BharatNotes Why this chapter matters for UPSC: This is Chapter 4 (Unit III, "Climate and Vegetation") of India — Physical Environment in the current rationalised NCERT (Reprint 2026-27). It explains why India has a monsoon climate, how the monsoon sets in, breaks and retreats, what each of the four seasons looks like, and why rainfall varies from over 1,000 cm in the Khasi Hills to under 10 cm in western Rajasthan. Mains GS1 has asked about monsoon climate (2017), the changing behaviour of the monsoon (2015), El Niño (2014) and the "Purvaiya" (2023).
Contemporary hook: The monsoon dates in this NCERT chapter are the old normals, based on 149 stations for 1901–1940. Since 1 June 2020 the India Meteorological Department (IMD) has used revised normals: onset over Kerala is still 1 June, but the monsoon now covers the whole country by 8 July (old: 15 July) and starts withdrawing from western Rajasthan on 17 September (old: 1 September). IMD also revised the all-India rainfall normal to 116 cm (1971–2020 data) in April 2022.
🧠 First Principles — Read This First
Weather is the state of the atmosphere at a given moment: today's temperature, wind, humidity and rain. Climate is the average of weather over a long period. Weather can change within a day. A change in climate may become noticeable only after 50 years or more.
"Monsoon" means a climate in which the prevailing wind reverses direction with the seasons. Land heats up and cools down faster than water. In summer the northwestern part of the subcontinent becomes very hot and air pressure there falls. The ocean to the south stays cooler, so its pressure is higher. Moist air from the ocean is drawn towards the land, and this is the southwest monsoon. In winter the land is colder than the sea, the gradient reverses, and dry winds blow from land to sea. These are the northeast monsoon winds. Crossing the Bay of Bengal, they pick up moisture and bring rain to the Tamil Nadu coast in October and November.
Think of a kitchen in summer: the hot stove makes air rise, and cooler air from the open window moves in to replace it. The northwestern plains are the stove and the Indian Ocean is the window.
Two more ideas explain most of the chapter. The Inter Tropical Convergence Zone (ITCZ) is a belt of low pressure where winds converge and rise. It moves north in summer, reaching 20°–25°N over the Gangetic plain in July. Relief then decides where the rain actually falls: windward slopes such as the Western Ghats get heavy rain, while the land behind them stays dry.
UPSC asks about this chapter because the monsoon sets India's crop calendar, its droughts and floods, and its regional differences in rainfall.
PART 1 — Quick Reference
Table 1: Factors Determining the Climate of India (NCERT)
| Factor | Effect | NCERT example |
|---|---|---|
| Latitude | Tropic of Cancer runs through central India. The south is tropical: high temperatures, small daily and annual range. The north is sub-tropical/temperate: extreme climate, high daily and annual range | — |
| Himalayan mountains | Shield the subcontinent from cold northern winds that originate near the Arctic Circle; trap the monsoon winds so they shed their moisture within the subcontinent | — |
| Distribution of land and water | Differential heating of land and sea creates different pressure zones in different seasons, which reverses the monsoon winds | Indian Ocean on three sides, mountain wall in the north |
| Distance from the sea | Coasts have an equable climate; the interior has extremes | Mumbai and the Konkan coast vs Delhi, Kanpur, Amritsar |
| Altitude | Temperature decreases with height | January: Agra 16 °C, Darjiling 4 °C (same latitude) |
| Relief | Windward slopes get heavy rain; leeward areas stay dry | Windward Western Ghats and Assam wet in June–September; southern plateau dry |
Source: NCERT, India — Physical Environment, Ch. 4 "Climate", Reprint 2026-27, pp. 29–30 (kegy104.pdf).
Table 2: The Four Seasons (as recognised by meteorologists)
| Season | Months | Main features (NCERT) |
|---|---|---|
| Cold weather | Sets in by mid-November in the north; December and January are the coldest months | Feeble high pressure over the northern plain; light outblowing winds; western disturbances bring winter rain to the northwest; northeast monsoon rain on the Tamil Nadu coast in October–November |
| Hot weather | March–May (April, May and June are the summer months in north India) | Heat belt moves north; ITCZ shifts north; loo, dust storms, nor'westers, mango showers |
| Southwest monsoon | June–September | Sudden "burst"; Arabian Sea and Bay of Bengal branches; breaks |
| Retreating monsoon | October–November | Clear skies, "October heat"; tropical cyclones; rain on the eastern Peninsula |
Source: NCERT kegy104, pp. 31–37.
Table 3: Normal Onset and Withdrawal Dates of the Southwest Monsoon
IMD's normals in use since 1 June 2020 are set against the old normals (1901–1940), which match NCERT's text. For the exam, know both: NCERT is the textbook answer, IMD is the current official figure.
| Place / stage | IMD normal (since 2020) | Old normal (1901–1940) | NCERT wording |
|---|---|---|---|
| Onset over Kerala (Thiruvananthapuram) | 1 June | 1 June | "by 1st June" |
| Mumbai | 11 June | 10 June | Mumbai and Kolkata "between 10th and 13th June" |
| Kolkata | 11 June | 10 June | as above |
| New Delhi | 27 June | 23 June | — |
| Covers the whole country (extreme northwest, e.g. Jaisalmer) | 8 July | 15 July | "By mid-July" |
| Withdrawal begins, western Rajasthan (Jaisalmer, Bikaner) | 17 September | 1 September | "by the first week of September" |
| Withdrawal from New Delhi | 25 September | 22 September | — |
| Withdrawal from Mumbai | 8 October | 29 September | — |
| Final withdrawal from south India | 15 October (unchanged) | 15 October | NCERT instead tracks the low-pressure centre, which is "completely removed from the Peninsula" by mid-December |
Source: IMD, "New Normal Dates of Onset/Progress and Withdrawal of Southwest Monsoon over India" (onset normals from 1961–2019 data, withdrawal normals from 1971–2019; in use from 1 June 2020), Table 1 (IMD PDF); NCERT kegy104, pp. 31, 37.
Table 4: All-India Rainfall Normals (IMD, 1971–2020)
| Season | Normal (mm) | Share of annual rainfall |
|---|---|---|
| Winter (January–February) | 39.8 | 3.4% |
| Pre-monsoon (March–May) | 130.6 | 11.3% |
| Southwest monsoon (June–September) | 868.6 | 74.9% |
| Post-monsoon (October–December) | 121.0 | 10.4% |
| Annual | 1,160.1 | 100% |
July is the wettest month (280.4 mm), followed by August (254.9 mm). The previous normal (1961–2010) was 1,176.9 mm annual and 880.6 mm for June–September. NCERT gives the average annual rainfall as "about 125 cm"; IMD's current figure is about 116 cm.
Source: IMD Press Release, "Updated Rainfall Normal based on data of 1971-2020", New Delhi, 14 April 2022, Tables 1(a)–1(c) (IMD PDF).
Table 5: Distribution of Annual Rainfall (NCERT)
| Band | Annual rainfall | Areas |
|---|---|---|
| High | Over 200 cm (over 1,000 cm in parts of the Khasi and Jaintia hills) | West coast, Western Ghats, sub-Himalayan areas of the northeast, hills of Meghalaya. The Brahmaputra valley and adjoining hills get less than 200 cm |
| Medium | 100–200 cm | Southern Gujarat, east Tamil Nadu, the northeastern Peninsula (Odisha, Jharkhand, Bihar, eastern Madhya Pradesh), northern Ganga plain along the sub-Himalaya, Cachar valley, Manipur |
| Low | 50–100 cm | Western Uttar Pradesh, Delhi, Haryana, Punjab, Jammu and Kashmir, eastern Rajasthan, Gujarat, Deccan Plateau |
| Inadequate | Below 50 cm | Parts of the Peninsula (Andhra Pradesh, Karnataka, Maharashtra), Ladakh, most of western Rajasthan |
Snowfall is restricted to the Himalayan region. IMD's subdivision normals (1971–2020) put Konkan and Goa highest, at over 300 cm a year, and West Rajasthan lowest, at about 33 cm.
Source: NCERT kegy104, p. 38; IMD Press Release, 14 April 2022.
Table 6: Local Storms of the Hot Weather Season (NCERT)
| Name | Where | Feature |
|---|---|---|
| Mango shower | Kerala and coastal Karnataka | Pre-monsoon showers at the end of summer; help mangoes ripen early |
| Blossom shower | Kerala and nearby areas | Coffee flowers blossom |
| Nor'westers ("Kalbaisakhi"; "Bardoisila" in Assam) | Bengal and Assam | Dreaded evening thunderstorms; useful for tea, jute and rice |
| Loo | Northern plains from Punjab to Bihar, most intense between Delhi and Patna | Hot, dry, oppressive afternoon winds that often continue into midnight |
Source: NCERT kegy104, pp. 34–35.
Table 7: Traditional Indian Seasons (NCERT)
| Season | Indian calendar | Gregorian months |
|---|---|---|
| Vasanta | Chaitra–Vaisakha | March–April |
| Grishma | Jyaistha–Asadha | May–June |
| Varsha | Sravana–Bhadra | July–August |
| Sharada | Asvina–Kartika | September–October |
| Hemanta | Margashirsa–Pausa | November–December |
| Shishira | Magha–Phalguna | January–February |
This six-season cycle reflects the experience of north and central India. NCERT notes that it does not match south India, where seasonal variation is small. Source: NCERT kegy104, pp. 37–38.
PART 2 — Concepts & Narrative
Unity and Diversity in the Monsoon Climate
The monsoon links India with the rest of south and southeast Asia. NCERT stresses that this broad unity hides large regional differences, which it calls sub-types of the monsoon climate. Its examples, all from the textbook:
- Temperature range across places: the mercury occasionally touches 55 °C in western Rajasthan in summer and drops to minus 45 °C around Leh in winter. Churu may record 50 °C or more on a June day while Tawang (Arunachal Pradesh) hardly touches 19 °C. On a December night Drass (Ladakh) may fall to minus 45 °C while Thiruvananthapuram or Chennai records 20–22 °C.
- Daily range: in Kerala and the Andaman Islands day and night temperatures differ by hardly 7–8 °C. In the Thar desert a 50 °C day can be followed by a night of 15–20 °C.
- Rainfall: Cherrapunji and Mawsynram (Khasi Hills) receive over 1,080 cm a year, while Jaisalmer rarely gets more than 9 cm. Tura (Garo Hills) may receive in a single day as much rain as Jaisalmer gets in 10 years. Annual precipitation is under 10 cm in the northwest Himalaya and the western deserts but exceeds 400 cm in Meghalaya.
- Timing: the Ganga delta and coastal Odisha are hit by rain-bearing storms every third to fifth day in July and August, while the Coromandel coast a thousand km to the south stays generally dry. Most of the country gets its rain in June–September; the Tamil Nadu coast gets it at the beginning of winter.
Inter Tropical Convergence Zone (ITCZ) and monsoon trough. The ITCZ is a low-pressure zone near the equator where the trade winds converge, so air tends to rise. In July it lies at about 20°N–25°N over the Gangetic plain, where it is called the monsoon trough. The trough encourages a thermal low over north and northwest India. Because the ITCZ has shifted north, the southern hemisphere trade winds cross the equator between 40°E and 60°E longitude and, turned by the Coriolis force, blow from the southwest: the southwest monsoon. In winter the ITCZ moves south and the winds reverse to blow from the northeast: the northeast monsoon. (NCERT kegy104, p. 30)
The Nature of the Indian Monsoon
NCERT is careful to say that no single theory explains the monsoon fully, and that understanding improved when it was studied at the global rather than the regional level. The chapter examines two aspects: onset and break.
How the monsoon sets in (NCERT's account).
- Differential heating. In April and May the sun shines vertically over the Tropic of Cancer. The large landmass north of the Indian Ocean heats intensely and an intense low pressure forms over the northwestern subcontinent. Pressure over the ocean to the south stays high because water heats slowly.
- Trades pulled across the equator. The low-pressure cell attracts the southeast trade winds of the southern hemisphere across the equator, between 40°E and 60°E. After crossing, they are deflected and blow from the southwest. The southwest monsoon is therefore a continuation of the southeast trades, which NCERT also calls "displaced" equatorial easterlies.
- ITCZ moves north. These conditions help shift the ITCZ northward. By July it is centred at about 25°N. The monsoon trough extends from the Thar desert in the northwest to Patna and the Chotanagpur plateau in the east-southeast.
- Jet streams switch. The northward shift of the ITCZ is linked to the withdrawal of the westerly jet stream from its winter position over the north Indian plain, south of the Himalayas. Only after the westerly jet has withdrawn does an easterly jet stream set in along 15°N. NCERT holds this easterly jet responsible for the burst of the monsoon.
- Burst. The rains begin abruptly, with violent thunder and lightning, and bring temperatures down sharply. The burst may come in the first week of June on the coasts of Kerala, Karnataka, Goa and Maharashtra, and be delayed to the first week of July in the interior. Day temperatures fall by 5–8 °C between mid-June and mid-July.
Source: NCERT kegy104, pp. 30–31, 34–35.
Break in the monsoon. If rain fails for one or more weeks after a few rainy days in the southwest monsoon period, it is a break. NCERT gives two causes for two regions:
- In northern India, rains are likely to fail when rain-bearing storms along the monsoon trough (the ITCZ) are infrequent.
- On the west coast, dry spells come on days when the winds blow parallel to the coast.
El Niño and the Southern Oscillation
NCERT describes El Niño as a complex weather system that appears once every three to seven years and brings drought, floods and other extremes to different parts of the world. Warm water appears off the coast of Peru in the eastern Pacific, temporarily replacing the cold Peruvian (Humboldt) current and raising coastal water temperature by 10 °C. The results: distorted equatorial atmospheric circulation, irregular evaporation, and fewer plankton and fish. The name means "Child Christ" because the current appears around Christmas, a summer month in Peru. NCERT notes that El Niño is used in India to forecast long-range monsoon rainfall, and that in 1990-91 a wild El Niño delayed the onset of the southwest monsoon over most of the country by five to twelve days.
The Southern Oscillation is measured, among other ways, by the pressure difference between Tahiti (about 20°S, 140°W, French Polynesia) and Port Darwin (12°30'S, 131°E, northern Australia). NCERT states that IMD can forecast the likely behaviour of the monsoon on the basis of 16 indicators.
El Niño raises the risk of a poor monsoon; it does not decide it. Two IMD records show this. In 1997, the El Niño that began around April–May 1997 and peaked around December 1997 was widely expected to cause a severe deficit, yet June–September rainfall over India was 2% above normal (Asnani, MAUSAM 52(1), 2001, IMD). In 2015, by contrast, rainfall was 86% of the long period average, a deficient season for which IMD's end-of-season report names the strong El Niño as one of the most important reasons (IMD, 2015 Southwest Monsoon End of Season Report). The correct exam phrasing is that El Niño years are more likely, not certain, to be below normal.
The Cold Weather Season
Temperature. The season sets in by mid-November in northern India. December and January are the coldest months on the northern plain, where the mean daily temperature stays below 21 °C. Night temperatures can fall below freezing in Punjab and Rajasthan. NCERT gives three reasons for the cold of north India:
- Punjab, Haryana and Rajasthan are far from the sea's moderating influence and have a continental climate.
- Snowfall in the nearby Himalayan ranges creates cold-wave conditions.
- Around February, cold winds from the Caspian Sea and Turkmenistan bring cold waves, frost and fog to the northwest.
The Peninsula has no well-defined cold season. The sea and the nearness to the equator keep coastal temperatures almost unchanged through the year, and the hills of the Western Ghats stay comparatively cool.
Pressure and winds. Around 22 December the sun is vertically over the Tropic of Capricorn. Feeble high pressure lies over the northern plain and pressure is slightly lower in the south. NCERT places the 1019 mb isobar over northwest India and the 1013 mb isobar in the far south. Light winds of about 3–5 km per hour blow outward. Relief shapes their direction: westerly or northwesterly down the Ganga valley, northerly in the Ganga–Brahmaputra delta, and clearly northeasterly over the Bay of Bengal.
Western disturbances. The pleasant winter weather is broken at intervals by shallow cyclonic depressions from the east Mediterranean. They travel east across West Asia, Iran, Afghanistan and Pakistan, picking up moisture from the Caspian Sea in the north and the Persian Gulf in the south, before reaching northwestern India. NCERT asks students to work out the role of the westerly jet stream in steering them into India.
Rainfall. Winter monsoon winds blow from land to sea, carry little humidity and sit under anticyclonic circulation, so most of India is dry. NCERT lists four exceptions:
- Weak temperate cyclones from the Mediterranean bring rain to Punjab, Haryana, Delhi and western Uttar Pradesh, and snow to the lower Himalaya. The amount is small but highly beneficial for rabi crops, and the snow sustains Himalayan rivers in summer. Precipitation decreases from west to east on the plains and from north to south in the mountains. Delhi's average winter rainfall is about 53 mm; Punjab and Bihar get between 25 mm and 18 mm respectively.
- Central India and the northern parts of the southern Peninsula occasionally get winter rain.
- Arunachal Pradesh and Assam get 25–50 mm in the winter months.
- In October and November the northeast monsoon picks up moisture over the Bay of Bengal and causes torrential rain over the Tamil Nadu coast, southern Andhra Pradesh, southeast Karnataka and southeast Kerala.
The Hot Weather Season
Temperature. With the sun's apparent northward movement in March, temperatures rise in north India. NCERT's figures: about 38 °C in the Deccan plateau in March; 38–43 °C in Gujarat and Madhya Pradesh in April; around 48 °C not uncommon in the northwest in May. South India is milder, at 26–32 °C, and the hills of the Western Ghats stay below 25 °C. On the coasts the isotherms run north–south, parallel to the coast, so temperature rises from the coast to the interior rather than from south to north. Mean daily minimum temperatures in summer rarely fall below 26 °C.
Pressure and winds. Heating and falling pressure in the northern half of the country move the ITCZ north. The surface winds it draws in are southwesterly on the west coast and along the coast of West Bengal and Bangladesh, and easterly or southeasterly over north Bengal and Bihar. Their influx by mid-June brings the change to the rainy season. In the heart of the ITCZ in the northwest the loo blows in the afternoon. Evening dust storms are common in May in Punjab, Haryana, eastern Rajasthan and Uttar Pradesh and bring light rain and relief. Where moist air is drawn towards the edge of the trough and meets dry air, intense local storms form with violent winds, torrential rain and hail.
The Southwest Monsoon Season
By early June the low over the northwestern plains is strong enough to draw in the southern hemisphere trades. They cross the equator, enter the Bay of Bengal and the Arabian Sea, pick up moisture over warm equatorial currents, and blow from the southwest. Relief and the thermal low over northwest India modify their direction, and they approach the land in two branches.
The Arabian Sea branch splits into three.
- Western Ghats sub-branch. The winds climb the slopes of the Western Ghats (900–1,200 m), cool, and give the windward Sahyadris and the western coastal plain 250–400 cm of rain. Descending the eastern side, they warm and lose humidity, leaving a rain-shadow east of the Ghats. NCERT asks students to compare Kozhikode and Mangalore with Pune and Bengaluru.
- Narmada–Tapi sub-branch. It strikes the coast north of Mumbai and moves along the Narmada and Tapi valleys, bringing rain to large parts of central India. The Chotanagpur plateau gets 15 cm from it. It then enters the Ganga plains and mingles with the Bay of Bengal branch.
- Saurashtra–Kachchh sub-branch. It strikes Saurashtra and Kachchh, then passes over west Rajasthan and along the Aravallis, giving only scanty rain. In Punjab and Haryana it too joins the Bay of Bengal branch. Together they bring rain to the western Himalaya.
The Bay of Bengal branch. It strikes the coast of Myanmar and southeast Bangladesh, but the Arakan Hills along the Myanmar coast deflect a large part of it towards India. It therefore enters West Bengal and Bangladesh from the south and southeast, not the southwest. Under the influence of the Himalayas and the thermal low in the northwest it splits again. One arm moves west along the Ganga plains as far as Punjab. The other moves up the Brahmaputra valley, bringing widespread rain, and a sub-branch strikes the Garo and Khasi hills of Meghalaya. Mawsynram, on the crest of the Khasi hills, receives the highest average annual rainfall in the world.
Source: NCERT kegy104, pp. 35–37.
Why the Tamil Nadu coast stays dry in June–September. NCERT gives two reasons: the coast lies parallel to the Bay of Bengal branch, and it lies in the rain-shadow of the Arabian Sea branch. IMD's normals show the result: Tamil Nadu gets 48% of its annual rainfall in the northeast monsoon season (October–December) and 36% in the southwest monsoon season (IMD, 14 April 2022).
The Season of Retreating Monsoon
October and November are the months of the retreating monsoon. By the end of September the southwest monsoon weakens as the low-pressure trough of the Ganga plain moves south with the sun. NCERT's sequence: the monsoon retreats from western Rajasthan by the first week of September and from Rajasthan, Gujarat, the western Ganga plain and the Central Highlands by the end of the month. By early October the low pressure covers the northern Bay of Bengal, by early November it is over Karnataka and Tamil Nadu, and by mid-December its centre has left the Peninsula.
IMD's current normals (Table 3) put the start of withdrawal from western Rajasthan later, at 17 September, and the final withdrawal of the southwest monsoon from south India at 15 October, after which the northeast monsoon sets in. NCERT's mid-December date is the departure of the low-pressure centre, not the withdrawal date of the southwest monsoon, so the two do not conflict.
The retreating season has clear skies and rising temperatures over still-moist land. The heat and humidity make the weather oppressive, a condition known as the "October heat". In the second half of October temperatures fall quickly, especially in the north. The season is dry in north India but wet on the eastern Peninsula, where October and November are the rainiest months.
Cyclones. The widespread rain of this season comes from cyclonic depressions that originate over the Andaman Sea and cross the east coast of the southern Peninsula. NCERT names the thickly populated deltas of the Godavari, Krishna and Kaveri as their preferred targets; a few also strike West Bengal, Bangladesh and Myanmar. Much of the Coromandel coast's rain comes from these depressions and cyclones. They are less frequent in the Arabian Sea.
Post-monsoon and pre-monsoon cyclones: dated IMD examples.
- Post-monsoon (October–December): Very Severe Cyclonic Storm Phailin crossed the Odisha coast near Gopalpur around 22:30 IST on 12 October 2013. Hudhud crossed the north Andhra Pradesh coast over Visakhapatnam between 12:00 and 13:00 IST on 12 October 2014.
- Pre-monsoon (April–May): the Bay of Bengal also has a pre-monsoon cyclone season. Extremely Severe Cyclonic Storm Fani crossed the Odisha coast close to Puri between 08:00 and 10:00 IST on 3 May 2019. Super Cyclonic Storm Amphan crossed the West Bengal–Bangladesh coast across the Sundarbans as a very severe cyclonic storm between 15:30 and 17:30 IST on 20 May 2020.
Sources: IMD/RSMC New Delhi reports on Phailin, Hudhud and Fani; IMD report on Amphan as released by PIB, June 2020 (links in Sources below).
Monsoons and the Economic Life of India
NCERT lists eight links between the monsoon and the economy. In summary:
- The monsoon is the axis of India's agricultural cycle. NCERT puts the share of people depending on agriculture at about 64% (the text gives no year) and notes that agriculture itself depends on the southwest monsoon.
- Except in the Himalayas, temperatures everywhere are above the threshold for growing crops all year.
- Regional variations in climate allow a wide variety of crops.
- Rainfall variability brings drought or flood to some part of the country every year.
- Agricultural prosperity depends on timely and well-distributed rain; failure hurts most where irrigation is undeveloped.
- A sudden monsoon burst causes soil erosion over large areas.
- Winter rain from temperate cyclones in north India is highly beneficial for rabi crops.
- Regional climatic variation shows in the variety of food, clothes and house types.
Global Warming (NCERT)
NCERT notes that climate has changed in the past at global and local levels and is changing now. Besides natural causes, large-scale industrialisation and polluting gases add to global warming. Carbon dioxide from burning fossil fuels is the main concern. Methane, chlorofluorocarbons and nitrous oxide are present in much smaller concentrations, but NCERT notes that they absorb long-wave radiation better than carbon dioxide; together these are the greenhouse gases. The mean annual surface temperature of the earth has risen over the past 150 years. NCERT's stated projections are a rise of about 2 °C in global temperature by 2100 and an average sea-level rise of 48 cm by the end of the twenty-first century. It expects more annual flooding, more insect-borne diseases such as malaria, and shifting climatic boundaries that make some regions wetter and others drier. These are the textbook's figures. For the exam, quote them as NCERT's projections, not as current IPCC estimates.
PART 3 — UPSC Integration
Cross-paper relevance
- GS1 (Geography): monsoon mechanism, seasons, regional rainfall, El Niño, local winds. Bank questions: gs1-pyq-2017-17 (monsoon climate and Monsoon Asia), gs1-pyq-2015-18 (monsoon behaviour and the humanised landscape), gs1-pyq-2014-10 (El Niño), gs1-pyq-2023-11 ("Purvaiya" in Bhojpur).
- GS3 (Disaster management, Environment): urban floods from high-intensity rain (gs3-pyq-2016-55), effects of climate change on Himalayan and coastal states (gs3-pyq-2017-37), global warming and greenhouse gases (gs3-pyq-2022-18), sea-level rise in the Indian Ocean region (gs3-pyq-2023-05).
- Prelims: the 2026 GS1 paper asked about the climate of the Andaman and Nicobar Islands, including whether they get rain from both monsoons (prelims-2026-gs1-023).
Southwest Monsoon vs Western Disturbances
| Feature | Southwest monsoon | Western disturbances |
|---|---|---|
| Season | June–September | Cold weather season |
| Origin | Southeast trades of the southern hemisphere, drawn across the equator by the low over northwest India | Shallow cyclonic depressions from the east Mediterranean |
| Path | Arabian Sea and Bay of Bengal branches | Across West Asia, Iran, Afghanistan and Pakistan |
| Area affected | Most of India | Punjab, Haryana, Delhi, western Uttar Pradesh; snow in the lower Himalaya |
| Use | Kharif agriculture | Rabi crops; snow feeds Himalayan rivers in summer |
Source: NCERT kegy104, pp. 30–35.
Two Frames for Mains Answers
- Rhythm and variability. Most of the year's rain comes in four months (74.9% in June–September, IMD 1971–2020), and both onset and breaks vary from year to year. This is why NCERT links rainfall failure to the absence of irrigation.
- Relief decides where rain falls. The Western Ghats rain-shadow, the Aravallis lying along the path of the Saurashtra–Kachchh sub-branch, the Arakan Hills turning the Bay branch, and the Tamil Nadu coast lying parallel to it explain most of the rainfall map.
Exam Strategy
Prelims fact-traps:
- Onset dates: Kerala 1 June in both old and new normals. "Covers the whole country" is 8 July in IMD's normals since 2020 but "mid-July" (15 July) in NCERT. Read the question for which source it follows.
- Withdrawal: begins from western Rajasthan (17 September now; first week of September in NCERT) and is complete over south India by 15 October. The "mid-December" date in NCERT is the low-pressure centre leaving the Peninsula.
- Retreating SW monsoon vs NE monsoon: the retreating southwest monsoon season is October–November (NCERT). The northeast monsoon season used by IMD is October–December and sets in after the southwest monsoon withdraws. Tamil Nadu gets 48% of its rain in October–December.
- Why the Tamil Nadu coast is dry in June–September: two reasons, not one. The coast is parallel to the Bay of Bengal branch and in the rain-shadow of the Arabian Sea branch.
- Nor'westers: Bengal and Assam (Bardoisila in Assam), useful for tea, jute and rice.
- Breaks: north India (few storms along the trough) and the west coast (winds parallel to the coast) have different causes.
- Cross-equatorial flow: 40°E–60°E. Easterly jet: along 15°N. ITCZ in July: 20°N–25°N.
- Average annual rainfall: NCERT about 125 cm; IMD 1971–2020 normal 116 cm (1,160.1 mm).
- El Niño: about once every three to seven years (NCERT). An El Niño year can still have normal rain (1997: +2%).
Mains patterns in the bank: characteristics of the monsoon climate (2017), human influence on monsoon behaviour (2015), how far El Niño explains unusual weather (2014), and the cultural effect of a seasonal wind (2023). The usual demand is mechanism plus regional consequence, with an Indian example.
Practice Questions
- UPSC CSE Mains 2017, GS Paper I (gs1-pyq-2017-17), 15 marks: "What characteristics can be assigned to monsoon climate that succeeds in feeding more than 50 percent of the world population residing in Monsoon Asia?"
- UPSC CSE Mains 2023, GS Paper I (gs1-pyq-2023-11), 10 marks: "Why is the South-West Monsoon called 'Purvaiya' (easterly) in Bhojpur Region? How has this directional seasonal wind system influenced the cultural ethos of the region?"
- UPSC CSE Mains 2014, GS Paper I (gs1-pyq-2014-10), 10 marks: "Most of the unusual climatic happenings are explained as an outcome of the El Nino effect. Do you agree?"
- Practice (UPSC-pattern, not a past paper): Consider the following statements: (1) The southwest monsoon is a continuation of the southeast trade winds deflected after crossing the equator. (2) Under IMD's normals in use since 2020, the monsoon covers the whole of India by 15 July. (3) Dry spells on the west coast during the monsoon are associated with winds blowing parallel to the coast. Which are correct? (Answer: 1 and 3. Statement 2 is the old normal; the current IMD normal is 8 July.)
- Practice (UPSC-pattern, not a past paper): Explain why the Coromandel coast receives most of its rainfall in October–November while the rest of India gets its rain in June–September.
📦 Revision Capsule
Hard Facts
- Factors (NCERT): latitude, Himalaya, land–water distribution, distance from sea, altitude (Agra 16 °C vs Darjiling 4 °C in January), relief
- ITCZ in July: 20°N–25°N over the Gangetic plain (monsoon trough); SE trades cross the equator between 40°E and 60°E
- Westerly jet withdraws from south of the Himalaya; easterly jet then sets in along 15°N and is held responsible for the burst (NCERT)
- Onset: Kerala 1 June; whole country 8 July (IMD normal since 2020; NCERT/old: mid-July, 15 July)
- Withdrawal: western Rajasthan 17 September (IMD; NCERT/old: first week of September); complete over south India 15 October
- Rainfall normal (IMD 1971–2020): annual 1,160.1 mm; June–September 868.6 mm = 74.9%; NCERT average about 125 cm
- Cherrapunji/Mawsynram over 1,080 cm; Jaisalmer rarely over 9 cm; Mawsynram has the highest average annual rainfall in the world (NCERT)
- Arabian Sea branch: three sub-branches (Western Ghats 250–400 cm; Narmada–Tapi, Chotanagpur 15 cm; Saurashtra–Kachchh–Aravallis scanty)
- Bay branch: deflected by the Arakan Hills, enters West Bengal from the south/southeast
- Tamil Nadu: 48% of annual rain in October–December, 36% in June–September (IMD)
- Nor'westers: Kalbaisakhi (Bengal), Bardoisila (Assam); mango showers (Kerala, coastal Karnataka); blossom showers (coffee)
- Southern Oscillation: pressure difference Tahiti (about 20°S, 140°W) – Port Darwin (12°30'S, 131°E)
Core Concepts
- Monsoon = seasonal reversal of winds caused by differential heating of land and sea
- The southwest monsoon is the southeast trade wind pulled north of the equator and turned by the Coriolis force
- Relief decides where the rain falls: windward slopes wet, leeward rain-shadow dry
- El Niño raises the risk of a weak monsoon but does not decide it
Confused Pairs
- Retreating SW monsoon (October–November) vs NE monsoon season (October–December, IMD)
- Burst (abrupt onset) vs break (dry spell of a week or more within the season)
- IMD withdrawal complete 15 October vs NCERT low-pressure centre leaves the Peninsula by mid-December
- Western disturbances (winter, Mediterranean origin) vs monsoon rain (summer, Indian Ocean origin)
Data Points
- Seasonal shares (IMD 1971–2020): winter 3.4%, pre-monsoon 11.3%, SW monsoon 74.9%, post-monsoon 10.4%
- 1997: El Niño year, monsoon 2% above normal; 2015: 86% of LPA (IMD)
PYQ Pattern
- Mains GS1: monsoon climate (gs1-pyq-2017-17), monsoon behaviour (gs1-pyq-2015-18), El Niño (gs1-pyq-2014-10), Purvaiya (gs1-pyq-2023-11); GS3 links on floods and climate change
- Prelims 2026: climate of Andaman and Nicobar Islands, rain from both monsoons (prelims-2026-gs1-023)
Sources
Sources
- NCERT, India — Physical Environment, Textbook in Geography for Class XI, Chapter 4 "Climate", rationalised edition, Reprint 2026-27 — kegy104.pdf.
- India Meteorological Department, "New Normal Dates of Onset/Progress and Withdrawal of Southwest Monsoon over India" (in use from 1 June 2020) — IMD PDF.
- India Meteorological Department, Press Release "Updated Rainfall Normal based on data of 1971-2020", New Delhi, 14 April 2022 — IMD PDF.
- Asnani, G.C., "El Nino of 1997-1998 and Indian Monsoon", MAUSAM 52(1), 2001 (IMD journal) — article page.
- India Meteorological Department, 2015 Southwest Monsoon End of Season Report — PDF (copy hosted on the NOAA/NCEP monsoon desk site).
- IMD/RSMC New Delhi, report on Very Severe Cyclonic Storm Phailin (October 2013) — PDF; report on Very Severe Cyclonic Storm Hudhud (October 2014) — PDF; report on Extremely Severe Cyclonic Storm Fani (April–May 2019) — PDF.
- PIB, "IMD releases a report on Super Cyclonic Storm 'Amphan' that crossed West Bengal coast during 16th–21st May, 2020…" — PIB.
- Mains PYQ texts: BharatNotes Mains GS1 and GS3 question banks (UPSC CSE Mains 2014, 2015, 2016, 2017, 2022, 2023); Prelims: BharatNotes Prelims 2026 GS1 bank.
