The monsoon

The first principle

A monsoon is a seasonal reversal of wind, and the rain comes with the wind that blows from the sea. It happens because land and sea store heat differently and because the belt of low pressure that follows the overhead sun swings far north over Asia in summer. When the hottest air, and so the lowest pressure, sits over the land, moist ocean air is drawn in and rises, cools and rains; when the land cools in winter the pressure gradient turns round and dry air flows out to sea.

The core

Why the wind reverses

• Water has a high specific heat and mixes its heat through a deep layer, while land heats only its top few centimetres, so in May the plains of north western India reach 45°C while the Arabian Sea stays near 30°C. Warm air over land expands and rises, surface pressure falls into a heat low, and air flows from the higher pressure over the sea towards it.
• As the air crosses the equator the Coriolis force, which deflects moving air to the right in the northern hemisphere, turns the southeast trades of the southern hemisphere into southwest winds. That is why India's summer monsoon blows from the southwest.

Theories, from simple to modern

• Thermal theory. Edmond Halley (1686) explained the monsoon as a giant sea breeze, which captures the land and sea contrast but cannot explain a sudden onset, long breaks or a drought year.
• Dynamic theory. Hermann Flohn (1950s) showed that the Inter Tropical Convergence Zone (ITCZ), the belt where trade winds meet, migrates north with the sun to about 20° to 25°N over India in July, where it is called the monsoon trough, so the monsoon is the planetary wind belt shifting seasonally.
• Jet streams. In winter a subtropical westerly jet flows south of the Himalaya and keeps the north dry. In late May and June it jumps north of the Tibetan plateau, and a tropical easterly jet sets up at about 150 hectopascals over peninsular India. This jump coincides with the burst of the monsoon, which is why onset is abrupt.
• Tibetan plateau. The plateau, about 4,500 m high, heats the middle atmosphere directly, so a high forms aloft over it; air flowing out of this high to the south becomes the easterly jet.
• Somali jet. J Findlater (1969) found a strong low level jet crossing the equator off the coast of East Africa, which carries much of the moisture that falls on the Western Ghats.

Ocean and atmosphere links that make one year differ from the next

• El Nino Southern Oscillation (ENSO). In an El Nino year the trade winds over the Pacific weaken, warm water spreads east, and the rising branch of the Walker circulation moves to the central Pacific, so air sinks over India and the monsoon tends to weaken; La Nina does the opposite. Not every El Nino brings drought, as 1997 showed.
• Indian Ocean Dipole (IOD). A positive dipole, with the western Indian Ocean warmer than the east near Sumatra, favours Indian rain and can offset El Nino, as in 2019.
• Madden Julian Oscillation (MJO). An eastward moving pulse of cloud and rain that circles the tropics in 30 to 60 days; its passage over the Indian Ocean often brings active spells and its absence breaks.

Onset, breaks and withdrawal

• The India Meteorological Department (IMD) puts normal onset over Kerala on 1 June and full coverage of India on 8 July. The current then advances in two branches: the Arabian Sea branch strikes the Western Ghats, and the Bay of Bengal branch strikes the north east and is then turned westward.
• Breaks occur when the monsoon trough shifts north to the Himalayan foothills, so the plains dry out while the hills and the north east flood. Withdrawal from north western India normally begins around 17 September.

Dimensions

Why the monsoon blows from the east in Bhojpur: the Purvaiya

The Bay of Bengal branch moves north towards Bangladesh and the north east, but the Himalaya and the Meghalaya and Arakan hills block it. The monsoon trough lies along the Ganga plain from Punjab to the Bay, with its lowest pressure in the north west, so the deflected air turns left and flows west up the plain. In Bihar and eastern Uttar Pradesh this rain bearing wind therefore arrives from the east, and the Bhojpuri word for it is Purvaiya (from purab, east), while the dry, hot westerly of summer is the Pachhua. Because the air loses moisture as it travels, rain falls from about 119 cm at Kolkata to about 56 cm at Delhi.

How the Purvaiya shaped the cultural ethos of the region

• Agricultural calendar. The arrival of the Purvaiya in Ashadha starts paddy nurseries and ploughing; transplanting in Shravan is done by groups of women singing ropni songs. Folk weather lore, such as the sayings attributed to Ghagh and Bhaddari, reads the direction of the wind to forecast rain.
• Songs. Kajri, sung in Shravan around Varanasi, Mirzapur and Bhojpur, celebrates swings, green fields and the longing of a woman whose husband is away; chaiti in spring carries a similar mood of waiting. In both, the east wind is a messenger of rain and of separation.
• Migration and Bidesia. Small rainfed holdings forced men to migrate, from the nineteenth century to Calcutta, Assam and the plantation colonies and today to Delhi, Surat and Mumbai. Bhikhari Thakur's Bidesia theatre and songs of virah (separation) grew out of this seasonal absence.
• Festivals. Teej and the Shravani mela fall in the rains, and Chhath in Kartik is offered to the sun at ponds and rivers filled by the monsoon, with new rice, sugarcane and fruit of the kharif harvest. It is also when migrants return home.

In practice

How the monsoon climate feeds Monsoon Asia

More than half the world's people live in Monsoon Asia, from Pakistan through India and South East Asia to southern China and Japan. The monsoon climate sustains them for several reasons.

• Rain arrives with heat. The wet season coincides with the hottest months, so water and warmth needed for growth come together, unlike Mediterranean climates where rain falls in the cool season.
• Rice. Paddy needs standing water and high temperature, and it yields more calories per hectare than most cereals, which allows dense rural populations, as in the Ganga delta, the Mekong and the Yangtze.
• Double cropping. Monsoon rain recharges soil moisture, tanks and groundwater, so a second (rabi) crop can follow, which in India is wheat in the north and pulses or oilseeds elsewhere.
• Rivers and plains. Monsoon floods built and still renew the fertile alluvial plains, and Himalayan snowmelt keeps the Ganga, Brahmaputra and Indus flowing before the rains.
• Predictable rhythm. Onset usually varies by only a week or two, so farming and markets can be organised around it.

Risks of depending on it

• The all India rainfall varies by about 10 per cent from year to year, and droughts such as 2002, 2009 and 2015 cut farm output and rural incomes; about half of India's net sown area is rainfed.
• The Intergovernmental Panel on Climate Change (IPCC), in its Sixth Assessment Report (2021), projects that South Asian monsoon rain will increase in the long run, with greater variation between years and more extreme daily rain.

Schemes and forecasts

• The Monsoon Mission (Ministry of Earth Sciences, 2012) built the dynamical model behind IMD's seasonal and extended range forecasts.
• The Pradhan Mantri Fasal Bima Yojana (Ministry of Agriculture and Farmers Welfare, 2016) insures farmers against crop loss from drought and flood.
• Case: 2023. A strong El Nino brought a long August break, yet a positive dipole helped the season end at about 94 per cent of normal.

Issues and way forward

• Rainfed farming and drought. Low irrigation in central India leaves output hostage to rain; watershed development, farm ponds and micro irrigation, urged by the Mihir Shah Committee on restructuring water institutions (2016), spread the water over time.
• Breaks and extremes. Fewer rainy days and more intense spells cause both dry spells and urban floods, because cities have paved their drains; IMD's district and block level warnings need to reach farmers through local language alerts.
• Distress migration. Monsoon failure pushes men out of Bihar and eastern Uttar Pradesh; local crop diversification and rural non farm jobs reduce this.

Common confusions

• Monsoon and rain. A monsoon is a wind system that reverses; rain is its result where the wind comes from the sea.
• Northeast monsoon and retreating monsoon. The northeast winter winds are dry over most of India and rain only on Tamil Nadu after crossing the Bay; the retreating monsoon is the withdrawal of the southwest current in October.
• Monsoon trough and ITCZ. The monsoon trough is the Indian name for the ITCZ when it lies over land in summer.
• El Nino and IOD. El Nino is a Pacific event; the dipole is an Indian Ocean temperature difference, and the two can act together or against each other.

For the exam

• Keyword bank: differential heating, monsoon trough, ITCZ migration, tropical easterly jet, subtropical westerly jet, Somali jet, Tibetan high, Walker circulation, El Nino, Indian Ocean Dipole, Madden Julian Oscillation, break in monsoon, Purvaiya, kajri, Bidesia, double cropping.
• Previous year questions: Practise the previous year questions on this topic →
• Use in an answer:
  • Introduction (data): The southwest monsoon brings about three quarters of India's annual rain in just four months, and about half of India's sown area depends on it.
  • Introduction (judgement, quote or report): The IPCC Sixth Assessment Report expects South Asian monsoon rain to rise but to become more variable, which makes understanding the monsoon a matter of food security.
  • Conclusion: The monsoon is not a single wind but a coupled system of land, ocean and atmosphere, and better forecasting with water storage turns its variability from a threat into a manageable risk.
  • Conclusion: In the Ganga plain the east wind is more than weather; it sets the calendar of work, song, migration and worship, which is why geography and culture cannot be read apart.