoon arrival could help reduce uncertainty in agricultural decisions.

The approach could also provide useful information for policymakers responsible for food security, water resource management and flood preparedness.