The monsoon is the most heavily examined topic in Indian geography, and the reason is simple: it is the only natural event that the Government of India forecasts in public, categorises numerically, and builds policy around. That makes it the rare geography topic with official definitions attached — and official definitions are what examiners turn into options.
The Hinge: the Monsoon Is Not Rain, It Is a Reversal
Most people picture the monsoon as heavy rain. It is not. The monsoon is a seasonal reversal of wind direction — the rain is a consequence. The word itself comes from the Arabic mausim, meaning season, which sailors used for the winds long before anyone measured the rainfall.
Hold the reversal and everything else follows. In summer the Indian landmass heats far faster than the Indian Ocean, creating low pressure over the land and relatively higher pressure over the sea; wind blows from sea to land, carrying moisture, and India gets its rain. In winter the land cools faster than the ocean and the gradient flips; wind blows from land to sea, dry.
So there are two monsoons, not one, and they are opposite in direction. This single fact answers more questions than any list of dates:
| Southwest monsoon | Northeast monsoon | |
|---|---|---|
| Also called | Advancing monsoon | Retreating monsoon or winter monsoon |
| Months | June to September | October to December |
| Wind direction | Sea to land (south-west) | Land to sea (north-east) |
| Who gets the rain | Most of India | Tamil Nadu, coastal Andhra Pradesh, south-east Kerala and Puducherry |
The trap is Tamil Nadu. It is the one large region that gets most of its rainfall from the northeast monsoon, because the retreating winds blow across the Bay of Bengal, pick up moisture and strike the Coromandel coast. A question asking which State depends on the retreating monsoon has one answer, and candidates who have learnt only “June to September” get it wrong.
Why the Winds Reverse: the Four Drivers
- Differential heating of land and sea. Land heats and cools faster than water, which creates and then destroys the pressure gradient.
- The shift of the ITCZ. The Inter-Tropical Convergence Zone is the belt near the equator where the trade winds of the two hemispheres meet. In the northern summer it moves north over the Gangetic plain, and the south-east trade winds of the southern hemisphere cross the equator to reach it — turning right as they do so, which is what makes them south-westerly.
- The Tibetan Plateau. This vast high surface heats intensely in summer and warms the air far above the plains, strengthening the circulation that draws moist air inland.
- The jet streams. The subtropical westerly jet sits south of the Himalaya in winter and withdraws north of the range in early summer; its withdrawal is a signal that the monsoon can advance. The tropical easterly jet then forms over peninsular India and helps the system along.
Jargon, defined once. A jet stream is a narrow ribbon of very fast wind high in the atmosphere. The Coriolis effect is the apparent deflection of moving air caused by the Earth’s rotation — to the right in the northern hemisphere — and it is why a wind crossing the equator from the south-east arrives in India from the south-west.
The Two Branches
Having crossed the equator, the southwest monsoon splits at the tip of the peninsula into two branches, and almost every rainfall pattern in India is a consequence of which branch reaches where:
| Branch | Path | What it explains |
|---|---|---|
| Arabian Sea branch | Strikes the Western Ghats, crosses into the peninsula, and a part moves up to Gujarat and Rajasthan | Very heavy rain on the windward Konkan and Malabar coasts; the dry rain shadow on the leeward side, which is why interior Karnataka and Maharashtra are dry |
| Bay of Bengal branch | Moves towards Myanmar and north-east India, is deflected west by the Himalaya and travels up the Ganga plain | Extremely heavy rain in Meghalaya; rainfall that decreases westwards along the plain, from Kolkata towards Delhi and on into the desert |
Two consequences worth memorising together. Mawsynram and Cherrapunji (Sohra) in Meghalaya receive the heaviest rainfall because the funnel-shaped Khasi hills force the Bay branch sharply upward. And rainfall falls off from east to west across the Ganga plain because the branch loses moisture as it travels — the same branch, progressively emptier.
Our capsule on Rivers, Dams and Lakes of India maps the catchments that this rain fills, and Soils and Crops of India covers what is sown on the strength of it.
The Official Dates, and a Genuine Disagreement
These are the normal dates, and they are asked directly:
| Event | Normal date |
|---|---|
| Onset over Kerala | 1 June |
| Monsoon covers the whole country | 8 July — but see the note below |
| Withdrawal begins, from north-west India | 17 September |
⚠ The two figures for full coverage do not agree, and the disagreement is inside the IMD’s own material. The India Meteorological Department’s December 2020 report gives 8 July as the normal date for the monsoon covering the entire country, while IMD’s own monsoon-onset web page still says the monsoon covers the country around 15 July. The 8 July figure comes from the revised normal dates; the 15 July figure is the older one. If an option offers 15 July, it is quoting the older set — which is still printed on an official page. This is a real hazard and not a trick: when an official source is revised, not every official page is updated at the same time.
On withdrawal, the IMD report gives 17 September as the normal start from north-west India but no normal date for complete withdrawal; in 2020 the monsoon actually withdrew from the whole country on 28 October. Learn the start of withdrawal, and treat complete withdrawal as a year-by-year fact rather than a constant.
How Onset Is Declared: Three Conditions, All Required
This is the part almost nobody studies and it is pure marks, because it is precise, official and stable. IMD does not declare the onset over Kerala because it has rained. Three conditions must be met together:
- Rainfall. After 10 May, at least 60% of 14 listed stations must report 2.5 mm or more of rain on two consecutive days. Onset is declared on the second day.
- Wind field. Westerlies must extend up to 600 hPa over the box from the equator to 10°N and 55°E to 80°E, with wind speed of about 15 to 20 knots at 925 hPa over 5–10°N and 70–80°E.
- Outgoing Longwave Radiation. Satellite-measured OLR must be below 200 watts per square metre over 5–10°N and 70–75°E.
Why the extra two conditions exist is the insight. Pre-monsoon thunderstorms can easily deliver two days of rain over Kerala without the monsoon having arrived at all. The wind test proves the circulation has actually reversed, and the low OLR proves there is deep cloud rather than a passing shower. A low reading means less heat escaping to space, because thick high cloud is blocking it. Rain alone can lie; wind and cloud together cannot.
Reading a Monsoon Forecast: LPA and the Five Categories
Every monsoon headline is a percentage of something, and that something is the Long Period Average. The LPA is 870 mm, the average June-to-September rainfall for the country over 1971 to 2020. IMD then sorts a season into five categories:
| Category | Rainfall as % of LPA |
|---|---|
| Deficient | Less than 90 |
| Below Normal | 90 to 95 |
| Normal | 96 to 104 |
| Above Normal | 105 to 110 |
| Excess | More than 110 |
The band for “normal” is narrow and asymmetric around 100, and that is the fact. A season at 95% is below normal, not normal. Options built on 90–110 or 95–105 look plausible and are wrong.
The 2026 season is the live illustration. IMD’s April forecast put it at 92% of LPA — below normal. The season actually ended at about 87% of LPA, roughly 759 mm against a normal near 869 mm, which falls in the deficient band and was the weakest since 2015. The South Peninsula was at about 76% and East and North-East India at about 74%, while the monsoon core zone held up better at about 94%.
El Niño, La Niña and the Indian Ocean Dipole
Two ocean patterns dominate the forecast, and candidates routinely mix up their effects.
| Pattern | What it is | Usual effect on the Indian monsoon |
|---|---|---|
| El Niño | Unusual warming of the central and eastern equatorial Pacific | Tends to weaken the monsoon |
| La Niña | The opposite phase, with cooler eastern Pacific waters | Tends to strengthen the monsoon |
| Positive Indian Ocean Dipole | Western Indian Ocean warmer than the eastern side | Tends to favour the monsoon, and can offset an El Niño |
The memory hook: El Niño is the Pacific pulling moisture away from India; a positive IOD is the Indian Ocean pulling it back. ENSO — the El Niño Southern Oscillation — is the name for the whole oscillation including its neutral state, so “ENSO-neutral” means neither El Niño nor La Niña.
The relationship is a tendency, not a rule. IMD states it in terms of likelihood, and a question claiming El Niño always produces drought is overstating it.
Two More Terms Worth Knowing
- Break in the monsoon. A spell of several days during the season when rainfall largely stops over most of the country. Breaks are normal; a long one at the wrong moment in the crop cycle does more damage than a lower seasonal total.
- Monsoon core zone. The mainly rain-fed agricultural belt across central India. It is reported separately because its rainfall matters more to the harvest than the national average does — in 2026 it was at 94% while the country was at 87%.
Why a Reader Should Care
The monsoon turns up in geography, in environment, in agriculture and in the economy section, and it carries one of the few sets of exactly defined numbers in the whole syllabus: 1 June, 17 September, 870 mm, 96 to 104 per cent. Those four are worth more than any amount of general description.
It also has unusual reach backwards and forwards. A weak monsoon four thousand years ago is the leading explanation for the end of the Harappan cities — see our capsule on the Indus Valley Civilisation, which sets out the pollen evidence. And a weak monsoon this year feeds straight into the crop base behind today’s Current Affairs capsule on the new E100 flex-fuel notification and ethanol blending. Rain-fed districts are also the target of PM Dhan-Dhaanya Krishi Yojana.
Practice Questions
Q1. The normal date for the onset of the southwest monsoon over Kerala is
(a) 15 May (b) 1 June (c) 10 June (d) 15 June
Answer: (b) 1 June IMD’s criteria can only be applied after 10 May, which is what makes the first option tempting to anyone who half-remembers the rule.
Q2. Under IMD’s rainfall criterion for declaring monsoon onset over Kerala, what share of the 14 listed stations must report 2.5 mm or more on two consecutive days?
(a) 40% (b) 50% (c) 60% (d) 75%
Answer: (c) 60% The rainfall test alone is never enough: the wind-field and outgoing longwave radiation conditions must be satisfied as well.
Q3. A southwest monsoon season is classified by IMD as ‘Normal’ when rainfall is
(a) 90% to 110% of LPA (b) 95% to 105% of LPA (c) 96% to 104% of LPA (d) 98% to 102% of LPA
Answer: (c) 96% to 104% of LPA The band is deliberately narrow, so a season at 95% is Below Normal and one at 105% is Above Normal.
Q4. Which region of India receives most of its annual rainfall from the retreating or northeast monsoon?
(a) Konkan coast (b) Meghalaya (c) Punjab (d) Tamil Nadu and the Coromandel coast
Answer: (d) Tamil Nadu and the Coromandel coast The retreating winds blow across the Bay of Bengal, gather moisture and strike that coast between October and December.
Q5. The Long Period Average used by IMD for the southwest monsoon is
(a) 870 mm, based on 1971 to 2020 (b) 1,000 mm, based on 1951 to 2000 (c) 750 mm, based on 1981 to 2010 (d) 920 mm, based on 1961 to 2010
Answer: (a) 870 mm, based on 1971 to 2020 Every monsoon headline expressed as a percentage is a percentage of this figure, so the base period is worth carrying with the number.
30-Second Revision
- The monsoon is a wind reversal, not rain. Southwest (advancing) June to September for most of India; northeast (retreating) October to December for Tamil Nadu and the Coromandel coast. Two branches: Arabian Sea (Western Ghats, rain shadow) and Bay of Bengal (Meghalaya, then drying westwards up the Ganga plain).
- Dates: onset over Kerala 1 June; withdrawal begins 17 September. Onset needs three conditions together — rainfall at 60% of 14 stations, the wind field, and OLR below 200 W/m².
- LPA = 870 mm (1971–2020); Normal = 96 to 104% of LPA. 2026 ended near 87% — deficient, the weakest since 2015. El Niño weakens, La Niña strengthens, a positive IOD favours the monsoon.
Sources
- India Meteorological Department, Monsoon Information — Onset — the three onset criteria, the 14 stations, and the normal onset date of 1 June.
- India Meteorological Department report of December 2020 — normal dates of onset over Kerala (1 June), coverage of the whole country (8 July) and the start of withdrawal from north-west India (17 September).
- Press Information Bureau, Long Range Forecast for the 2026 Southwest Monsoon Season Rainfall — the Long Period Average of 870 mm for 1971 to 2020, the five rainfall categories with their ranges, and the ENSO and Indian Ocean Dipole outlook.
- India Meteorological Department end-of-season figures for 2026, as reported — about 87% of LPA nationally, with the South Peninsula near 76% and the monsoon core zone near 94%.
