Climate of India and the Monsoon

1. Weather, climate and the monsoon: unity and diversity

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• Weather is the state of the air at one moment: temperature, pressure, wind, humidity and rain. Climate is the average weather over a long time. Class 11 says climate change shows only after 50 years or more. Class 7 and Class 9 use 30 years or more.
• A monsoon is a seasonal reversal of winds (the winds change direction with the season). The word comes from the Arabic mausim, "season". Arab sailors named it because they used the wind reversal to sail to India.
• A monsoon climate has wet summers, dry winters and a seasonal wind reversal. India has a hot monsoon climate, like the rest of South and Southeast Asia. Class 7 notes that Australia and Africa also have monsoon systems.
• Unity and diversity of the monsoon climate: the whole country follows one monsoon rhythm, but the regions are very different.
• Temperature: 55°C in western Rajasthan vs −45°C around Leh. Churu can touch 50°C on a June day while Tawang reaches only 19°C. On a December night Drass can be at −45°C while Chennai is at 20-22°C.
• Daily range: only 7-8°C in Kerala and the Andamans, but 50°C by day and 15-20°C by night in the Thar.

• Rain: over 1,080 cm at Mawsynram and Cherrapunji vs under 9 cm at Jaisalmer. Tura (Garo hills) can get Jaisalmer's ten-year rainfall in a single day. Tamil Nadu gets its rain at the start of winter.

• All the same, India's climate is "monsoonal in rhythm and character" (Class 11). Class 9's station table asks why Shillong is wetter than Kolkata and why Chennai's rain peaks late.

2. Factors of India's climate: location and relief

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• Factors determining India's climate: latitude, the Himalayas, land-water distribution, distance from the sea, altitude and relief. Pressure, winds, jets and ocean links also play a part (Sections 3-5).
• Latitude: the Tropic of Cancer runs through the middle of India.
• The south is tropical: hot all year, with a small daily and yearly range.

• The north is sub-tropical and temperate: an extreme climate with a large range.

• Himalayas as a climatic divide: the range blocks cold Arctic and Central Asian winds in winter. It also traps the monsoon winds, so they drop their moisture inside India. Beyond NCERT: without the Himalayas, north India would have Central Asian winters.

• Differential heating of land and water: land heats and cools faster than the sea. In summer a low (an area of low air pressure) forms over the land and a high over the ocean. In winter it is the reverse. This flips the wind direction.
• Distance from the sea: coasts are mild and the interior is extreme. Mumbai's yearly range is about 14°C, while Nagpur's is about 34°C (Class 7).
• Altitude: temperature falls with height. In January, Agra is at 16°C but Darjiling is at 4°C, though both are on the same latitude. Similarly, Ooty is 10-25°C while Coimbatore is 25-38°C.
• Relief: the windward Western Ghats and Assam are wet, while the Deccan, in the lee, is dry. Class 7 also covers microclimates, such as urban heat islands (cities warmer than the land around them).

3. Pressure, winds and the upper air: the ITCZ, jet streams, Tibet and the Mascarene High

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• The Inter Tropical Convergence Zone (ITCZ) is a low-pressure belt near the equator. Here the trade winds of both hemispheres meet and air rises, giving calm winds (the doldrums) and convectional rain (rain from rising warm air).
• In July it moves north to about 20°-25°N over the Ganga plain.
• Southern trade winds then cross the equator between 40° and 60°E. The Coriolis force (the turning effect of Earth's spin) bends them, and they arrive as the southwest monsoon.

• In winter the ITCZ moves south, and the winds reverse to the northeast monsoon.

• The monsoon trough is a long low-pressure belt marking the ITCZ over north India. It runs from the Thar to Patna and the Chotanagpur plateau. Storms along it bring rain. When the trough is weak or shifts, the rain breaks.

• Jet streams (fast, narrow winds high in the air):
• In winter the subtropical westerly jet lies south of the Himalaya. It steers western disturbances.
• In early summer the westerly jet moves north of Tibet. Only after that does the tropical easterly jet set in along about 15°N. This jet is "held responsible for the burst".

• Beyond NCERT: the low-level Somali (Findlater) jet carries moisture to the west coast.

• Tibetan Plateau heating and the monsoon (beyond NCERT): the plateau, about 4,000 m high, heats the upper air in summer. This creates high pressure aloft, which helps form the easterly jet and strengthens the monsoon (Koteswaram, 1958). Heavy spring snow over Eurasia has long been linked with a weaker monsoon.

• The Mascarene High is a semi-permanent high near Mauritius in the southern Indian Ocean. When it is strong, the flow across the equator is stronger and the monsoon is more vigorous.

4. How the monsoon works: theories, onset, burst and advance

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• Monsoon theories:
• Halley (1686), thermal theory: the monsoon is a giant sea breeze driven by the land-sea heating contrast.
• Flohn (1951), dynamic theory: the ITCZ and the planetary winds shift with the sun through the year.
• 1950s, jet stream and Tibet theories: the upper-air controls explained in Section 3.
• Later, teleconnections: links with distant oceans, such as ENSO and the IOD (Section 5).

• Class 11 says no single theory explains the monsoon fully; studying it at the global scale was the real breakthrough.

• Ancient Indian observation (Class 9): Krishiparashara (positions of the sun and moon); Varahamihira's Brihatsamhita (lunar mansions); Kalidasa's Meghadutam, around the 5th century CE (onset date and cloud path); Kautilya's Arthashastra (measuring rainfall for revenue and relief). Class 7 adds folk signs: fish surfacing off the Konkan coast, the Golden Shower tree, and crows' nests.

• Onset of the monsoon (late 19th-century view):
• In April-May the land north of the Indian Ocean heats up and an intense low forms over the northwest.
• The cooler ocean keeps higher pressure, so the low pulls the southeast trades across the equator.

• Turned by the Coriolis force, these trades become the southwest monsoon.

• Burst of the monsoon: heavy rain starts suddenly with thunder and lightning. It reaches the Kerala-Maharashtra coasts in the first week of June and the interior by the first week of July. Day temperatures fall 5-8°C.

• Normal arrival dates: Kerala on 1 June; Mumbai and Kolkata on 10-13 June. The whole country is covered by about 8 July (NCERT: mid-July; IMD revised this in 2020). In 2025 the monsoon reached Kerala on 24 May, the earliest since 2009 (verify current).

5. Teleconnections: El Nino, the Southern Oscillation, the Indian Ocean Dipole and the MJO

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A teleconnection is a link between weather in faraway places.

• El Nino comes every 3-7 years. Warm water appears off Peru around Christmas, which is why it is called "Christ Child". The Peru coast warms by up to 10°C, fish catches fall, and the equatorial circulation is distorted.
• Correction to NCERT: El Nino does not "replace" the Humboldt current. Weaker trade winds let warm western-Pacific water spread east, which stops the cold upwelling (cold deep water rising to the surface).
• The 1990-91 El Nino delayed India's onset by 5-12 days.

• Beyond NCERT: El Nino years were often drought years (2002, 2009, 2015), but 1997 was normal. La Nina (a cool Pacific) usually helps the monsoon.

• Southern Oscillation: air pressure rises in one place as it falls in the other, between Tahiti (about 20°S, 140°W) and Darwin (12°30'S, 131°E). The pressure difference tracks monsoon strength. El Nino and the Southern Oscillation together are called ENSO.

• The Indian Ocean Dipole (IOD; Saji et al., 1999) is a difference in sea temperature between the western Arabian Sea and the eastern Indian Ocean near Sumatra.
• A positive IOD (warmer west) usually helps the monsoon and can cancel out El Nino, as in 1997 and 2019.

• A negative IOD hurts the monsoon.

• The Madden-Julian Oscillation (MJO; Madden and Julian, 1971-72) is a pulse of cloud and rain that moves east around the tropics every 30-60 days.

• When it is active over the Indian Ocean, monsoon spells are active and onset or cyclones can be triggered.
• When it is absent, breaks are likely.

6. The rhythm of seasons and the cold weather season

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• Seasons of India: the IMD recognises four seasons. The month ranges differ slightly between textbooks.

IMD season	Class 11	Class 9
Cold weather	Dec-Feb	Dec-early April
Hot weather / pre-monsoon	Mar-May	April-June (to July in NW)
Southwest monsoon	June-Sept	June-Sept
Retreating / post-monsoon	Oct-Nov	Oct-Dec

• Traditional Indian seasons: Vasanta (March-April), Grishma (May-June), Varsha (July-August), Sharada (September-October), Hemanta (November-December), Shishira (January-February). They fit north and central India but not the south.
• Cold weather season: it begins by mid-November in the north. December and January are coldest, with mean temperatures below 21°C and frost in Punjab and Rajasthan. There are three reasons:
• continentality (being far from the sea's moderating effect)
• snow on the nearby Himalaya

• cold February winds from the Caspian Sea and Turkmenistan

• The south has no well-defined cold season. NCERT gives Thiruvananthapuram's January mean maximum as "21°C" against 29.5°C in June. This is probably a misprint for about 31°C; the point is that January is warmer there.

• Pressure and winds: a weak high (1019 mb isobar) lies over the northwest, and light offshore winds blow at 3-5 km/h. They are westerly in the Ganga valley and northeasterly over the Bay of Bengal.
• Western disturbances are shallow lows from the east Mediterranean, steered by the westerly jet across Iran and Pakistan. They bring light rain to the northwest (Delhi about 53 mm), which is good for rabi wheat, and Himalayan snow that feeds rivers in summer. The rain decreases from west to east in the plains.

7. The hot weather season and its local storms

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• Hot weather season: the heat belt follows the sun north. It is about 38°C on the Deccan in March, 38-43°C in Gujarat and Madhya Pradesh in April, and about 48°C in the northwest in May.
• The south is milder, at 26-32°C. On the coasts, temperature rises from the coast inland rather than from north to south.
• Pressure and winds: pressure falls in the north and the ITCZ moves north. Evening dust storms in May (Punjab, Haryana, eastern Rajasthan, Uttar Pradesh) bring brief relief. Where moist and dry air meet, violent storms with hail form.

Local storm	Where	Note
Loo	Northern plains, Punjab to Bihar; strongest Delhi-Patna	Hot, dry afternoon wind, May-June; causes heatstroke
Mango showers	Kerala, coastal Karnataka	End-of-summer showers; ripen mangoes early
Blossom showers	Kerala and nearby Karnataka	Coffee flowers bloom (cherry or coffee showers)
Nor'westers	Bengal, Assam	Evening thunderstorms: Kalbaisakhi ("calamity of Baisakh") in Bengal, Bardoisila in Assam; help tea, jute and rice

• Beyond NCERT: heat waves are becoming more frequent (verify current IMD data). Heat-wave policy is covered in natural-hazards-disasters.

8. The southwest monsoon season: the two branches, rain shadow and breaks

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• Southwest monsoon season (June-September): the strong northwest low pulls the southern trades across the equator. They pick up moisture over warm seas and arrive as two branches. This season brings about three-quarters of India's yearly rain.
• The Arabian Sea branch of the monsoon splits into three:
• Western Ghats arm: it rises 900-1,200 m and gives 250-400 cm on the windward side. Descending, it warms and dries, creating a rain shadow area (a dry zone behind a mountain) over Pune and Bengaluru.
• Narmada-Tapi arm: it enters north of Mumbai and rains over central India. Chotanagpur gets 15 cm from it.

• Kachchh-Rajasthan arm: it runs parallel to the Aravallis, so it gives scanty rain.

• The Bay of Bengal branch of the monsoon:

• It strikes Myanmar and Bangladesh.
• The Arakan Hills turn it towards India, so it enters West Bengal from the south and southeast.

• It then splits: one arm goes west up the Ganga plain to Punjab, the other up the Brahmaputra valley.

• Mawsynram, on the crest of the Khasi hills, gets the world's highest average yearly rainfall. Class 7 gives about 11,000 mm; funnel-shaped valleys trap the moist winds. Nearby Cherrapunji (Sohra) is famous for its records, and the heavy rain has left its surface bare and rocky.

• Dry Tamil Nadu coast in the southwest monsoon: the coast lies parallel to the Bay branch and in the rain shadow of the Arabian Sea branch.
• Break in the monsoon: a dry spell of a week or more.
• In the north, it happens when few storms form along the trough, or when the trough shifts to the foothills.

• On the west coast, it happens when winds blow parallel to the coast.

• A monsoon depression is a low that forms over the head of the Bay and moves west-northwest across central India, bringing much of the season's rain.

9. The retreating monsoon and the northeast monsoon

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• Retreating monsoon: the trough moves south with the sun.
• Withdrawal starts from western Rajasthan, in the first week of September (NCERT) or about 17 September (IMD revised normal, 2020).
• By the end of September it has left Gujarat, the western Ganga plain and the Central Highlands.

• The low reaches Karnataka and Tamil Nadu by early November and has left the Peninsula by mid-December. IMD's revised normal date for leaving the country is about 15 October.

• October heat: skies clear and temperatures rise while the land is still wet. The heat and humidity together make the weather oppressive. It ends as temperatures fall in the second half of October, especially in the north.

• Northeast monsoon: dry offshore winds blow as the ITCZ moves south. They pick up moisture over the Bay of Bengal. In October-December they drench the Tamil Nadu coast, southern Andhra Pradesh, southeast Karnataka and southeast Kerala.
• Cyclones: cyclonic depressions from the Andaman Sea cross the east coast and hit the crowded Godavari, Krishna and Kaveri deltas. A few strike West Bengal, Bangladesh and Myanmar.
• These systems give the Coromandel coast most of its rain, which is why Chennai's wettest months are October-November.
• Cyclones are rarer in the Arabian Sea, though beyond NCERT they are increasing there. Cyclone mechanics are covered in air-masses-fronts-cyclones.

10. Distribution and variability of rainfall; climatic regions of India

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• Annual rainfall of India: the average is about 125 cm, and it varies greatly from place to place. About half of India gets less than 75 cm.

Zone	Areas
High (>200 cm)	West coast, Western Ghats, sub-Himalayan North-East, Meghalaya (>1,000 cm in parts of the Khasi-Jaintia hills)
Medium (100-200 cm)	South Gujarat, east Tamil Nadu, Odisha, Jharkhand, Bihar, east Madhya Pradesh, northern Ganga plain, Cachar, Manipur
Low (50-100 cm)	West Uttar Pradesh, Delhi, Haryana, Punjab, Jammu and Kashmir, east Rajasthan, Gujarat, Deccan
Inadequate (<50 cm)	Parts of Andhra Pradesh, Karnataka and Maharashtra; Ladakh; most of west Rajasthan

• The 100 cm isohyet (a line joining places with equal rainfall) roughly separates rice lands from wheat lands.
• Variability of rainfall: this is how much rain changes from year to year. It is under 25% on the west coast and in the North-East, but over 50% in west Rajasthan, Kachchh and the Thar (earlier edition of Class 11). Variability is highest where rain is least. As a result, some part of India faces drought or flood every year.
• Climatic regions of India:
• Class 7's seven types: alpine, temperate, subtropical, arid, tropical wet, semi-arid and tropical.
• Koeppen (earlier edition of Class 11): Amw (west coast south of Goa), As (Coromandel), Aw (most of the Peninsula), BShw (semi-arid northwest), BWhw (extreme west Rajasthan), Cwg (Ganga plain, most of the North-East), Dfc (Arunachal), E (Jammu and Kashmir, Himachal Pradesh, Uttarakhand).

11. The monsoon, the economy and forecasting

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• Monsoon and the Indian economy: the monsoon is the axis of the farm cycle.
• NCERT says 64% of people depend on farming. The current figure is about 46% of the workforce (PLFS 2023-24; verify current).
• The rain's timing and spread decide kharif output, rural incomes, reservoir levels and food prices.

• A sudden burst erodes soil, and winter rain helps rabi crops.

• Monsoon failure means a deficient season or a long break at sowing time. Class 7 lists its effects: crop loss, long walks for water (mostly by women), migration to cities and food inflation.

• Monsoon forecasting by IMD (the IMD was founded in 1875):
• NCERT mentions a 16-indicator model. That model was used from 1988 to 2002 and is now outdated.

• Since 2021 the IMD uses a Multi-Model Ensemble with dynamical models (computer models of the ocean and air). It gives a forecast in mid-April and an update at the end of May (verify current).

• The National Monsoon Mission (MoES, 2012, led by IITM Pune) built a coupled ocean-atmosphere model that forecasts at all time scales, from days to whole seasons.

• Mission Mausam (approved September 2024, about ₹2,000 crore; verify current) aims for a "Weather Ready, Climate Smart" India.
• The Long Period Average of monsoon rainfall (LPA) is the all-India June-September average used as the benchmark. It is about 87 cm (868.6 mm, based on 1971-2020).
• IMD monsoon rainfall categories (as a percentage of LPA):

Category	% of LPA
Deficient	below 90
Below normal	90-95
Normal	96-104
Above normal	105-110
Excess	above 110

• Recent seasons: 2023 was 94% of LPA (an El Nino year), and 2024 was 108%. Figures for 2025 and 2026 should be checked against IMD reports (verify current).

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Exam angles

Prelims — high-yield facts and traps

• The ITCZ lies at about 20°-25°N in July. The southeast trades cross the equator at 40°-60°E. The easterly jet runs at about 15°N and causes the burst. The westerly jet steers western disturbances.
• Normal dates: Kerala 1 June; Mumbai and Kolkata 10-13 June; whole country by about 8 July (2020 revision; NCERT says mid-July); withdrawal begins about 17 September; out of the country by about 15 October.
• Local storm matching: mango showers – Kerala and coastal Karnataka; blossom showers – coffee; Kalbaisakhi – Bengal; Bardoisila – Assam; loo – the Delhi-Patna belt.
• Branch facts: the Western Ghats get 250-400 cm; Pune and Bengaluru lie in the rain shadow; the Aravallis run parallel to the winds, so Rajasthan stays dry; the Arakan Hills turn the Bay branch towards India; Mawsynram gets the highest average rainfall.
• The Tamil Nadu coast is dry in June-September and wet in October-December. Statement "Tamil Nadu gets most rain from the SW monsoon" — FALSE.
• Traditional seasons in order: Vasanta, Grishma, Varsha, Sharada, Hemanta, Shishira.
• ENSO and other links: ENSO is measured by Tahiti-Darwin pressure. A positive IOD helps the monsoon. The MJO repeats every 30-60 days. The LPA is about 87 cm, and "normal" is 96-104%.
• "El Nino always causes drought" — FALSE (1997). "El Nino means the cold Humboldt current replaces warm water" — FALSE (it means warm water suppresses cold upwelling).
• Rainfall facts: the average is about 125 cm; about half of India gets under 75 cm; variability is over 50% in the Thar.
• Factor pairs: Agra vs Darjiling (altitude); Mumbai vs Nagpur (distance from the sea); Ooty vs Coimbatore (altitude).
• When the sun is over the Tropic of Capricorn, a high forms over northwest India.

Mains — GS-I themes

1. "In spite of regional variations, the climate of India is monsoonal in rhythm." Discuss with examples.
2. Explain the monsoon's mechanism: the ITCZ shift, jet streams, Tibetan heating and the Mascarene High.
3. Why does the Coromandel coast get its rain in winter while the rest of India gets it in summer?
4. How do El Nino, the IOD and the MJO affect the monsoon? How reliable are forecasts based on them?
5. Breaks in the monsoon and rainfall variability: their causes and their effects on farming and water.
6. Is the Indian monsoon changing (more extreme spells, fewer rainy days, shifting onset)?
7. The economy's dependence on the monsoon: weak points and ways to build resilience, such as irrigation, crop insurance and contingency plans (GS-III link).

Current-affairs hooks

• IMD's April and May forecasts, onset and withdrawal dates, and end-of-season rainfall as a percentage of LPA (verify current).
• El Nino, La Nina and IOD status in NOAA and IMD bulletins.
• Heat waves and heat action plans; extreme rain and urban floods; the 2025 Punjab floods, where western disturbances added to the monsoon rain.
• The rollout of Mission Mausam: radars, supercomputers and cloud-seeding research.
• Winter western-disturbance deficits and their effect on rabi crops; Himalayan cloudbursts.
• The monsoon's links with food inflation, kharif sowing and reservoir levels.