Explain the orographic influence of the Western Ghats on the southwest monsoon, and discuss factors affecting the amount of rainfall actually reaching the surface.

Q. Explain the orographic influence of the Western Ghats on the southwest monsoon, and discuss factors affecting the amount of rainfall actually reaching the surface. (15 marks, 250-350 words)

The Western Ghats, running almost perpendicular to the moisture-laden southwesterly flow of the Arabian Sea branch, act as India's principal orographic barrier, converting monsoon winds into the country's heaviest rainfall belt. Yet the rain a mountain generates is not the rain a farmer receives — sub-cloud and surface processes decide the final yield.

Orographic mechanism of the Ghats

• Forced ascent: moist winds striking the steep western scarp are lifted, cool adiabatically and condense, producing heavy windward rainfall along the Konkan–Malabar coast [1].
• Windward–leeward contrast: descending, warming air on the eastern side creates a pronounced rain-shadow — Pune, on the leeward side, receives a fraction of the crestline total [1][2].
• The barrier also anchors the offshore trough and low-level jet, sustaining rainfall through June–September and shaping the west coast's short, intense spells [1].

Factors reducing rainfall reaching the surface

• Sub-cloud raindrop evaporation: an IITM Pune study using stable isotopes of rain and vapour — India's first observational estimate — found that on average about 23% of monsoon rain mass evaporates mid-air, with daily values ranging from 4% to 61% [2].
• Atmospheric humidity and drop size: drier sub-cloud air and smaller droplets falling through a deeper column raise evaporative loss, explaining the wide leeward variability [2].
• Model under-representation: climate and forecast models poorly parameterise this "sub-cloud process", so simulated surface rainfall carries an evaporation bias — a gap institutions like IITM under the Ministry of Earth Sciences are addressing through the Monsoon Mission framework [3][4].
• Interception and runoff: dense Ghat canopy interception and steep-slope runoff further reduce infiltration and effective water availability.

Thus the Ghats amplify monsoon rainfall, but evaporative and terrain losses mediate what the ground actually gains. Extending IITM's nine-site isotope network pan-India would let planners estimate effective rainfall rather than gauge totals, strengthening water budgeting, crop advisories and SDG-6 goals — and confirming that accurate monsoon science begins with measuring what the mountains give, and what the air takes back.

(~330 words)

Sources: 1. S.K. Raghavan, "Influence of the Western Ghats on the Monsoon Rainfall at the coastal boundary of Peninsular India", MAUSAM (IMD journal) — orographic lifting, windward-leeward rainfall contrast, west coast monsoon rainfall 2. Nimya et al., "Assessing raindrop evaporation over northern Western Ghats from stable isotope signature of rain and vapour", Atmospheric Chemistry and Physics, 2026 — isotope method, ~23% average evaporation, 4–61% daily range, Pune leeward site 3. Indian Institute of Tropical Meteorology (IITM), Pune — Ministry of Earth Sciences — institutional mandate, rainwater-isotope observation network 4. PIB, "National Monsoon Mission", Ministry of Earth Sciences — model development for improved monsoon rainfall prediction