Indian Monsoon

Covers onset, branches, rainfall patterns, and key NDA exam traps.

NDA • Geography • Moderate • 1–2 Questions • Factual + Conceptual

Quick Overview

Nature: Seasonal wind reversal — SW Monsoon (Jun–Sep) and NE Monsoon (Oct–Dec).
Rainfall: SW Monsoon alone delivers ~75% of India’s annual rainfall.
Branches: Arabian Sea (Western Ghats, Konkan) and Bay of Bengal (NE India, Gangetic plain).
Exception: Tamil Nadu does NOT get SW monsoon rain — depends entirely on NE (Retreating) Monsoon.
Cause: Differential land-sea heating + ITCZ shift + Tropical Easterly Jet + Somali Jet.
Winter Rain: NW India (Punjab, J&K) rain comes from Western Disturbances, not monsoon.

Core Concept

What

A seasonal reversal of prevailing winds — the SW Monsoon (summer, Jun–Sep) and the NE Monsoon (retreating/winter, Oct–Dec).

Cause

Differential heating between the Indian landmass and the Indian Ocean, combined with the northward shift of the ITCZ in summer.

Key Factors

Tropical Easterly Jet (TEJ) and Somali Jet strengthen SW winds
Withdrawal of Sub-Tropical Westerly Jet Stream from the plains triggers onset
El Niño weakens monsoon; La Niña strengthens it
Western Disturbances bring separate winter rain to NW India (Dec–Feb)

Duration

SW: Jun–Sep (onset Kerala 1 June, full coverage by 15 July)
NE: Oct–Dec
Western Disturbances: Dec–Feb

Exam Relevance

SW Monsoon delivers ~75% of India’s annual rainfall. Questions test onset dates, branch coverage, rain-shadow areas, and the NE monsoon exception for Tamil Nadu.

Important Facts & Data

SW Monsoon onset (Kerala)	1 June
Covers entire country by	15 July
SW Monsoon withdrawal	Sep (NW India) → Oct (South)
NE (Retreating) Monsoon	October – December
SW rainfall share	~75% of annual rainfall
Wettest place in India	Mawsynram, Meghalaya (~11,872 mm)
Driest region	Western Rajasthan (Thar, <25 cm)
SW Monsoon branches	Arabian Sea & Bay of Bengal
Tamil Nadu rainfall source	NE (Retreating) Monsoon
Winter rain in NW India from	Western Disturbances
Key jet stream	Tropical Easterly Jet (TEJ)
El Niño effect	Weakens Indian Monsoon

Monsoon Branches & Coverage

Branch	Regions Covered	Exam Insight
Arabian Sea Branch	Western Ghats, Konkan, Kerala	Causes heavy rainfall due to orographic uplift
Bay of Bengal Branch	NE India, Gangetic plains	Deflected by Himalayas → heavy rainfall in Mawsynram
Retreating (NE) Monsoon	Tamil Nadu & Coromandel Coast	Only source of rain here — frequent exam trap
Western Disturbances	NW India (Punjab, J&K)	Brings winter rainfall — not monsoon

Memory Hook

1 June · 75% · Mawsynram · Tamil Nadu · NE

Onset 1 June · 75% rainfall · Mawsynram wettest · Tamil Nadu gets NE monsoon