One quarter of India's monsoon rains evaporate into the air, says a new study

Researchers at IIITM, Pune, have calculated that, on average, about 25% of rainfall evaporates; the actual amount varies from day to day and ranges from 4% to 61% during the four monsoon months from June to September. | Photo credit: Murali Kumar K

Nearly a quarter of the rain that falls on the Northwest Ghats during the southwest monsoon evaporates in the air, according to a study by the Indian Institute of Tropical Meteorology (IITM) in Pune.

This is the first time this fraction has been measured through an experiment in India. This will help scientists improve weather and climate models and better understand the monsoon, according to an accompanying press release.

“This is the first observational estimate of raindrop evaporation over the Western Ghats and the technique can be used across India,” corresponding author of the study Saikat Sengupta told The Hindu in a phone interview. The work was published in the peer-reviewed journal Atmospheric Chemistry and Physics.

The Pune result is the first step in a wider effort to map the process across India, where Mr Sengupta expects it to change sharply with temperature and humidity, from arid Rajasthan to the rain-soaked coast. IITM already operates a rainwater isotope network of nine sites — from the Himalayas to the Northeast and Port Blair — where sampling has been ongoing for a decade.

The decisive measure

Evaporation rate measurement is not just an academic exercise. As a raindrop evaporates on its way down, it absorbs heat from the surrounding air and cools the subcloud layer, driving downdrafts and creating cold pools of air at the surface. This reshapes the convection itself, which generates another burst of rain. It’s a process that climate and monsoon models have long struggled to capture. If it gets it wrong, it skews their precipitation forecast and the associated potential for cooling the atmosphere and triggering storms.

A rain weight loss of about a quarter is elsewhere at the low end of global estimates. Measurements based on satellite data put evaporation in the tropics at nearly 20%; over Zurich it was about 40% and near Barbados about 60%. This is due to the smaller drops in Barbados and the drier air; evaporation eliminates the small drops of light rain and barely touches the large drops of an intense downpour. In the case of Zurich, turning off evaporation in the model increased precipitation by about 75%, a measure of how sharply cooling chokes convection.

To calculate the rate of evaporation, Mr. Sengupta and his colleagues used the fact that most substances have different combinations of isotopic atoms. For example, most water is plain H2O, but a small fraction carries a “heavier isotope” (more neutrons), heavy oxygen or heavy hydrogen. These heavier molecules are slightly slower, so when the droplet evaporates, the lighter molecules preferentially escape, leaving the surviving droplet enriched in heavy isotopes. Rain that evaporates less retains a lighter signature; in contrast, rain that has evaporated more carries a heavier composition.

During the 2019 monsoon, the team collected rainwater and atmospheric vapor at ground level in Pune, read their isotope ratios on a laser spectrometer, and fed the results into a one-dimensional subcloud interaction model that tracks a single droplet falling from cloud base to the ground. Collecting the vapor is challenging—each sample took six to seven hours to capture by freezing atmospheric moisture. The group began acquiring portable analyzers that read vapor isotopes in real time and deploying them around the country.

Mr Sengupta stressed that the results point to a way to improve the way weather and climate models represent precipitation. Representation in modeling means how faithfully the equations within the model reproduce the actual physical process; a model must correctly capture a process before its predictions of the effects of that process can be trusted.

According to him, all the work was done internally. “All the observational and modeling work at IITM was done by PhD students, postdocs and project collaborators”.

Mr. Sengupta noted that quantitative estimates of evaporation are rare for India. Even non-isotopic ones are hard to find in the literature because the amounts needed to measure evaporation in air are not easy to measure, so they use isotopes as tracers to fingerprint the process, he said.

Published – 11 Jul 2026 22:49 IST