A new IITM study reveals that 25% of India's monsoon rain evaporates mid-air, aiding climate model accuracy.
New Delhi, India Jul 12, 2026 ALN: Nearly a quarter of the rain that falls over the north Western Ghats during the Southwest monsoon evaporates mid-air, according to a study conducted by researchers at the Indian Institute of Tropical Meteorology (IITM) in Pune. This groundbreaking research marks the first time that such a fraction has been measured through experimental methods in India, which is significant for understanding the dynamics of the monsoon, a critical weather phenomenon for the country.
The findings from the IITM indicate that on average, about 25% of the rain mass evaporates before it reaches the ground. However, this percentage is not constant; it varies from day to day and can range anywhere from 4% to 61% over the four months of the monsoon season, which spans from June to September. This variability highlights the complex interactions between atmospheric conditions and precipitation patterns.
“This is the first observational estimate of raindrop evaporation over the Western Ghats, and the technique can be used over the whole of India,” stated Saikat Sengupta, the study’s corresponding author. The research was published in the peer-reviewed journal Atmospheric Chemistry and Physics, which underscores its scientific validity and relevance.
The results from Pune represent just the initial phase of a broader initiative aimed at mapping the evaporation process across India. Sengupta anticipates that evaporation rates will differ significantly across the country, influenced by varying temperatures and humidity levels, from the arid landscapes of Rajasthan to the lush, rain-soaked coasts of Kerala. The IITM has been operating a rainwater-isotope network consisting of nine sites across India, from the Himalayan region to the northeastern states and Port Blair, where sampling has been conducted for over a decade. This extensive network will aid in understanding the regional differences in evaporation rates.
The measurements of evaporation rates are not merely an academic exercise; they have practical implications for weather forecasting and climate modeling. When raindrops evaporate while descending, they absorb heat from the surrounding air. This process cools the sub-cloud layer, which can influence downdrafts and create cold pools of air at the surface. These changes can reshape the convection processes that lead to subsequent rainfall events. Historically, climate and monsoon models have struggled to accurately capture these dynamics. Incorrect representations of evaporation can lead to skewed predictions of rainfall, atmospheric cooling, and the potential for storm development.
The estimated rain mass loss of approximately one-quarter sits at the lower end of global estimates from other regions. For instance, satellite data suggests that evaporation rates in tropical regions hover around 20%, while studies conducted over Zurich report rates of about 40%. In contrast, areas like Barbados experience evaporation rates close to 60%. The differences can be attributed to factors such as the size of raindrops and the humidity of the surrounding air; smaller drops are more susceptible to evaporation, while larger drops from intense downpours are less affected. In Zurich, for example, disabling evaporation in climate models resulted in a significant increase in predicted rainfall, highlighting the critical role that evaporation plays in regulating convection and precipitation.
The researchers employed innovative techniques to calculate the evaporation rate. They utilized the concept of isotopes, which are variants of chemical elements that have differing numbers of neutrons. While most water is composed of the standard H₂O molecules, a small fraction consists of heavier isotopes, such as heavy oxygen or heavy hydrogen. These heavier molecules evaporate less readily due to their greater mass. Consequently, when a raindrop evaporates, the lighter molecules escape first, leaving the remaining drop enriched in heavier isotopes. This isotopic signature serves as a valuable indicator of the extent of evaporation: rain that has experienced less evaporation retains a lighter isotopic signature, while more evaporated rain exhibits a heavier composition.
During the 2019 monsoon season, the research team collected samples of rainwater and atmospheric vapor at ground level in Pune. They analyzed the isotopic ratios using a laser spectrometer and incorporated the findings into a one-dimensional Below Cloud Interaction Model designed to track the journey of a single raindrop from the cloud base to the ground. The collection of atmospheric vapor proved to be a challenging task; each sampling process took approximately six to seven hours as the team froze atmospheric moisture to capture the vapor. To enhance their research capabilities, the group is in the process of acquiring portable analyzers that can provide real-time readings of vapor isotopes, which will be stationed at various locations throughout the country.
Sengupta emphasized that the study's results offer a pathway to enhance the accuracy of weather and climate models regarding rainfall representation. In modeling, representation refers to how effectively the equations within a model replicate real-world physical processes. For a model to yield trustworthy predictions about the effects of these processes, it must accurately capture them. The IITM's work in this area is crucial, as it lays the groundwork for future improvements in meteorological predictions.
It is worth noting that quantitative estimates of evaporation are relatively rare in India. Even non-isotopic measurements are scarce in the scientific literature, primarily due to the difficulties in measuring the quantities necessary to assess mid-air evaporation accurately. This research, which employs isotopes as tracers, represents a significant advancement in our understanding of the evaporation process and its implications for weather patterns in India.
As climate change continues to impact global weather systems, understanding the intricacies of the monsoon and the factors influencing rainfall becomes increasingly vital. The findings from this study not only contribute to the scientific community's knowledge of monsoonal dynamics but also have far-reaching implications for agriculture, water resource management, and disaster preparedness in India, where the monsoon season is a critical determinant of the agricultural calendar and overall economic stability.
To learn more about the latest developments in Science & Environment, stay updated with our exclusive reports and analyses on AiLensNews.
