When IMD issues a "red alert" for heavy rainfall, it's never for all of India. It's for a specific meteorological subdivision — West Rajasthan, or Coastal Karnataka, or North Interior Tamil Nadu. These 36 subdivisions aren't arbitrary bureaucratic lines. They reflect real climatic boundaries defined by geography, and understanding why they exist explains a lot about why rainfall forecasts differ so dramatically across short distances in this country.
The Himalayas Shape Everything to Their North
The Himalayas are the most consequential geographic fact in South Asian meteorology. They block cold, dry continental air from Central Asia during winter — without them, northern India would experience winters considerably more severe than it currently does, more similar to the steppes of Central Asia than the relatively mild winters of the Punjab plains. In summer, the mountains trap moisture within the Indian subcontinent. Monsoon winds that would otherwise dissipate further north are forced upward by the Himalayan barrier, triggering massive rainfall along the foothills — the reason Cherrapunji in Meghalaya holds the world record for wettest place on earth.
The Himalayas also create their own weather systems. Western disturbances — extratropical storm systems from the Mediterranean — travel along the Himalayan ridge in winter, depositing snow in Kashmir, Himachal, and Uttarakhand. These subdivisions follow mountain geography specifically because the weather there is orographically driven in ways that have no parallel in the plains below.
The Western Ghats and the Rain Shadow
Stand on the west side of the Western Ghats during southwest monsoon season and you'll experience relentless, intense rainfall. Cross to the east side — sometimes literally a 30-minute drive — and you're in the Deccan Plateau rain shadow, which receives 60–70% less rainfall than the windward side. This dramatic contrast within 50 kilometres is why Coastal Karnataka is a separate subdivision from Inland Karnataka, and why Mumbai (Konkan coast) and Pune (Deccan Plateau edge) have such strikingly different monsoon experiences despite being under 150 km apart.
The Ghats force warm, moist southwest monsoon air to rise rapidly. Rising air cools, condenses, and rains. By the time the air crosses the mountain crest and descends on the eastern side, it has lost most of its moisture and warms as it descends — the rain shadow effect. The Deccan Plateau and interior Andhra Pradesh exist in this rain shadow, dependent on secondary monsoon systems and the northeast monsoon for much of their rainfall.
The Desert and Semi-Arid Zones
West Rajasthan and North Gujarat are the most arid of India's meteorological subdivisions, shaped by proximity to the Thar Desert. Rainfall here is sparse, unreliable, and concentrated in short bursts during the monsoon's deepest western penetration. The desert also influences temperatures across much of northwestern India — the hot, dry loo winds that blast Delhi and Haryana in May originate from Rajasthan's heated plains.
These 36 subdivisions mean that when you hear a national monsoon forecast, you always need to locate your region's specific projection. "Normal monsoon nationally" often conceals above-normal conditions in one subdivision and below-normal in another. The regional detail is where the forecast actually becomes useful.
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Dr. Arun Sharma
Author & ResearcherPh.D. in Atmospheric Sciences, M.Sc. Meteorology
Dr. Arun Sharma has over 15 years of experience researching tropical meteorology, monsoon dynamics, and atmospheric modeling in South Asia. He oversees WeatherPulse's weather data verification standards and climate trend models.
