Evaporation of Southwest Monsoon Rain Over Western Ghats

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A new study by the **Indian Institute of Tropical Meteorology (IITM), Pune** shows that almost **25%** of India's Southwest monsoon rain over the **Western Ghats** evaporates in mid-air before reaching the ground.

During the **Southwest monsoon** months from June to September, an average of **25%** of rain mass evaporates mid-air over the **north Western Ghats**.
The daily mid-air evaporation rate changes greatly from day to day, varying between **4% and 61%** based on atmospheric temperature and humidity.

As raindrops evaporate below cloud level, they absorb heat from the air around them, which cools the lower layer and creates strong downdrafts along with **surface cold pools**.
These cold pools alter local atmospheric convection and shape future rainfall patterns, making this evaporation process crucial for predicting storms and rain accurately.

India's **25%** evaporation rate is relatively low compared to global figures because intense monsoon rains have larger drops and the surrounding air is very humid.
In contrast, rain evaporation rates reach nearly **40%** over **Zurich** and **60%** near **Barbados**, where the air is much drier and raindrops are smaller.

Scientists could not measure mid-air evaporation directly, so they used **stable isotopes** as natural chemical markers to track the process.
Falling raindrops lose lighter water molecules first during evaporation, which increases the proportion of **heavier isotopes** left in the remaining droplets.
The **IITM, Pune** runs a **nine-site rainwater-isotope network** extending from the **Himalayas** to the Northeast and down to **Port Blair** in the **Andaman and Nicobar Islands** to trace how monsoon rains form and move.

Traditional climate models have often failed to represent mid-air evaporation accurately in weather simulations.
By using the **one-dimensional Below Cloud Interaction Model**, researchers can now refine hydrological models and significantly improve forecasts for rainfall and **extreme weather events**.