Geography
Climate and the Indian Monsoon
Mechanism of the Indian Monsoon
How the Indian monsoon works: climate controls, ITCZ and jet streams, Tibet, Somali jet, onset dates, Arabian Sea and Bay of Bengal branches, breaks and withdrawal.
Prerequisites
- Physiography of India — the Himalaya, the Western Ghats and the plateaus decide where monsoon winds drop their rain.
- Basic world climatology: pressure belts, the Coriolis force and the idea that air moves from high to low pressure.
The Hook: The Rain That Arrives on a Schedule
Connect
Every June, farmers across the Chhattisgarh plain watch the sky before they sow paddy. The first heavy showers usually reach Bastar first and then move north to Raipur, Bilaspur and Surguja. If the rain is a fortnight late, nurseries dry up; if it pauses for three weeks in July, transplanting stops.
The rain does not come by chance. It is carried by a giant seasonal wind system, the monsoon (मानसून), which reverses direction between summer and winter. What drives this reversal, why does it start near 1 June in Kerala, and why does it sometimes stop in the middle of the season?
Why it matters: Prelims asks onset dates, the branches of the monsoon, jet streams and “which factor” questions. Mains GS-III (P-V) asks you to explain the mechanism and link it to agriculture, drought and floods in India and Chhattisgarh.
What a Monsoon Is
The word comes from the Arabic mausim (मौसम), meaning “season”. A monsoon is a seasonal reversal of winds over a large region, bringing a wet season and a dry season.
| Season | Prevailing surface wind over India | Character |
|---|---|---|
| Summer (June–September) | South-west (from the Indian Ocean towards land) | Moist, rain-bearing: the south-west monsoon |
| Winter (December–February) | North-east (from land towards sea) | Mostly dry, except where it crosses the Bay of Bengal and reaches the Tamil Nadu coast: the north-east monsoon |
India’s climate is classed as tropical monsoon type (उष्णकटिबंधीय मानसूनी जलवायु), even though the northern half of the country lies north of the Tropic of Cancer. The Himalaya shut out cold Central Asian air, so most of the country behaves climatically like a tropical region.
Controls of Indian Climate
| Factor | Effect on India |
|---|---|
| Latitude | The Tropic of Cancer (23.5° N) runs through the middle of the country, including the northern part of Chhattisgarh (Surguja–Koriya region). South of it the climate is tropical; north of it, subtropical, with greater seasonal range |
| The Himalaya | Block cold, dry Central Asian winds in winter and force monsoon winds to shed rain in the north and north-east in summer |
| Distribution of land and water | The large Asian landmass heats and cools faster than the Indian Ocean, creating seasonal pressure contrasts that drive the monsoon |
| Distance from the sea | Coastal places have an equable (maritime) climate; interior places such as Delhi or Raipur have a continental climate with a large annual range of temperature |
| Altitude | Temperature falls with height, so hill stations such as Mainpat in Chhattisgarh are cooler than the plains |
| Relief | Windward slopes get heavy rain, leeward slopes stay dry (rain shadow). The Western Ghats and the Khasi hills are classic examples |
| Upper-air circulation | The westerly jet stream, the easterly jet stream and the shift of the ITCZ control the timing of onset and withdrawal |
| Western disturbances and tropical cyclones | Bring winter rain to north-west India and heavy post-monsoon rain to the east coast |
How the Monsoon Works: The Theories
No single theory explains the monsoon. Examiners expect you to know the classical idea, the modern explanation and the supporting factors.
1. Thermal concept (classical, land–sea contrast)
The oldest explanation, associated with Edmund Halley (1686), treats the monsoon as a giant sea breeze:
- In summer, the land heats faster, a low-pressure area develops over north-west India and Pakistan, and moist air flows in from the high-pressure Indian Ocean.
- In winter, the land cools faster, high pressure builds over the land, and dry winds blow out towards the sea.
Limitation: it cannot explain why the onset is sudden, why the monsoon has breaks, or why rainfall varies so much from year to year.
2. Dynamic concept: the shifting ITCZ
The Inter-Tropical Convergence Zone (अंतर-उष्णकटिबंधीय अभिसरण क्षेत्र, ITCZ) is a belt of low pressure where the trade winds of the two hemispheres meet. It follows the overhead sun.
- In summer, the ITCZ moves north to about 20°–25° N over the Ganga plain. Here it is called the monsoon trough.
- The south-east trade winds of the southern hemisphere cross the equator, are deflected to the right by the Coriolis force, and arrive over India as south-west winds.
- In winter, the ITCZ moves south of the equator, and north-east winds prevail over India.
This explanation is associated with the German meteorologist H. Flohn (1950s).
3. Jet streams and the Tibetan Plateau
| Feature | Role |
|---|---|
| Subtropical westerly jet stream (पश्चिमी जेट धारा) | In winter it flows south of the Himalaya over north India and brings western disturbances. In early summer it shifts north of the Himalaya; this shift is a key signal for the onset of the monsoon over India |
| Tibetan Plateau heating | The plateau, heated in summer, acts as an elevated heat source. Rising air creates an upper-level high and helps set up the easterly jet |
| Tropical easterly jet (TEJ) (पूर्वी जेट धारा) | Forms in summer in the upper troposphere over the peninsula, at roughly 15° N. Its strength is linked to the strength of the monsoon and to the westward movement of tropical depressions that bring rain to India |
4. Other contributing factors
| Factor | What to remember |
|---|---|
| Mascarene High | A high-pressure cell over the south Indian Ocean, near Madagascar. Its strength pushes moist air towards India |
| Somali Jet (Findlater Jet) | A strong low-level cross-equatorial wind off the coast of East Africa (Somalia) that carries moisture into the Arabian Sea branch |
| El Niño–Southern Oscillation (ENSO) | Warm Pacific (El Niño) years tend to weaken the Indian monsoon. See El Niño and La Niña |
| Indian Ocean Dipole (IOD) | A positive IOD (warmer western Indian Ocean) tends to help the monsoon |
Mains framing
Write the mechanism as a chain, not a list: differential heating → low over north-west India and the monsoon trough → northward shift of the ITCZ → withdrawal of the westerly jet to the north of the Himalaya and formation of the easterly jet → cross-equatorial south-east trades turn into south-west winds (Coriolis force), helped by the Somali jet and the Mascarene High → onset over Kerala. Then add that ENSO and IOD modulate the strength from year to year.
graph LR
A["Summer heating of land
and Tibetan Plateau"] --> B["Low pressure over NW India
Monsoon trough at 20-25 N"]
B --> C["ITCZ shifts north
Westerly jet moves north
of the Himalaya"]
C --> D["SE trades cross the equator
Coriolis turns them SW
Somali jet, Mascarene High"]
D --> E["Onset over Kerala
about 1 June"]
E --> F["Two branches advance
whole country by about 8 July"]Onset and Advance
| Event | Normal date (IMD) |
|---|---|
| Onset over Kerala | 1 June (with a variability of about a week either way) |
| Covers the entire country | 8 July (revised normal; earlier treated as about 15 July) |
| Withdrawal begins from north-west India (west Rajasthan) | 17 September (revised normal; earlier 1 September) |
| Withdrawal from the entire country | 15 October |
The 2020 revision
In 2020 the India Meteorological Department (भारत मौसम विज्ञान विभाग, IMD) revised its normal onset and withdrawal dates using more recent data. The Kerala onset date stayed at 1 June, but the monsoon now normally covers the whole country by 8 July and withdraws from the whole country by 15 October. Older books still print 15 July and “end of September”; use the revised dates.
“Burst” of the monsoon: the onset is often sudden, with a spell of heavy rain, thunder and lightning lasting several days. This sudden arrival is called the burst of the monsoon.
The Two Branches
The peninsula’s shape splits the south-west monsoon into two branches.
Arabian Sea branch (अरब सागर शाखा)
| Sub-branch | Where it goes | Result |
|---|---|---|
| 1. Western Ghats | Strikes the windward slopes (Sahyadri) | Very heavy rain on the west coast and the windward Ghats (generally more than 250 cm a year); the leeward Deccan (Pune, interior Karnataka, Marathwada) lies in the rain shadow |
| 2. Narmada–Tapi corridor | Enters north of Mumbai and moves east through the rift valleys | Rain in central India, reaching as far as the Chhota Nagpur plateau and the Chhattisgarh plain |
| 3. Saurashtra–Kachchh–Rajasthan | Moves north parallel to the Aravalli range | Little rain, because the Aravalli run parallel to the wind and do not force it to rise; this is a key reason Rajasthan is dry |
The Arabian Sea branch is the stronger of the two (it carries more moisture), and it finally merges with the Bay of Bengal branch over the north-west plains.
Bay of Bengal branch (बंगाल की खाड़ी शाखा)
- Strikes the Myanmar coast and south-east Bangladesh first; the Arakan Yoma deflects much of it towards India.
- Enters West Bengal and Bangladesh from the south and south-east.
- In the north-east it is trapped by the Garo–Khasi–Jaintia hills, giving Mawsynram and Cherrapunji (Sohra) some of the heaviest rainfall on Earth.
- Blocked by the Himalaya, it turns westward up the Ganga plain. Rainfall therefore decreases from east to west in north India (Kolkata > Patna > Prayagraj > Delhi).
- Along the monsoon trough, low-pressure systems and monsoon depressions formed over the head of the Bay travel west-north-west across Odisha, Chhattisgarh and Madhya Pradesh. They bring much of central India’s rain.
Chhattisgarh angle
Chhattisgarh receives rain from both branches, but the Bay of Bengal branch and its depressions are the main source. The monsoon normally reaches southern Chhattisgarh (Bastar) in mid-June and spreads north over the next week or two. A season with few Bay depressions, or one in which they track elsewhere, means a weak season for the state’s paddy belt. See Climate and Soils of Chhattisgarh.
Why does Tamil Nadu stay dry in June–September?
The Coromandel (Tamil Nadu) coast lies in the rain shadow of the Western Ghats for the Arabian Sea branch, and it lies parallel to the Bay of Bengal branch. It therefore gets little south-west monsoon rain and receives most of its rain in October–December from the north-east (retreating) monsoon.
Breaks in the Monsoon (मानसून विच्छेद)
The monsoon does not rain continuously. It has active spells and break spells.
| Situation | What happens |
|---|---|
| Active spell | The monsoon trough lies over the central Ganga plain and central India; depressions keep forming over the Bay; widespread rain |
| Break | The monsoon trough shifts north to the Himalayan foothills. Rain falls on the Himalayan foothills and north-east India (sometimes causing floods there) and on parts of the south-east peninsula, while most of the country stays dry for one to three weeks |
Other causes of breaks: fewer Bay depressions, a weaker cross-equatorial flow, and, on the west coast, winds blowing parallel to the coast.
Why breaks matter: a long break in July–August, during transplanting and tillering of paddy, can do more damage than a low seasonal total. This is why the distribution of rain within the season matters as much as the total.
Withdrawal (Retreat)
- Withdrawal starts from west Rajasthan around 17 September and is complete from the whole country by about 15 October (normal dates).
- As the sun moves south, the monsoon trough weakens and shifts south; a high-pressure area builds over north-west India; winds slowly turn north-easterly.
- Withdrawal is gradual, unlike the sudden onset.
- The retreat brings the October heat to north India and the north-east monsoon to Tamil Nadu. See Seasons of India.
Analysis: Why the Mechanism Matters (Mains material)
- Agriculture: about half of India’s net sown area is rain-fed, and kharif sowing follows the onset. In Chhattisgarh, the “rice bowl” plains depend heavily on the timing of onset and the absence of long breaks.
- Water and power: reservoirs such as the Minimata (Hasdeo) Bango and Gangrel dams are filled almost entirely by monsoon inflows; hydropower and irrigation for the rabi season depend on them.
- Economy: a weak monsoon lowers rural incomes, raises food inflation and slows demand for industrial goods.
- Forecasting: IMD issues long-range forecasts in April with an update in late May/June, and uses dynamical models developed under the Monsoon Mission. Better forecasts help farmers choose crops and sowing dates.
- Climate change: studies report more extreme rainfall days and longer dry spells within the season, which make the mechanism less predictable at the local scale.
Memory Tricks
The mechanism chain: 'Hot Land Invites Jets, Somali Sends Rain'
Heating of land and Tibet → Low pressure / monsoon trough → ITCZ moves north → Jet streams shift (westerly jet north, easterly jet forms) → Somali jet and Mascarene High push moisture → Rain begins over Kerala.
Dates: 1-8-17-15
1 June Kerala onset → 8 July whole country → 17 September withdrawal starts → 15 October withdrawal complete.
Arabian Sea branch: 'Ghats, Gap, Gujarat'
The three sub-branches go to the Western Ghats, through the Narmada–Tapi gap into central India, and along Gujarat–Rajasthan parallel to the Aravalli.
Don’t Confuse
| This | vs | That |
|---|---|---|
| Subtropical westerly jet — winter, south of the Himalaya; brings western disturbances | vs | Tropical easterly jet — summer, over the peninsula; linked to monsoon depressions |
| Monsoon trough — the ITCZ over north India in summer | vs | Break in monsoon — the trough shifts to the Himalayan foothills |
| Somali jet — low-level, off East Africa | vs | Mascarene High — surface high pressure near Madagascar |
| Onset — sudden (“burst”), from 1 June | vs | Withdrawal — gradual, from about 17 September |
| Arabian Sea branch — stronger, west coast and central India | vs | Bay of Bengal branch — north-east India, Ganga plain, depressions over Odisha–Chhattisgarh |
Common Mistakes
MCQ traps
- “The monsoon covers the whole country by 15 July.” That is the old normal. Since 2020 IMD uses 8 July.
- “Rajasthan is dry because it is far from the sea.” The textbook reason is that the Aravalli lie parallel to the Arabian Sea branch and do not lift the winds.
- “The westerly jet brings the monsoon.” The westerly jet’s northward shift (away from India) is linked to onset; in winter it brings western disturbances, not the monsoon.
- “Tamil Nadu gets most of its rain in July.” No. Most of its rain comes in October–December from the north-east monsoon.
- “During a break, it does not rain anywhere.” During a break, the Himalayan foothills and north-east India often get heavy rain.
Practice
All questions below are original practice questions written in CGPSC style. They are not previous-year questions.
Level 1: Foundation
- The word “monsoon” comes from the Arabic word ______, meaning season.
- The normal date of onset of the south-west monsoon over Kerala is ______.
- In summer, the ITCZ over the Ganga plain is called the ______.
Answers
- Mausim.
- 1 June.
- Monsoon trough.
Level 2: Prelims MCQs
Q1. Consider the following statements about the Indian monsoon:
- The northward shift of the subtropical westerly jet stream to the north of the Himalaya is associated with the onset of the monsoon over India.
- The tropical easterly jet develops over peninsular India in winter.
- The Somali jet is a low-level cross-equatorial wind off the coast of East Africa.
Which of the statements given above is/are correct?
- (a) 1 only
- (b) 1 and 3 only
- (c) 2 and 3 only
- (d) 1, 2 and 3
Answer and explanation
(b) 1 and 3 only. Statement 1 is correct: the westerly jet moves north of the Himalaya in early summer, a key signal for onset. Statement 2 is wrong: the tropical easterly jet forms in summer (the monsoon season), not winter. Statement 3 is correct: the Somali (Findlater) jet carries moisture across the equator into the Arabian Sea.
Q2. Match List I (Feature) with List II (Description):
| List I | List II |
|---|---|
| A. Mascarene High | 1. Sudden onset with heavy rain |
| B. Monsoon trough | 2. High pressure near Madagascar |
| C. Burst of monsoon | 3. Northern position of the ITCZ over the Ganga plain |
| D. Break in monsoon | 4. Trough shifts to the Himalayan foothills |
- (a) A-2, B-1, C-3, D-4
- (b) A-3, B-2, C-1, D-4
- (c) A-2, B-3, C-1, D-4
- (d) A-4, B-3, C-1, D-2
Answer and explanation
(c) A-2, B-3, C-1, D-4. The Mascarene High lies over the south Indian Ocean near Madagascar. The monsoon trough is the ITCZ over north India in summer. The burst is the sudden, heavy-rain onset. A break occurs when the trough moves north to the Himalayan foothills.
Q3. Western Rajasthan receives little rain from the Arabian Sea branch of the monsoon mainly because:
- (a) It lies in the rain shadow of the Western Ghats
- (b) The Aravalli range lies parallel to the direction of the moisture-bearing winds
- (c) The Bay of Bengal branch exhausts its moisture before reaching it
- (d) The westerly jet stream blocks the monsoon winds over it
Answer and explanation
(b). The Saurashtra–Kachchh sub-branch moves north parallel to the Aravalli, so the winds are not forced to rise and cool, and little rain falls. Option (c) is partly true of the Bay branch but is not the main reason for western Rajasthan’s dryness; options (a) and (d) are wrong.
Q4. According to the normal dates revised by IMD in 2020, the south-west monsoon normally covers the entire country by:
- (a) 1 July
- (b) 8 July
- (c) 15 July
- (d) 31 July
Answer and explanation
(b) 8 July. The earlier normal was about 15 July (option c, a common trap). The revision also moved the start of withdrawal to 17 September and complete withdrawal to 15 October.
Q5. Which one of the following is the main source of monsoon rain over the Chhattisgarh plain?
- (a) Western disturbances moving in from the Mediterranean
- (b) The north-east monsoon of October–December
- (c) Low-pressure systems and depressions of the Bay of Bengal branch moving along the monsoon trough
- (d) The Saurashtra–Kachchh sub-branch of the Arabian Sea branch
Answer and explanation
(c). Depressions forming over the head of the Bay of Bengal travel west-north-west across Odisha and Chhattisgarh along the monsoon trough and bring much of the state’s rain. The Narmada–Tapi sub-branch of the Arabian Sea branch also contributes, but the Saurashtra–Kachchh sub-branch (d) goes to Rajasthan.
Level 3: Mains Answer Writing
Q1. (150–200 words) Explain the mechanism of the south-west monsoon in India. Why is the thermal concept alone insufficient?
Model answer outline
- Intro: The monsoon is a seasonal reversal of winds; India receives about three-quarters of its annual rain from the south-west monsoon (June–September).
- Body:
- Thermal concept (Halley): land–sea heating contrast creates summer low over north-west India; moist air flows in.
- Its limits: cannot explain sudden onset, breaks, or year-to-year variability.
- Dynamic concept: ITCZ shifts to 20°–25° N as the monsoon trough; south-east trades cross the equator and turn south-west (Coriolis).
- Upper air: westerly jet shifts north of the Himalaya; Tibetan heating sets up the tropical easterly jet.
- Other drivers: Somali jet, Mascarene High; ENSO and IOD modulate strength.
- Chhattisgarh angle: the state’s rain comes largely from Bay of Bengal depressions travelling along the monsoon trough, so the trough’s position decides good and bad years for paddy.
- Conclusion: The monsoon is a coupled ocean–land–atmosphere system; no single theory explains it, which is why forecasting combines several signals.
Q2. (60 words) What is a “break in the monsoon”? Why is it important for agriculture?
Model answer outline
- Definition: a dry spell of one to three weeks during the monsoon, when the monsoon trough shifts to the Himalayan foothills; rain then falls mainly in the foothills and north-east India.
- Importance: a long July–August break stresses transplanted paddy and other kharif crops, even when the seasonal total is normal.
- Example: rain-fed paddy in the Chhattisgarh plain.
Quick Revision Table
| Fact | Detail |
|---|---|
| Meaning | Arabic mausim = season; seasonal reversal of winds |
| India’s climate type | Tropical monsoon |
| Tropic of Cancer | Crosses the middle of India, including northern Chhattisgarh |
| Thermal concept | Halley (1686); land–sea contrast |
| Dynamic concept | ITCZ shift (Flohn); monsoon trough at 20°–25° N |
| Westerly jet | Winter, south of the Himalaya; shifts north before onset |
| Tropical easterly jet | Summer, upper air over the peninsula |
| Somali jet | Low-level cross-equatorial jet off East Africa |
| Mascarene High | High pressure near Madagascar |
| Kerala onset | 1 June (normal) |
| Whole country | 8 July (revised 2020) |
| Withdrawal | Starts about 17 September (west Rajasthan); complete about 15 October |
| Arabian Sea branch | Stronger; three sub-branches (Ghats, Narmada–Tapi, Saurashtra–Rajasthan) |
| Bay of Bengal branch | North-east India, Ganga plain (rain decreases east to west), depressions over Odisha–Chhattisgarh |
| Break | Trough moves to the Himalayan foothills |
Key Takeaways
Key Takeaways
- The monsoon is a seasonal reversal of winds driven by differential heating, the northward shift of the ITCZ and changes in the jet streams.
- The westerly jet moves north of the Himalaya and the easterly jet forms before onset; the Somali jet and Mascarene High feed the flow.
- Normal dates: 1 June (Kerala), 8 July (whole country), withdrawal 17 September to 15 October.
- The Arabian Sea branch is stronger; the Bay of Bengal branch and its depressions water Chhattisgarh and the Ganga plain.
- Breaks in July–August can hurt crops more than a low seasonal total.
What’s Next
- Seasons of India — the four IMD seasons that follow from this mechanism
- Rainfall Distribution in India — where the monsoon rain actually falls
- El Niño, La Niña and the Indian Ocean Dipole — why some years are weak
- Climate and Soils of Chhattisgarh — the state-level picture
- Agriculture in India — kharif and rabi seasons tied to the monsoon
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