Onset of the monsoon

Also called: Monsoon onset · Topic: Climate of India and the Monsoon · NCERT: Class 7, Ch 3 "Climates of India"; Class 9, Ch 3 "Atmosphere and Climate"; Class 11, Ch 4 "Climate"

Meaning

Onset of the monsoon is the official arrival of the southwest monsoon over mainland India. The India Meteorological Department (IMD) declares it when the monsoon sets in over Kerala. The normal date is 1 June [6]. It marks the change from the hot, dry season to the rainy season [2].

It matters because the southwest monsoon brings about 75 per cent of India's total rainfall [5]. The onset date decides when kharif sowing starts and when reservoirs begin to fill.

Explanation

How the onset happens (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 trade winds are pulled north.

• The low over northwest India pulls the southeast trade winds (steady winds of the Southern Hemisphere that blow towards the equator) 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 waters, pick up a lot of moisture and bring heavy rain to India.

• The modern ITCZ view:

• ITCZ (Inter Tropical Convergence Zone) is the low-pressure belt near the equator where the trade winds of the two hemispheres meet. Air rises there, clouds form and rain falls [5].
• In summer the ITCZ moves north with the sun and pulls Southern Hemisphere trades towards India [5].
• Over land this belt is called the monsoon trough. In July it can reach the Gangetic plains [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].

What controls the timing

• Thermal theory (Edmund Halley, 1686): the monsoon is a giant sea breeze. It explains why the winds reverse, but not why the onset date changes from year to year.
• Dynamic theory (H. Flohn, 1951): the ITCZ and the wind belts shift north and south with the sun.
• Jet stream and Tibet theories (1950s): controls high up in the air set the timing of onset. These include the westerly jet stream (a narrow band of very fast wind high in the atmosphere) moving north of the Himalayas, the heating of the Tibetan Plateau and the tropical easterly jet.
• Teleconnections (links between weather in places far apart):
• El Niño is the warming of the eastern equatorial Pacific, off Peru. It disturbs the moist winds flowing towards India and often gives a weak or late monsoon [5].
• IOD (Indian Ocean Dipole) is the temperature difference between the western and eastern Indian Ocean. A positive IOD (warmer west) usually helps Indian rainfall.

How IMD declares onset over Kerala

• Signs IMD checks (as seen at the 2025 onset) [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 over the south Arabian Sea reach up to 4.5 km above sea level.
• Wind speed: lower-level westerlies of about 20–30 knots.

• Rain: widespread rain over Kerala.

• Northern Limit of Monsoon (NLM): the northernmost line the monsoon has reached on a given day. IMD marks it by latitude–longitude points and by town names [6].

Normal dates and how the date varies

• IMD revised the normal dates on 15 April 2020 [2].
• Onset dates are based on data for 1961–2019 [2].

• Withdrawal dates are based on data for 1971–2019 [2].

• Normal timeline:

• Andaman Sea: about 22 May [2]
• Kerala: 1 June, unchanged in the revision [6]
• Most of northeast India: about 5 June [6]
• Mumbai and Kolkata: 10–13 June (NCERT)

• Whole country: 8 July, earlier 15 July [3]. NCERT says "mid-July".

• Recent onsets over Kerala:

• 2015: 5 June. 2016: 8 June. 2017: 30 May. 2018: 29 May. 2019: 8 June [2].
• 2020: 1 June, exactly normal [3].
• 2025: 24 May, 8 days early. This was the earliest since 2009 (23 May) [6].

• 2026: 4 June, 3 days after normal [7].

• Lesson: the onset date moves by a week or more either side of 1 June. An early onset does not promise a good rainy season.

Where it occurs

• South Andaman Sea: the monsoon rains arrive here first, about 22 May. From there they move northwest across the Bay of Bengal [2].
• Kerala: onset here marks the official start of the monsoon over mainland India [2].
• Southeast and south Arabian Sea: IMD watches the clouds (OLR) and westerly winds here before declaring onset [6].
• Off East Africa and Somalia (40°E–60°E): the southeast trades cross the equator here.
• Northwest India (Rajasthan–Punjab) and Pakistan: the summer low-pressure area that pulls the winds in forms here.
• West coast (Kerala, Karnataka, Goa, Maharashtra): the monsoon "bursts" here in the first week of June. Moist winds hit the Western Ghats, rise, cool and pour rain on the windward side.
• The two branches:
• The Arabian Sea branch brings rain to the west coast and central India [5].

• The Bay of Bengal branch reaches eastern and northeastern India [5].

• NLM examples:

• On 24 May 2025 the NLM passed through Karwar, Shimoga, Dharmapuri and Chennai [6].

• On 4 June 2026 it passed through Mangalore, Udhagamandalam, Kodaikanal and Thoothukudi [7].

• Mizoram: the 2025 monsoon reached here 12 days early [6].

Don't confuse with

• Burst of the monsoon: onset is the official arrival date over Kerala. Burst is the sudden start of heavy rain with thunder and lightning on the west coast, as the moist winds hit the Western Ghats.
• Advance of the monsoon: onset is the arrival at one point, Kerala on about 1 June. Advance is the spread across the country until it covers the whole of India, normally by 8 July [3].
• Withdrawal (retreat) of the monsoon: onset is the start of the season. Withdrawal begins from northwest India around 17 September (IMD normal) [4], and the monsoon leaves the whole country by about 15 October.
• First arrival over the Andaman Sea: the rains reach the Andaman Sea first, about 22 May. However, the "onset" IMD declares for mainland India is over Kerala [2].

Prelims Hooks

• Normal onset over Kerala is 1 June. IMD did not change this date in its 2020 revision [6].
• Trap: the monsoon first appears over the Andaman Sea (about 22 May), not over Kerala [2].
• Whole country covered: 8 July under IMD's 2020 normals (earlier 15 July). NCERT says "mid-July" [3].
• 2025 onset: 24 May, the earliest since 23 May 2009 [6]. 2026 onset: 4 June [7].
• Southwest monsoon = continuation of the southeast trades. They cross the equator between 40°E and 60°E and are bent by the Coriolis force.
• Onset signs used by IMD: OLR below 200 W/m², westerlies up to 4.5 km deep, wind speed of 20–30 knots, and widespread rain over Kerala [6].

Mains Points

• Onset date and the farm economy (GS-I/III):
• About 75 per cent of India's rain falls in the southwest monsoon [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].

• Why the onset date varies (GS-I):

• The "giant sea breeze" idea explains why the winds reverse. It does not explain why the onset date shifts from year to year.
• The modern view combines the northward shift of the ITCZ, upper-air controls (jet streams and Tibetan heating) and ocean links (ENSO, IOD).

• El Niño often brings a weak or late monsoon. Even so, only 7 of 16 El Niño years since 1950 had below-normal rain [5].

• Variable timing and disaster preparedness (GS-III):

• Onset over Kerala ranged from 24 May (2025) to 8 June (2016, 2019) [2][6].
• In 2020 the monsoon covered the whole country on 26 June, 12 days early [3].
• So advisories for farmers should follow the onset IMD actually declares and the NLM it tracks, not fixed calendar dates.

Related concepts

• Monsoon theories
• Southwest monsoon
• Burst of the monsoon

Read more

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