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

Detail

1. What the word "monsoon" means

• Monsoon: a wind system that changes direction with the season. In summer the winds blow from sea to land. In winter they blow from land to sea.
• India has two monsoons:
• The southwest monsoon (June to September) is the main rainy season. It brings about 75 per cent of India's total rainfall [5].

• The northeast (retreating) monsoon comes around October–December.

• Long Period Average (LPA): the average rain a place gets over a long period, usually 30 years. It is the yardstick for calling a season "normal". The all-India southwest-monsoon LPA is 868.6 mm (1971–2020 average) [5].

2. Theories of the monsoon

• Class 11 NCERT says no single theory explains the monsoon fully. The real breakthrough came when scientists studied it at the global scale, not just as a local Indian event.

Theory	When / who	Core idea	Weak point
Thermal theory	Edmund Halley, 1686	The monsoon is a giant sea breeze. Land heats faster than sea in summer → low pressure over land → moist sea air flows in. The flow reverses in winter.	Cannot explain why rain comes in bursts and breaks, or why it varies so much from year to year
Dynamic theory	H. Flohn, 1951	The ITCZ and the planetary wind belts shift north and south with the sun. The southwest monsoon is the southeast trades pulled across the equator.	Does not explain the upper-air controls on its own
Jet stream and Tibet theories	1950s	Upper-air controls: the westerly jet stream moving north of the Himalayas, the heating of the Tibetan Plateau, and the tropical easterly jet set the timing of onset (see the upper-air section of this topic)	Explain timing better than the total rain
Teleconnections	Later 20th century	The monsoon is linked to distant oceans: ENSO in the Pacific and the IOD in the Indian Ocean	Give probabilities, not certainties

• Key terms:
• ITCZ (Inter Tropical Convergence Zone): the low-pressure belt near the equator where the trade winds of the two hemispheres meet. Warm air rises here, so clouds form and rain falls [5].
• Coriolis force: the bending of moving air caused by the Earth's rotation. It bends air to the right in the Northern Hemisphere.
• Jet stream: a narrow band of very fast wind high up in the atmosphere (upper troposphere).
• Teleconnection: a link between weather in places that are very far apart.
• ENSO (El Niño–Southern Oscillation): a cycle of warming (El Niño) and cooling (La Niña) of the central and eastern Pacific Ocean, linked to changes in air pressure.

• IOD (Indian Ocean Dipole): a temperature difference between the western and eastern Indian Ocean. A positive IOD (warmer west) usually helps Indian rainfall.

• The ITCZ view in official material:

• In summer the ITCZ moves north with the sun. In July it can reach the Gangetic plains [5].
• Over land this low-pressure belt is called the monsoon trough [5].
• Around October the ITCZ moves south again. The winds reverse and the northeast monsoon begins [5].

• The Government of India calls the ITCZ "a switch" that turns the monsoon on and off. It says the ITCZ explains the monsoon and its variation better than the "gigantic sea breeze" theory [5].

• ENSO evidence:

• El Niño (warm water off Peru and in the eastern equatorial Pacific) disturbs the moist winds flowing towards India. This often gives a weak or late monsoon [5].
• Since 1950 there have been 16 El Niño years. In 7 of them India's monsoon rain was below normal [5]. So El Niño raises the risk of drought but does not guarantee one.
• La Niña (a cooler central and eastern Pacific) usually gives normal to above-normal rain. It can also cause floods [5].

3. Ancient and traditional ways of reading the monsoon

• Classical texts (Class 9 NCERT):
• Krishiparashara: forecast rain from the positions of the sun and the moon.
• Varahamihira's Brihatsamhita: linked rain to the lunar mansions (nakshatras, the groups of stars the moon passes through).
• Kalidasa's Meghadutam (around the 5th century CE): records the date of onset and the path of the monsoon clouds over central India.

• Kautilya's Arthashastra: describes measuring rainfall. Rainfall records were used to fix land revenue and to plan relief in years of low rain.

• Folk signs (Class 7 NCERT):

• Fish surfacing in large numbers off the Konkan coast (Maharashtra–Goa).
• The flowering of the Golden Shower tree (amaltas).

• Where and how crows build their nests.

• These signs come from careful watching of nature over many generations. They are not instrument-based forecasts.

4. Onset of the monsoon: the late 19th-century explanation (Class 11, Fig. 4.1)

• 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 to the south 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 from the subtropical high towards the equator in the Southern Hemisphere) across the equator.

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

• Step 4: the winds turn.

• After crossing the equator, the Coriolis force bends them to the right.
• They now blow from the southwest as the southwest monsoon.

• So the southwest monsoon is a continuation of the southeast trades.

• Step 5: they pick up moisture.

• The winds pass over warm equatorial currents, take up a great deal of moisture, and bring heavy rain to India.

• The official modern account explains it the same way. Trade winds from the Southern Hemisphere are drawn north as the ITCZ moves north. They bend because of the Earth's rotation and reach India from the southwest [5].

• Class 7 and Class 9 in simple terms: land heats faster than sea → low pressure over land → moist air flows from sea to land → rain.

5. How IMD declares the onset over Kerala today

• Onset over Kerala marks the official start of the monsoon over mainland India. It is the change from the hot, dry season to the rainy season [2].
• Signs IMD checks (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 gives it by latitude and longitude points and by towns. On 24 May 2025 it passed through Karwar, Shimoga, Dharmapuri and Chennai [6].

• Where the rains first arrive: monsoon rain first reaches the south Andaman Sea. It then moves northwest across the Bay of Bengal. The normal date for the Andaman Sea is about 22 May [2].

6. Burst of the monsoon

• Burst (or "break") of the monsoon: the sudden start of heavy rain with thunder and lightning. It is not a slow, gradual increase in rain.
• Where and when:
• First week of June: the coasts of Kerala, Karnataka, Goa and Maharashtra.

• Interior parts: may have to wait until the first week of July.

• Effect on temperature: day temperatures fall by 5–8°C between mid-June and mid-July.

• Why it is sudden: the moist southwest 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).

7. Advance of the monsoon across India (Class 11, Fig. 4.2)

• Two branches carry the monsoon inland [5]:
• Arabian Sea branch: brings rain to the west coast and central India.
• Bay of Bengal branch: reaches eastern and northeastern India. Weather systems that form over the warm Bay then carry rain into north India.

Stage	NCERT (Class 11)	IMD normal date, revised 2020
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

• The 2020 revision: IMD released the new 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].

• What changed in practice: the monsoon now reaches extreme northwest India (Rajasthan, Punjab, Haryana) about a week earlier than in the old normal. It also stays longer in the northwest before it withdraws.

8. Recent onset and advance dates

Year	Onset over Kerala	Note
2015	5 June	[2]
2016	8 June	[2]
2017	30 May	[2]
2018	29 May	[2]
2019	8 June	[2]
2020	1 June (exactly normal)	Covered the whole country on 26 June 2020, 12 days early [3]. Began withdrawing from west Rajasthan and Punjab on 28 September 2020, against the normal 17 September [3][4]
2025	24 May	8 days early. The earliest since 2009, when onset came on 23 May. It reached Mizoram 12 days early [6]
2026	4 June	3 days after normal. That day the NLM passed through Mangalore, Udhagamandalam, Kodaikanal and Thoothukudi [7]

• Earliest full coverage in recent years: the monsoon covered the whole country on 16 June 2013 [3].
• Withdrawal varies a lot: from west Rajasthan it ranged from 4 September (2015) to 9 October (2019) [4].
• Lesson: the onset date moves by a week or more either way from 1 June. An early onset does not mean the season will have good rain.

Prelims Hooks

• Halley (1686) gave the thermal (sea breeze) theory. Flohn (1951) gave the dynamic (ITCZ-shift) theory. Do not swap them.
• 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.
• Normal onset over Kerala is 1 June. IMD did not change it in the 2020 revision [6].
• Whole country covered: 8 July (IMD, 2020 revision; earlier 15 July). NCERT says "mid-July" [3].
• Withdrawal from northwest India begins about 17 September (IMD normal) [4].
• 2025 onset over Kerala: 24 May, the earliest since 23 May 2009 [6]. 2026 onset: 4 June [7].
• Burst order (first week of June): coasts of Kerala → Karnataka → Goa → Maharashtra. Mumbai and Kolkata get the monsoon about 10–13 June.
• Match the text to its content: Meghadutam (Kalidasa) – onset date and cloud path; Brihatsamhita (Varahamihira) – lunar mansions; Arthashastra (Kautilya) – rain measurement for revenue and relief; Krishiparashara – sun and moon positions.
• Trap: the monsoon first appears over the Andaman Sea (about 22 May), not over Kerala [2].
• 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 theories changed (GS-I): the thermal "giant sea breeze" idea explains the reversal of the winds. It cannot explain breaks in the rain, the role of the jet streams, or why rain changes from year to year. Modern understanding joins three things: the shift of the ITCZ, upper-air controls (jet streams and Tibetan heating), and ocean teleconnections (ENSO and IOD). The official view now says the ITCZ explains the monsoon better than the sea breeze theory [5].
• Onset date and the 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].
• More variable timing (GS-III, disasters): onset over Kerala ranged from 24 May (2025) to 8 June (2016, 2019) [2][6]. Withdrawal from west Rajasthan ranged from 4 September to 9 October [4]. Farmers therefore need forecasts based on the actual onset, not fixed calendar dates.
• Traditional knowledge as a resource: ancient texts such as the Arthashastra (rain measurement for revenue and relief) and folk signs from the Konkan coast show a long Indian habit of watching the monsoon. This local knowledge can support community-level early warning.