996 mb
Tibetan Low

Mechanism of Indian Monsoon

Comprehensive breakdown for UPSC GS-I & Geography Students

1. The Thermal Theory (Classical)

Proposed by Edmund Halley (1686), this treats the monsoon as a giant land-sea breeze.

During summer, the sun shines vertically over the Tropic of Cancer. The massive landmass of North India heats up intensely, while the Indian Ocean remains cool. This creates a steep Pressure Gradient.

[Image of sea breeze and land breeze diagram]
[ Diagram Placeholder: Sea Breeze Mechanism ]
  • Low Pressure: Forms over the Tibetan Plateau due to intense heat.
  • High Pressure: Remans over the Indian Ocean (Mascarene High).
  • Result: Moisture-laden winds rush from Sea to Land.

2. The Dynamic Theory (ITCZ Shift)

The Inter-Tropical Convergence Zone (ITCZ) is a low-pressure belt near the equator. In July, it shifts North to the Gangetic Plains (20-25°N).

[ Diagram Placeholder: ITCZ Shift & Coriolis Force ]

This shift pulls the South-East Trade Winds across the equator. Under Ferrel's Law (Coriolis Force), they deflect to the RIGHT, becoming the South-West Monsoon.

3. The Jet Stream Theory

Explains the sudden "Burst" of the monsoon. It relies on upper-air circulation (9-12km altitude).

[ Diagram Placeholder: STWJ and TEJ Positions ]
  • Westerly Jet (STWJ): Must withdraw North of the Himalayas for the monsoon to enter India.
  • Easterly Jet (TEJ): Aligns over 15°N and drives the depression that causes rainfall.