Monsoon

Also called: Seasonal wind · Topic: Climate of India and the Monsoon · NCERT: Class 7, Ch 3 "Climates of India"; Class 7, Ch 1 "The Story of Indian Farming"; Class 9, Ch 3 "Atmosphere and Climate"; Class 11, Ch 4 "Climate"; Class 11, Ch 6 "Natural Hazards and Disasters: Causes, Consequences and Management"; Class 11, Ch 9 "Atmospheric Circulation and Weather Systems"; Class 11, Ch 11 "World Climate and Climate Change"

Meaning

Monsoon is a seasonal reversal of winds: the main wind direction over a large area flips between summer and winter, and this brings wet summers and dry winters. In India, the south-west monsoon brings heavy rain from June to September. The north-east monsoon blows from land to sea in winter and is mostly dry, but it brings rain to the Tamil Nadu coast in October-December.

It matters because the monsoon sets the rhythm of India's year: its seasons, its farming and its water supply. Class 11 NCERT says India's climate is "monsoonal in rhythm and character".

Explanation

How the monsoon works (cause → process → result)

• The word: it comes from the Arabic mausim, meaning "season".

• Arab sailors and traders gave it this name. Their sailing ships used the south-west wind in summer to reach India and the north-east wind in winter to sail home.

• The basic cause: land and sea heat up and cool down at different speeds.

• Summer (south-west monsoon):
• Land heats up faster than the sea.
• Low pressure forms over north-west India.

• Moist south-west winds are pulled in from the Indian Ocean, which gives heavy rain from June to September.

• Winter (north-east monsoon):

• Land cools faster than the sea.
• High pressure forms over the land.

• Dry north-east winds blow from land to sea. There is little rain, except on the Tamil Nadu coast.

• Monsoon climate: a climate with wet summers, dry winters and a seasonal reversal of winds. India has a hot (tropical) monsoon climate.

The two branches and how relief shapes rain

• Bay of Bengal branch:
• It moves north over the plains of Bangladesh and meets the funnel-shaped Khasi and Garo hills in Meghalaya.
• The hills push the moist air up. Rising air cools, its water vapour condenses, and it rains heavily on the side facing the wind (the windward side). This is orographic rain (rain caused when mountains lift the air).

• It then moves west along the Ganga plain and gets weaker as it goes. That is why Delhi gets less rain than places further east.

• Arabian Sea branch:

• In Rajasthan it runs parallel to the Aravalli range, so no hills lift it. Jodhpur and the Thar therefore get very little rain.

• Rain shadow: the rain shadow is the dry side behind a mountain. In June-September the Coromandel coast lies in the rain shadow of the Western Ghats, and the Bay branch runs parallel to that coast. So Chennai stays fairly dry in these months.

The seasons and the retreating monsoon

• India has four seasons in a fixed order:
• the cold weather season
• the hot weather season
• the south-west monsoon

• the retreating monsoon

• The retreating / north-east monsoon (October-December):

• The winds cross the Bay of Bengal and pick up moisture.
• They then hit the Coromandel coast directly. Cyclones add more rain.
• So Chennai's rain peaks in October-November.
• Tamil Nadu and Puducherry get about 44 cm in this season. This is about 48% of their annual rain [5].
• South Peninsular India as a whole gets about 30% of its annual rain in this season [5].

Unity and diversity under one monsoon

• Unity:
• The same monsoon winds reverse over the whole country.
• Every region follows the same order of seasons.
• The Himalaya works like a wall. It keeps the very cold Central Asian winds out in winter and traps the monsoon winds over the subcontinent in summer.

• NCERT says the monsoon gives India "a very strong framework" of unity.

• Diversity: the monsoon's rain is very uneven across the country.

• IMD measures each season against the LPA (Long Period Average: the average June-September rainfall for India as a whole).
• In 2019, India got 110% of the LPA. That LPA was 88 cm [3].
• In the same season, 5 of the 36 meteorological subdivisions had deficient (below-normal) rain. Haryana, Delhi and Chandigarh were 42% short [3].

Where it occurs

• In the world:
• India and the rest of South and Southeast Asia have a hot (tropical) monsoon climate.
• Monsoon systems are also found over northern Australia and Africa (Class 7 NCERT).

• The Arabian Sea was the historic sailing route of Arab traders who used the monsoon winds.

• In India: the wettest places

• Mawsynram and Cherrapunji (Sohra) in the Khasi Hills, Meghalaya get over 1,080 cm a year (NCERT).
• Mawsynram's average is about 1,187 cm (467 inches) a year, most of it in the monsoon season [4]. It lies just west of Cherrapunji, and its rain may be even higher than Cherrapunji's [4].

• Tura (Garo Hills, Meghalaya) can get as much rain in one day as Jaisalmer gets in ten years.

• In India: the driest places

• Jaisalmer (western Rajasthan) gets under 9 cm a year.

• The north-west Himalaya (Ladakh) and the western deserts get under 10 cm.

• Rain at different times in different places:

• In July-August, the Ganga delta and the Odisha coast are hit by strong rain storms every third to fifth day.

• At the same time, the Coromandel coast (Tamil Nadu), only about a thousand kilometres to the south, stays dry.

• Station contrasts (Class 9 NCERT climographs):

• Shillong (Meghalaya plateau) gets more rain than Kolkata (flat Ganga delta).
• Delhi gets more rain than Jodhpur (Thar).
• Chennai's rain peaks in October-December.

Don't confuse with

• Weather vs monsoon climate: weather is the state of the air at one place at one moment, and it can change within hours. The monsoon is part of climate, a pattern that repeats every year. Climate means 30+ years of weather (Class 7/9), and a change in climate shows only after 50+ years (Class 11).
• South-west monsoon vs north-east (retreating) monsoon: the south-west monsoon (June-September) blows from sea to land and brings rain to most of India. The north-east monsoon (October-December) blows from land to sea. It is mostly dry, but it is the main rain season for Tamil Nadu (about 48% of its annual rain [5]).
• Orographic rain vs rain shadow: in orographic rain, hills lift moist air and it rains on the windward side (for example Meghalaya). A rain shadow is the dry side behind the hills (for example the Coromandel coast behind the Western Ghats in June-September).
• Monsoon (seasonal reversal) vs a simple rainy season: a monsoon climate needs a reversal of wind direction between summer and winter. Rain alone does not make a monsoon.

Prelims Hooks

• "Monsoon" comes from the Arabic mausim ("season"). Arab sailors named it because they used the wind reversal to sail to India and back.
• Monsoon climate = wet summer + dry winter + seasonal reversal of winds. It is found in South and Southeast Asia, northern Australia and Africa.
• Trap: Tamil Nadu gets most of its rain from the north-east monsoon (October-December), about 44 cm or 48% of its annual rain [5], not from the south-west monsoon.
• Highest to lowest rainfall: Mawsynram (~1,187 cm [4]) > Cherrapunji (Khasi Hills) … Jaisalmer (<9 cm). Tura is in the Garo Hills, and Mawsynram is in the Khasi Hills. Both are in Meghalaya.
• Jodhpur/Thar is dry because the Arabian Sea branch runs parallel to the Aravalli, so nothing lifts the air. Shillong is wet because the Meghalaya plateau forces the Bay branch to rise.
• In 2019, India got 110% of the LPA (88 cm), yet 5 of 36 subdivisions had deficient rain [3].

Mains Points

• Unity in diversity (GS-I):
• One monsoon rhythm unites India: the wind reversal, the same order of seasons and the Himalayan barrier. This rhythm shapes farming, festivals and the economy across the country.

• Relief, latitude and distance from the sea create very different rainfall regimes within this one rhythm, from over 1,080 cm in Meghalaya to under 9 cm in Jaisalmer.

• Uneven rainfall in space and time (GS-I/GS-III):

• Even in a surplus year (2019: 110% of LPA), 5 subdivisions had deficient rain [3]. Drought in one region and floods in another can happen at the same time.

• India therefore needs water storage, inter-basin planning and drought-proofing that suit each region.

• Regional monsoon calendars (GS-III disasters and resources):

• The Coromandel coast depends on the October-December rains, about 48% of Tamil Nadu's annual rain [5]. Most of India depends on June-September.
• Crop plans, reservoir operation and cyclone preparedness must follow each region's own monsoon timing.
• Judging a monsoon against IMD's LPA and the WMO's 30-year normals (1991-2020) [2] helps tell a single bad season (weather) apart from a lasting shift in rainfall (climate change).

Related concepts

• Monsoon climate
• Unity and diversity of the monsoon climate

Read more

• Weather, climate and the monsoon: unity and diversity