Climate

Weather is the state of the atmosphere at a moment; climate is the average of weather over a long period. Weather can change within a day; climate changes imperceptibly and may be noticed only after 50 years or more. India has a hot monsoon climate, the climate of South and South-East Asia, in which the direction of winds reverses with the seasons.

• Builds on Chapters 1–3: latitude and the Tropic of Cancer, the Himalaya and the Western Ghats, and the rain-fed rivers.
• Leads into Chapter 5 (Natural Vegetation follows rainfall and temperature) and Chapter 6 (cyclones, floods, droughts).
• Physical Geography links: the ITCZ, jet streams, El Niño and cyclones are explained in Chapter 9; rain-shadow rainfall in Chapter 10; climate types in Chapter 11. This chapter applies them to India.
• UPSC: Prelims ask about seasonal rain, local storms, rainfall regions and the monsoon's drivers; GS1 Mains asks about monsoon behaviour and its cultural and economic role; GS3 about drought, floods and climate change.

• Monsoon = seasonal reversal of winds; India's climate is monsoonal in rhythm despite regional variety.
• Six controls: latitude · Himalaya · land–water distribution · distance from sea · altitude · relief.
• Onset: heat low in NW India + ITCZ shifts to 20°–25°N + easterly jet at 15°N once the westerly jet withdraws.
• Reaches Kerala by 1 June, Mumbai and Kolkata 10–13 June, the whole country by mid-July.
• Four IMD seasons: cold weather · hot weather · SW monsoon · retreating monsoon; Tamil Nadu's rain comes in Oct–Nov.
• Average rainfall ≈ 125 cm; over 200 cm on the west coast and in the north-east, under 50 cm in W Rajasthan, Ladakh and the rain-shadow Deccan.

The monsoon regime stresses India's unity with South-East Asia, but there are big regional variations in winds, temperature, rainfall, the rhythm of seasons and wetness. These can be called sub-types of the monsoon climate.

• Snow falls in the Himalaya; elsewhere precipitation is rain.
• In July–August the Ganga delta and coastal Odisha are hit by rain-bearing storms every third to fifth day, while the Coromandel coast, a thousand km south, stays largely dry.
• Most of India gets rain in June–September; coastal Tamil Nadu gets it at the start of winter.

No single theory fully explains the monsoon; a breakthrough came when it was studied at the global rather than regional level. In the older view (late 19th century), differential heating sets the stage: in April–May, when the sun is overhead near the Tropic of Cancer, the land north of the Indian Ocean heats up, an intense low forms over the north-west, and it attracts the south-east trades across the equator, helping the ITCZ shift north. The south-west monsoon is thus a continuation of the south-east trades, deflected towards India after crossing the equator between 40°E and 60°E.

Jet streams. The ITCZ's shift is linked to the withdrawal of the westerly jet stream from over the north Indian plain, south of the Himalaya. Only after it withdraws does the easterly jet stream set in along about 15°N — and that easterly jet is held responsible for the burst of the monsoon.

Break in the monsoon. If, after some days of rain, rain fails for one or more weeks, that is a break. In northern India breaks come when rain-bearing storms along the monsoon trough/ITCZ are infrequent; on the west coast when winds blow parallel to the coast.

• Temperature: sets in by mid-November; December–January coldest in the north; mean daily temperature below 21°C over most of the north; night frost possible in Punjab and Rajasthan.
• Why so cold in the north: (i) far from the sea — continental climate; (ii) snowfall on nearby Himalaya — cold waves; (iii) around February, cold winds from the Caspian Sea and Turkmenistan bring cold waves, frost and fog to the north-west.
• The Peninsula has no well-defined cold season; coastal temperatures hardly change.
• Pressure and winds: by 22 December the sun is over the Tropic of Capricorn; a feeble high over the northern plain (1019 mb isobar in the north-west, 1013 mb in the far south). Light winds (3–5 km/h) blow outwards: westerly or north-westerly down the Ganga valley, northerly in the Ganga–Brahmaputra delta, north-easterly over the Bay of Bengal.

Rainfall. Winter monsoons blow from land to sea, carry little moisture and come from an anticyclonic circulation, so most of India is dry in winter. Exceptions: (i) western disturbances give light rain to Punjab, Haryana, Delhi and western UP — valuable for rabi crops — and snow on the lower Himalaya; (ii) occasional rain in central India and the northern Peninsula; (iii) 25–50 mm in Arunachal Pradesh and Assam; (iv) in October–November the north-east monsoon, picking up moisture over the Bay of Bengal, gives torrential rain to the Tamil Nadu coast, southern Andhra Pradesh, south-east Karnataka and south-east Kerala.

• South India is milder: 26–32°C; Western Ghats hills below 25°C. Along the coasts isotherms run parallel to the coast — temperature rises from the coast inland, not from south to north. Summer night minimums rarely fall below 26°C.
• Pressure and winds: the ITCZ moves north to about 25°N in July; the monsoon trough stretches from the Thar to Patna and the Chotanagpur plateau. Winds are south-westerly on the west coast and along West Bengal–Bangladesh, easterly or south-easterly over north Bengal and Bihar — ‘displaced’ equatorial easterlies.
• Loo: hot, dry afternoon winds in the heart of the ITCZ in the north-west; dust storms in May evenings in Punjab, Haryana, E Rajasthan and UP bring brief relief. Where dry and moist air meet, violent local storms with hail form.

By early June the intensified low over the north-west attracts the southern trades, which cross the equator, pick up moisture over warm equatorial waters and arrive from the south-west. The rain begins abruptly — the burst — in the first week of June on the coasts of Kerala, Karnataka, Goa and Maharashtra, and as late as the first week of July in the interior. Day temperatures fall 5–8°C between mid-June and mid-July. The flow is shaped by relief and the north-western low into two branches.

• Arabian Sea branch: (i) climbs the Western Ghats (900–1,200 m) → 250–400 cm on the windward side, a rain shadow to the east; (ii) strikes north of Mumbai, moves up the Narmada and Tapi valleys into central India (Chotanagpur gets 15 cm from it), then mingles with the Bay branch; (iii) crosses Saurashtra and Kachchh and west Rajasthan along the Aravali with scanty rain, joining the Bay branch in Punjab–Haryana; together they bring rain to the western Himalaya.
• Bay of Bengal branch: hits Myanmar and south-east Bangladesh; the Arakan Hills deflect much of it to India, so it enters West Bengal and Bangladesh from the south and south-east. It splits: west up the Ganga plain to Punjab; north and north-east up the Brahmaputra valley; a sub-branch strikes the Garo and Khasi hills — Mawsynram.

• October–November. By end-September the Ganga plain's trough moves south; the monsoon leaves western Rajasthan by the first week of September, and Rajasthan, Gujarat, the western Ganga plain and the Central Highlands by month-end. The low reaches the northern Bay by early October, Karnataka and Tamil Nadu by early November, and leaves the Peninsula by mid-December.
• October heat: clear skies, rising temperature, moist land → oppressive weather; temperatures fall fast in the north from the second half of October.
• In the eastern Peninsula October–November are the rainiest months — from cyclonic depressions born over the Andaman Sea that cross the east coast. The crowded Godavari, Krishna and Kaveri deltas are their favourite targets; most of the Coromandel coast's rain comes from them. Cyclones are less frequent in the Arabian Sea.

• Monsoon is the axis of India's agricultural cycle (NCERT: about 64% of people depend on agriculture — an old figure; Beyond NCERT: Census 2011 put agriculture's share of workers at about 55%).
• Except in the Himalaya, temperatures allow crops all year; regional variety allows varied crops.
• Variability brings floods or droughts somewhere every year; prosperity depends on timely, well-spread rain, especially without irrigation.
• A sudden burst causes soil erosion; winter rain from temperate cyclones helps rabi crops; climate shows in food, clothes and houses.

• Climate has always changed (evidence of past ice ages); now human activities — industrialisation and polluting gases — add to natural causes.
• Carbon dioxide from burning fossil fuels is the main concern; methane, CFCs and nitrous oxide, though in smaller amounts, absorb long-wave radiation more effectively per molecule.
• Mean surface temperature has risen over the past 150 years. NCERT's projections: about +2°C by 2100 and sea level +48 cm by the end of the century — more flooding, more insect-borne disease (malaria), shifting climatic boundaries and farming patterns.

Try each question yourself first, then open the outline to compare.

Numbers

One-liners

• Monsoon = seasonal reversal of winds.
• SW monsoon = SE trades that cross the equator and turn right.
• Westerly jet withdraws → easterly jet at 15°N → burst of the monsoon.
• Break: few storms on the trough (north) or winds parallel to the coast (west).
• Arabian Sea branch: three arms; Bay branch: Ganga plain and Brahmaputra valley.
• Tamil Nadu: dry June–September, wettest October–November.
• Western disturbances: east Mediterranean → NW India → rabi rain and Himalayan snow.
• Mango shower (Kerala), blossom shower (coffee), Kalbaisakhi/Bardoisila (Bengal/Assam), Loo (Punjab–Bihar).

Traps to avoid

• The monsoon reaches the Kerala coast first, not West Bengal.
• Winter rain in Chennai = NE monsoon; in Punjab = western disturbances.
• Rainfall falls from east to west along the Ganga plain.
• Bengaluru is in the rain shadow; Mangaluru gets several times more rain.
• El Niño = warm water replacing the cold Peruvian current (NCERT's box words it backwards).
• IOD compares the western and eastern Indian Ocean, not the Pacific.
• The easterly jet (≈ 15°N) arrives only after the westerly jet withdraws.
• Mawsynram and Cherrapunji are in Meghalaya (Khasi hills), not Assam.
• IMD's seasons are four; the traditional seasons are six.

Prelims — what UPSC asks

• Seasonal rain sources are asked again and again: Chennai's winter rain (1979, 1991), north India's winter rain (1982), Punjab–Haryana (1989), westerlies and western disturbances (2015), the monsoon's onset (1991), the south-west monsoon itself (1996).
• Spatial patterns: areas over 200 cm (1986), Ajmer vs Shillong (1993), east-to-west decrease in the Ganga plain (1994, 2012), Bengaluru vs Mangaluru (2004), cities in order of rainfall (2005), the rain shadow of the Western Ghats (1989), isohyets (1985), the share of area above 750 mm (2002).
• Drivers and forecasting: El Niño/ENSO and weak monsoons (2002), the Indian Ocean Dipole (2017), the monsoon as India's climate type (1989) and seasonal reversal (2014), anticyclonic winters (2001 A–R), climatic conditions matched to causes (1997).
• Regions: the 2026 question on the Andaman & Nicobar climate (humid tropical coastal; rain from both monsoons) tests the same reasoning as Tamil Nadu's two-season rain — learn which coast gets which monsoon.

Mains — where it appears and how to answer

• GS1: changing monsoon behaviour due to ‘humanising landscapes’ — urbanisation, aerosols, land-use change (2015); ‘Purvaiya’ in Bhojpur and the monsoon's cultural imprint (2023); why monsoon climate feeds over half of the world's people (2017); Loo vs Chinook vs Foehn (2026).
• Answer frame: mechanism (heat low, ITCZ, trades crossing the equator, jets, ENSO/IOD) → spatial and temporal pattern with NCERT numbers → economic and cultural impact (agriculture, rabi/kharif, festivals) → variability and risk (droughts, floods, climate change) → adaptation (forecasting, irrigation, crop planning).
• Sketches that earn marks: the monsoon branches flow, the Western Ghats rain-shadow section, and an onset timeline.

Tap an option. NCERT's own exercise questions come first; the rest test the figures and common confusions.

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