25% of monsoon rain vanishes mid-air over Western Ghats, IITM study finds
A new study by the Indian Institute of Tropical Meteorology (IITM) in Pune reveals that nearly a quarter of the rain falling over the north Western Ghats during the Southwest monsoon evaporates before reaching the ground. This is the first time such a measurement has been made through direct observation in India, offering scientists a valuable tool to refine weather and climate models.
The study, published in the peer-reviewed journal Atmospheric Chemistry and Physics, found that on average about 25% of the rain mass evaporates, though the fraction varies widely from 4% to 61% over the four monsoon months of June to September. Lead author Saikat Sengupta stated, “This is the first observational estimate of raindrop evaporation over the Western Ghats, and the technique can be used over the whole of India.”
The research team used isotopic analysis to measure evaporation. When raindrops fall, lighter water molecules evaporate preferentially, leaving the remaining drops enriched in heavier isotopes. By collecting rainwater and atmospheric vapour at ground level in Pune during the 2019 monsoon and reading their isotope ratios with a laser spectrometer, the scientists were able to feed the data into a below-cloud interaction model that tracks a single drop’s journey from cloud base to ground.
The evaporation of raindrops mid-air is not merely an academic curiosity. As drops evaporate, they absorb heat from the surrounding air, cooling the sub-cloud layer, feeding downdrafts, and creating cold pools at the surface. This process reshapes convection that generates subsequent rainfall, yet it has been difficult for climate models to capture accurately. Errors in representing evaporation can skew rainfall predictions and associated atmospheric cooling.
The measured 25% loss sits at the lower end of global estimates. Satellite-based data show evaporation near 20% in the tropics, about 40% over Zurich, and roughly 60% near Barbados. The variation stems from differences in drop size and humidity: smaller drops and drier air lead to more evaporation, while large drops in intense downpours are barely affected. In the Zurich case, turning off evaporation in a model boosted rainfall by about 75%, illustrating how strongly this cooling effect throttles convection.
The IITM team now plans to extend the work across India using portable analysers that read vapour isotopes in real time. Sengupta noted that the results point to a way to improve how weather and climate models represent rainfall, a crucial step for better monsoon predictions. “All observation and modelling work was carried out at the IITM,” he said, emphasising the in-house nature of the research.
