One-fourth of India’s monsoon rain evaporates mid-air, says new study

1. At a Glance

• IITM Pune researchers produced India's first observational estimate of raindrop evaporation using stable isotope signatures of rain and vapour over the northern Western Ghats [S1][S4].
• On average, ~25% (nearly a quarter) of monsoon rain mass evaporates mid-air before reaching the ground; daily variation ranges from 4% to 61% across the four monsoon months (June–September) [S4].
• Findings will help refine weather and climate models, which currently poorly parameterize raindrop evaporation (a "sub-cloud process") [S1][S3].
• Relevant for UPSC as an example of Indian institutional climate science (IITM, Ministry of Earth Sciences) contributing to monsoon dynamics understanding — a recurring Prelims/Mains theme (monsoon mechanics, climate modelling, MoES agencies).

2. Why in the News

• Study published in the peer-reviewed journal Atmospheric Chemistry and Physics (ACP) in 2026, titled "Assessing raindrop evaporation over northern Western Ghats from stable isotope signature of rain and vapour," authored by Nimya, Rajaveni, Saikat Sengupta, Bhattacharya, and Ananthavel [S1].
• Reported by The Hindu (Jacob Koshy) on 12 July 2026, based on an IITM press statement and author interview [S4].

3. Background & Evolution

• Raindrop evaporation is a known but poorly quantified sub-cloud process that alters both rainfall amount reaching the surface and the isotopic composition of rainwater; prior Global Circulation Models (GCMs) have shown an "evaporation deficit" — i.e., they under-represent this process [S3].
• IITM Pune's Centre for Climate Change Research (CCCR) has run earlier related work: raindrop size distribution (DSD) studies over the Western Ghats (2019–2021) estimating ~23% evaporation in the humid Pune leeward region during the 2019 monsoon [S2].
• IITM operates a rainwater-isotope network of nine sites across India, feeding into a broader effort (e.g., a dataset of "Three Years of Stable Water Isotope Data" from three geomorphic regions of India) [S4][per ACP/Nature-linked isotope studies].
• The current 2026 ACP study is the first experimentally-measured (observational) estimate specific to the Western Ghats, as opposed to earlier model-inferred or DSD-based estimates.

4. Core Static Facts

5. Multi-Dimensional Analysis

Scientific/Technological - First-of-its-kind observational (not merely modelled) measurement of raindrop evaporation in India, using isotope tracing rather than indirect proxies [S1][S4]. - Technique is scalable — the corresponding author states it "can be used over the whole of India," enabling future spatial mapping from arid Rajasthan to the humid coast [S4].

Environmental/Climatic - Evaporation loss directly affects effective rainfall reaching the ground, impacting water availability estimates, hydrology, and agriculture planning. - Better quantification narrows the "evaporation deficit" in GCMs, improving monsoon and climate-change projection accuracy [S3].

Administrative/Governance - Demonstrates capacity-building within a MoES-funded domestic research institute (IITM) rather than reliance on foreign climate models — relevant to India's push for indigenous climate science capacity.

Geographic/Regional - Focus on the Western Ghats, a UNESCO World Heritage biodiversity hotspot and critical orographic rainfall zone for the southwest monsoon — ties into Physical Geography (monsoon mechanics, orographic rainfall).

6. Recent Developments (last 12–18 months)

• 2026: ACP publication of the northern Western Ghats raindrop evaporation study (Nimya et al., incl. Sengupta) [S1].
• 12 July 2026: The Hindu carries the report summarizing findings and quoting Sengupta on plans to extend the technique pan-India [S4].
• Continued expansion of IITM's isotope-network dataset collection across geomorphic regions of India (arid, coastal, humid) feeding into evaporation studies [S4].

7. Prelims Hooks

• Study conducted by the Indian Institute of Tropical Meteorology (IITM), Pune, under the Ministry of Earth Sciences [S1][S4].
• Average monsoon raindrop evaporation over northern Western Ghats: ~25% of rain mass [S4].
• Daily evaporation range: 4% to 61% during June–September monsoon [S4].
• This is India's first observational (experimental) estimate of raindrop evaporation [S4].
• Study published in journal Atmospheric Chemistry and Physics (ACP) [S1].
• Corresponding author: Saikat Sengupta [S4].
• Method used: stable isotope signatures of rain and vapour [S1].
• IITM maintains a rainwater-isotope network of 9 sites across India [S4].
• Earlier (2019) raindrop-size-distribution-based estimate for Pune region: ~23% evaporation [S2].
• Raindrop evaporation is classified as a sub-cloud process [S3].
• The technique is proposed to be extended to map evaporation variability across India, from Rajasthan (arid) to the west coast (humid) [S4].
• IITM Pune is located on the leeward side of the Western Ghats [S2].
• GCMs (General Circulation Models) reportedly show an "evaporation deficit" — under-representing raindrop evaporation [S3].

8. Mains Relevance

• GS-I: Geography — Indian monsoon mechanism, orographic rainfall, Western Ghats.
• GS-III: Science & Technology — indigenous research; Environment — climate modelling, water resource assessment, hydrological cycle.
• Possible question stems: 1. "Discuss the significance of sub-cloud processes like raindrop evaporation in refining climate and monsoon models. Illustrate with recent Indian research." (GS-III) 2. "Examine the role of institutions like IITM Pune in strengthening India's indigenous climate science capacity." (GS-III) 3. "Explain the orographic influence of the Western Ghats on the southwest monsoon, and discuss factors affecting the amount of rainfall actually reaching the surface." (GS-I)

9. Related Topics to Study Next

• Southwest Monsoon mechanics & orographic rainfall — core physical geography linkage to Western Ghats rainfall patterns.
• IITM Pune & Ministry of Earth Sciences institutional structure — recurring Prelims factoid on MoES bodies (IMD, NCMRWF, INCOIS, NCPOR, IITM).
• Climate model parameterization / GCMs — relevant to broader climate change assessment (IPCC linkages).
• Stable isotope hydrology — technique applicability across hydrology, paleoclimate, groundwater studies.
• Western Ghats as UNESCO World Heritage Site — biodiversity and ecological angle (Kasturirangan/Gadgil committee reports).
• India Meteorological Department (IMD) monsoon forecasting — contrast IMD's forecasting role vs IITM's research role.
• Water security & hydrological cycle — implications of evaporation losses for water budgeting in river basins.

10. Common Errors / Trap Areas

• Confusing IITM Pune (research institute under MoES) with IMD (India Meteorological Department, the operational forecasting agency) — different mandates.
• Assuming the study covers all of India — it currently covers only the northern Western Ghats; pan-India mapping is a stated future goal, not a completed fact.
• Misremembering the evaporation figure as a fixed 25% — the actual finding shows a wide daily range (4%–61%), with 25% being only the average.
• Attributing the study to a government scheme/policy — it is an independent peer-reviewed scientific study, not a policy announcement.
• Confusing this 2026 isotope-based study with the earlier (2019) raindrop-size-distribution-based ~23% estimate — they use different methodologies.

11. Sources

• [S1] ACP - Assessing raindrop evaporation over northern Western Ghats from stable isotope signature of rain and vapour — https://acp.copernicus.org/articles/26/9061/2026/ — (tier: 3)
• [S2] IITM/raindrop size distribution research context (via search aggregation, ScienceDirect/ACP related studies on Western Ghats DSD) — https://acp.copernicus.org/articles/21/4741/2021/ — (tier: 3)
• [S3] Quantifying Raindrop Evaporation Deficit in General Circulation Models — MDPI, Atmosphere journal — https://www.mdpi.com/2073-4433/14/7/1147 — (tier: 3)
• [S4] The Hindu, "One-fourth of India's monsoon rain evaporates mid-air, says new study," Jacob Koshy, 12 July 2026 — https://www.thehindu.com/todays-paper/2026-07-12/th_chennai/articleGIDG85QJV-15376052.ece — (tier: 4)