One-fourth of India’s monsoon rain evaporates mid-air, says new study
Researchers at the Indian Institute of Tropical Meteorology (IITM), Pune, a research institute under the Ministry of Earth Sciences, have found that roughly a quarter of the mass of monsoon rainfall evaporates in mid-air before reaching the ground, during the June-to-September southwest monsoon season
The study used stable isotope analysis of paired rain and water-vapour samples collected simultaneously during a monsoon season at Pune, on the leeward side of the Western Ghats
The team applied a Below-Cloud Interaction Model (BCIM) to quantify how much of a raindrop's mass is lost to evaporation as it falls through the sub-cloud atmospheric layer
The evaporation fraction was found to vary widely — from about 4% to 61% on different days — averaging around 18-23%, and was found to be higher for smaller raindrops, at higher temperatures, and in lower relative humidity
The findings are intended to help refine numerical weather prediction and climate models used for monsoon forecasting, which typically treat below-cloud evaporation as a fixed or poorly constrained parameter
Southwest Monsoon Mechanism and Moisture Processes
The Indian southwest monsoon is a seasonal reversal of wind systems driven by differential heating of land and ocean, bringing moisture-laden winds from the Arabian Sea and Bay of Bengal onto the subcontinent between roughly June and September. Rainfall generation involves several stages — moisture transport, convective uplift and condensation to form raindrops within clouds, and then the drop's fall through the sub-cloud layer to the surface. Below-cloud evaporation — the focus of this new study — is a physical loss process at this last stage, distinct from the evapotranspiration and moisture-recycling processes that return surface moisture to the atmosphere to feed subsequent rainfall.
Key Details
- Southwest monsoon onset over Kerala typically occurs around June 1 (IMD's operational date), with a normal date range and withdrawal typically completing by mid-October
- Stable isotopes of oxygen (δ18O) and hydrogen (δD) in precipitation are widely used tracers of moisture source, transport history, and phase-change (evaporation/condensation) processes — a technique standard in isotope hydrology
- Below-cloud (sub-cloud) evaporation is distinct from moisture recycling via evapotranspiration, and from large-scale climate drivers of monsoon variability such as the El Nino-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD)
The IITM study isolates and quantifies one specific loss term — sub-cloud raindrop evaporation — in the journey of monsoon moisture from cloud to ground, showing it removes a substantial and variable share (about a fifth to a quarter on average) of the rain mass before it is recorded at the surface.
Institutional Framework for Monsoon Science in India
Monsoon research and forecasting in India is coordinated by institutions under the Ministry of Earth Sciences (MoES), principally the India Meteorological Department (IMD) for operational forecasting and warnings, and the Indian Institute of Tropical Meteorology (IITM), Pune, for monsoon process research and model development. IITM also anchors India's Monsoon Mission, aimed at improving dynamical prediction models for the Indian monsoon.
Key Details
- IITM, Pune functions under the Ministry of Earth Sciences and focuses on atmospheric and monsoon process research
- IMD, also under MoES, is the operational agency responsible for monsoon onset/withdrawal announcements and seasonal forecasts
- The Monsoon Mission is a MoES initiative to improve monsoon forecasting skill through better dynamical/numerical models
- Findings such as this study feed into refining the physics (microphysics parameterisation) used inside these numerical weather and climate models
This study exemplifies the process-level research IITM undertakes (as distinct from IMD's operational forecasting role), providing empirical evaporation-fraction data that can be used to correct a known source of error in rainfall estimates within weather and climate models.
Rain Gauge Measurement and Model Validation
Surface rainfall is conventionally measured using rain gauges and, at larger scales, weather radar and satellite rainfall estimates. Because a meaningful fraction of rain mass evaporates before reaching the surface, the amount measured at ground level can differ from the amount actually generated within clouds — a discrepancy that models must account for when back-calculating cloud-level precipitation processes or validating simulated rainfall against observations.
Key Details
- Below-cloud evaporation fraction found to be inversely related to raindrop size and relative humidity, and positively related to temperature
- Smaller drops (associated with lower-intensity rainfall) lose proportionally more mass to evaporation than larger drops in heavy rainfall
- Such quantification supports better microphysics parameterisation in General Circulation Models (GCMs) and Numerical Weather Prediction (NWP) systems used for Indian monsoon forecasting
By quantifying how much rain "disappears" between cloud base and the ground gauge, the study gives modellers a correction factor to make simulated and satellite-derived rainfall estimates more consistent with what clouds actually produce, improving monsoon forecast accuracy.
- Average below-cloud evaporation fraction found: roughly 18-23% of rain mass (reported in news coverage as "about a quarter" or 25%); day-to-day range observed: approximately 4% to 61%
- Study site: Pune (18.53 degrees N, 73.85 degrees E), on the leeward side of the Western Ghats
- Method: stable isotope analysis (δ18O, δD) of paired rain and vapour samples with a Below-Cloud Interaction Model (BCIM)
- Conducted by IITM Pune, under the Ministry of Earth Sciences; published in the journal Atmospheric Chemistry and Physics in 2026
- Southwest monsoon season studied: June to September