Southwest monsoon

Also called: Summer monsoon, Advancing monsoon · 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"

Meaning

The southwest monsoon is the moist wind that blows over India from the southwest between June and September. It begins as the southeast trade winds of the Southern Hemisphere. These winds are pulled across the equator and then bent to the right, so they reach India from the southwest. It brings about 75 per cent of India's total rainfall [5]. Kharif sowing, reservoir levels and the crop calendar all depend on when it arrives and how it moves.

Explanation

How it forms (cause → process → result)

• Step 1: the land heats up.
• In April–May the sun shines directly over the Tropic of Cancer (23.5°N).
• The large landmass north of the Indian Ocean heats up strongly.

• An intense low-pressure area forms over northwest India (Rajasthan–Punjab–Pakistan).

• Step 2: the sea stays cooler.

• The Indian Ocean heats up more slowly, so it keeps higher pressure.

• Step 3: the trades are pulled north.

• The low over the northwest pulls the southeast trade winds (steady winds that blow towards the equator in the Southern Hemisphere) across the equator.

• They cross between 40°E and 60°E, off East Africa and Somalia.

• Step 4: the winds turn.

• The Coriolis force (the bending of moving air caused by the Earth's rotation) bends them to the right in the Northern Hemisphere.

• They now blow from the southwest. So the southwest monsoon is a continuation of the southeast trades.

• Step 5: they pick up moisture.

• The winds pass over warm equatorial currents and take up a great deal of moisture.

• They bring heavy rain to India.

• The ITCZ view:

• The ITCZ (Inter Tropical Convergence Zone, the low-pressure belt where the trade winds of the two hemispheres meet) moves north with the sun. In July it can reach the Gangetic plains [5].
• Over land this belt is called the monsoon trough [5].
• The Government of India calls the ITCZ "a switch" that turns the monsoon on and off. It says the ITCZ explains the monsoon better than the "gigantic sea breeze" theory [5].

Theories behind it

• Thermal theory (Edmund Halley, 1686): the monsoon is a giant sea breeze. Land heats faster than sea, low pressure forms over land, and moist sea air flows in. It cannot explain breaks in the rain or why rain changes from year to year.
• Dynamic theory (H. Flohn, 1951): the ITCZ and the wind belts shift north with the sun. The southeast trades are pulled across the equator.
• Jet stream and Tibet theories (1950s): upper-air controls set the timing of onset. These are the westerly jet stream (a narrow band of very fast wind high up) moving north of the Himalayas, the heating of the Tibetan Plateau, and the tropical easterly jet.
• Teleconnections (later 20th century): the monsoon is linked to distant oceans (a teleconnection is a link between weather in places far apart).
• ENSO (El Niño–Southern Oscillation, the warming and cooling cycle of the central and eastern Pacific).
• IOD (Indian Ocean Dipole, the temperature difference between the western and eastern Indian Ocean). A positive IOD (warmer west) usually helps Indian rain.
• El Niño (warm water off Peru) often gives a weak or late monsoon [5]. But only 7 of the 16 El Niño years since 1950 had below-normal rain [5].

• La Niña usually gives normal to above-normal rain and can cause floods [5].

• Class 11 NCERT says no single theory explains it fully.

Its two branches and how it advances

• Arabian Sea branch: brings rain to the west coast and central India [5].
• Bay of Bengal branch: reaches eastern and northeastern India. Weather systems that form over the warm Bay then carry rain into north India [5].
• Onset: IMD's official start of the rainy season over mainland India, declared over Kerala [2]. IMD checks for these signs [6]:
• Clouds: more clouds over the southeast Arabian Sea, with OLR below 200 W/m². OLR (Outgoing Longwave Radiation) is the heat the Earth gives off to space. Low OLR means thick, deep clouds.
• Wind depth: westerly winds reaching up to 4.5 km above sea level.
• Wind speed: lower-level westerlies of about 20–30 knots.

• Rain: widespread rain over Kerala.

• Burst: the sudden start of heavy rain with thunder and lightning, not a slow build-up.

• Moist winds hit the Western Ghats and are pushed up.
• Rising air cools, its water vapour turns into cloud, and it pours on the side facing the wind (the windward side).

• Day temperatures fall by 5–8°C between mid-June and mid-July.

• Northern Limit of Monsoon (NLM): the northernmost line the monsoon has reached on a given day [6].

• Long Period Average (LPA): the average rain over a long period, used to call a season "normal". For the all-India southwest monsoon it is 868.6 mm (1971–2020 average) [5].

Stage	NCERT (Class 11)	IMD normal date (2020 revision)
Andaman Sea	—	about 22 May [2]
Kerala (onset)	1 June	1 June (unchanged) [6]
Most of northeast India	—	about 5 June [6]
Mumbai and Kolkata	10–13 June	—
Whole country covered	mid-July	8 July (earlier 15 July) [3]
Withdrawal begins from northwest India	about 1 September	17 September [4]
Leaves the whole country	—	about 15 October

• IMD released the new normal dates on 15 April 2020. Onset dates use data for 1961–2019 and withdrawal dates use data for 1971–2019 [2].
• What changed: the monsoon now reaches Rajasthan, Punjab and Haryana about a week earlier. It also stays longer in the northwest before it withdraws.

Where it occurs

• Origin of the winds: the Southern Hemisphere trade-wind belt. The winds cross the equator between 40°E and 60°E, off East Africa and Somalia.
• First arrival: the south Andaman Sea, about 22 May. From there the rain moves northwest across the Bay of Bengal [2].
• Kerala: official onset over mainland India, with a normal date of 1 June [6].
• Burst zone (first week of June): the coasts of Kerala, Karnataka, Goa and Maharashtra, on the windward side of the Western Ghats. Interior parts may have to wait until the first week of July.
• Low-pressure heart: northwest India (Rajasthan–Punjab–Pakistan). The monsoon trough lies over the Gangetic plains in July [5].
• NLM examples:
• 24 May 2025: through Karwar, Shimoga, Dharmapuri and Chennai [6].

• 4 June 2026: through Mangalore, Udhagamandalam, Kodaikanal and Thoothukudi [7].

• Recent onset over Kerala:

• 2025: 24 May, 8 days early. It was the earliest since 23 May 2009 and reached Mizoram 12 days early [6].
• 2026: 4 June, 3 days late [7].

• 2020: 1 June. It covered the whole country on 26 June 2020, 12 days early [3].

• Withdrawal: begins from west Rajasthan and Punjab. In 2020 it began on 28 September [4]. The start has ranged from 4 September (2015) to 9 October (2019) [4].

Don't confuse with

• Northeast (retreating) monsoon: blows from land to sea around October–December, after the ITCZ moves south [5]. The southwest monsoon blows from sea to land in June–September.
• Southeast trade winds: the parent wind in the Southern Hemisphere. They become the southwest monsoon only after crossing the equator and being bent by the Coriolis force.
• Onset vs burst: onset is IMD's official declaration over Kerala, based on cloud, wind and rain signs [6]. Burst is the sudden heavy downpour with thunder on the west coast.
• Thermal theory vs dynamic theory: Halley (1686) saw a giant sea breeze. Flohn (1951) saw a shift of the ITCZ and wind belts. Do not swap the names.

Prelims Hooks

• The southwest monsoon is a continuation of the southeast trades. They cross the equator between 40°E and 60°E and are bent by the Coriolis force.
• Trap: the monsoon first appears over the Andaman Sea (about 22 May), not over Kerala [2].
• Normal onset over Kerala is 1 June and was unchanged in 2020 [6]. Whole country covered: 8 July (earlier 15 July) [3]. Withdrawal begins: 17 September [4].
• 2025 onset: 24 May, the earliest since 2009 [6]. 2026 onset: 4 June [7].
• Branch–region match: Arabian Sea branch → west coast and central India. Bay of Bengal branch → eastern and northeastern India [5].
• Trap: El Niño does not always cause drought. Only 7 of 16 El Niño years since 1950 had below-normal rain [5].

Mains Points

• Why the explanation changed (GS-I):
• The "giant sea breeze" idea explains why the winds reverse.
• It fails on breaks in the rain, the role of jet streams and changes from year to year.

• Modern understanding joins three things: the ITCZ shift, upper-air controls (jet streams and Tibetan heating), and ocean teleconnections (ENSO and IOD). The official view says the ITCZ explains it better [5].

• Onset and the economy (GS-I/III):

• About 75 per cent of India's rain comes in this season [5].
• A late or patchy onset delays kharif sowing and lowers reservoir levels.

• IMD's revised normal dates (2020) set the timing used for crop calendars and contingency plans [2][3][4].

• Variable timing and risk (GS-III, disasters):

• Onset over Kerala has ranged from 24 May (2025) to 8 June (2016, 2019) [2][6].
• Withdrawal from west Rajasthan has ranged from 4 September to 9 October [4].
• So farm advisories need forecasts based on the actual onset, not fixed calendar dates. An early onset does not promise a good season.

Related concepts

• Monsoon theories
• Onset of the monsoon
• Burst of the monsoon

Read more

• How the monsoon works: theories, onset, burst and advance