What Monsoon ‘Onset’ Means
What Monsoon ‘Onset’ Means, Why It Is Early – Explained for CLAT 2026
Introduction: Why in News?
- The Southwest monsoon arrived in Kerala on May 24, 2025, which is eight days earlier than its usual date of June 1.
- This was the earliest monsoon onset since 2009.
- This early onset was officially declared by the India Meteorological Department (IMD) and has drawn attention from climate scientists, agriculture experts, and policymakers alike.
- Important for CLAT Current Affairs 2026 and environment law and climate governance topics.
Point-Wise Summary
- How IMD Declares Monsoon Onset
- IMD uses 3 criteria:
- Rainfall: At least 60% of 14 core stations in Kerala, Karnataka, and Lakshadweep must receive ≥2.5 mm rain for 2 days.
- Wind Field: Westerly winds with speeds between 27–37 kmph must persist in the lower troposphere.
- Heat/Ocean Parameters: Outgoing Longwave Radiation (OLR) < 200 W/m² over the region must be observed.
- Why Did the Monsoon Arrive Early in 2025?
- Sea Surface Temperature (SST): Higher-than-normal SSTs in Arabian Sea and Bay of Bengal.
- Madden-Julian Oscillation (MJO): Active MJO phase enhanced moisture-laden convection.
- Low-Pressure System: A depression in Arabian Sea accelerated monsoon winds.
- Somali Jet: A low-level wind from west of Indian Ocean (Mauritius & Madagascar) intensified and helped monsoon surge.
- Does Early Onset Mean More Rainfall?
- No. Early arrival does not predict total rainfall.
- Early or delayed onset may only affect distribution, not overall quantity.
- Is Early Onset Unusual?
- Not uncommon. Previous early arrivals:
- 2024: May 30
- 2017: May 30
- 2009: May 23 (record early)
- Variations occur due to dynamic atmospheric-oceanic conditions.
- Monsoon Progress So Far
- As of May 26, the monsoon had covered:
- Kerala, Tamil Nadu, Tripura, Manipur, Nagaland, parts of Maharashtra, Karnataka, Andhra Pradesh, and Goa.
Explanation of Peculiar Terms (CLAT-Friendly Notes)
- Monsoon Onset: The formal beginning of monsoon, declared by IMD when specific rainfall, wind, and radiation conditions are met.
- Madden-Julian Oscillation (MJO): A tropical disturbance that enhances rainfall when active over Indian Ocean.
- Outgoing Longwave Radiation (OLR): A measure of heat emitted from Earth; low OLR means high cloudiness and likelihood of rain.
- Somali Jet: A fast, moisture-rich wind stream that helps initiate monsoon onset over the Indian subcontinent.
Relevance for CLAT 2026 Aspirants
- Environment & Climate Law: Monsoon pattern changes affect water law, crop insurance, and sustainable agriculture.
- General Knowledge: Understanding of IMD methods and monsoon dynamics can help answer CLAT GK and legal reasoning questions.
- Policy & Geography: Early monsoon onset influences government planning, drought prediction, and water sharing arrangements.
Timeline of Southwest Monsoon Onset Over Kerala (2009–2025)
Year | Date of Onset |
2025 | May 24 |
2024 | May 30 |
2023 | June 8 |
2022 | May 29 |
2019 | June 8 |
2018 | May 29 |
2016 | June 8 |
2013 | June 1 |
2010 | May 31 |
2009 | May 23 |
Conclusion
The 2025 early monsoon onset offers a timely reminder of the complexity of climate systems and the limitations of prediction models. While such early arrivals may seem promising, they don’t necessarily lead to surplus rainfall. CLAT 2026 aspirants must grasp the scientific, policy, and legal dimensions of such climate-related events as they increasingly influence India’s governance landscape.
Let this topic serve as a case study in environmental law, climate science, and data-based decision-making — all crucial to future legal professionals in a changing world.
