Every monsoon season, India waits. Farmers plan, reservoirs prepare, and meteorologists watch — all calibrated to what finally falls. But a new study from the Indian Institute of Tropical Meteorology (IITM), Pune, reveals a stubborn subtraction happening in plain sight: a significant share of the rain that forms in clouds simply evaporates on its way down, before it ever touches soil.
What the Falling Drop Loses Mid-Air
Researchers at IITM Pune analysed stable isotopes of hydrogen and oxygen in rain and vapour samples collected simultaneously from Pune during the 2019 summer monsoon. The technique works because evaporating raindrops leave a chemical fingerprint — lighter isotopes escape first, shifting the ratio in a measurable way.
What they found is striking. Raindrop evaporation fraction estimated from the model yields daily-scale values varying from 4% to 61%, averaging around 23% — roughly one in four drops never completes its journey. The range matters too: on dry, hot days, more than half the falling rain mass can vanish before it registers anywhere.
This evaporation influences the heat budget and affects monsoon convection, and the study also shows that drop evaporation reduces the rainfall amount considerably, especially in the lower range of precipitation. In other words, on lighter rain days — the kind that dominate the shoulder months of June and September — the loss is proportionally worse.
Raindrop evaporation is an important sub-cloud process that modifies rainfall amounts, and earlier studies have shown that various general circulation models do not incorporate this process properly. A precise quantification of raindrop evaporation is therefore required for validation of the models used routinely for monsoon rainfall predictions.
A Monsoon Already Under Stress
This finding lands in a context that is already unsettling. Over the last 50 years, there has been a decrease in mean monsoon rainfall over India, associated with the lowering of land-ocean thermal contrast — partly because the western Indian Ocean warmed rapidly while aerosol pollution cooled the Indian landmass.
The change isn't simply "less rain." Extreme rain events have increased quite significantly, specifically over the core monsoon zone. Observational evidence shows a decreasing trend in summer mean rainfall over northern India post-1950, alongside a threefold rise in extreme precipitation over central India. The monsoon, in short, is becoming more violent and less reliable at the same time.
A Wetter But Wilder Future
Looking ahead, the picture is paradoxical. With continued global warming and expected reductions of aerosol concentrations in the future, climate models project an increase in annual and summer monsoon mean rainfall, as well as the frequency of heavy rain events over most parts of India during the 21st century.
Projections predict an increase of about 6% under a moderate emissions scenario and 8% under a high-emissions scenario up to 2050 over India's core monsoon region, with the projected increase by end of century reaching approximately 10% to 14%, depending on the scenario.
More total rain, but arriving in fiercer bursts — with a larger share quietly lost in the air between cloud and earth. Understanding that airborne loss isn't just atmospheric trivia. It shapes how heat moves in the atmosphere, how clouds form next, and how much water actually reaches a field or a reservoir. The IITM study adds one more reason why India's monsoon is harder to model than it looks — and why getting that model right has never mattered more.
Sources
- ACP - Assessing raindrop evaporation over northern Western Ghats from stable isotope signature of rain and vapour
- Quantifying Raindrop Evaporation Deficit in General Circulation Models from Observed and Model Rain Isotope Ratios on the West Coast of India
- Indian Summer Monsoon Rainfall in a changing climate: a review | Journal of Water and Climate Change | IWA Publishing
- Precipitation Changes in India | Springer Nature Link
- How is Climate Change Altering India’s Monsoon Rainfall Patterns?