One-fourth of India's monsoon rain mass evaporates mid-air

Why in News?

A recent study by the Indian Institute of Tropical Meteorology (IITM), Pune, has revealed that approximately 25% of India's monsoon rain mass evaporates mid-air before ever touching the ground.

Key Findings of the Study

• Regional Focus - The primary measurements were gathered over the northern Western Ghats and Pune during the Southwest monsoon season.
• Massive Daily Swings- While the average mid-air loss sits at 25%, the exact evaporation rate fluctuates wildly from day to day—ranging from 4% up to 61% depending on ambient weather conditions.
• Atmospheric Drivers- The mid-air loss is dictated primarily by localised atmospheric factors like ambient temperature, heat, and dry air.
• Method - The research team utilised advanced environmental tracking techniques to fingerprint the falling water.
  • Isotopic Analysis - When water evaporates, lighter water molecules escape into the air first.
  • This leaves the remaining falling raindrop highly enriched with heavier hydrogen and oxygen isotopes.
  • Hardware Sampling- Researchers spent hours freezing and trapping atmospheric vapor alongside surface rainwater, analysing their distinct molecular signatures using a laser spectrometer.
  • Computer Simulation - They fed this empirical isotopic data into a below-cloud interaction model to track individual raindrops from the base of the clouds to the surface.

Global Comparison

• India's 25% mid-air evaporation rate sits on the lower end of global baselines.
• For context, satellite data measures tropical trade-wind evaporation at around 20%, Zurich averages 40% during cold fronts, and drier oceanic regions like Barbados can witness up to 63% of rain vanishing mid-air.
• The larger size of Indian monsoon drops and higher local humidity help shield India's rain from disappearing even faster.

Reference

The Hindu | One-fourth of India’s monsoon rain evaporates mid-air