GeographyCBSE Class 11 · Climate

The Nature of the Indian Monsoon: Onset, ITCZ, Breaks and El Nino (Class 11 Geography, India: Physical Environment, Chapter 4)

Updated 17 October 2026 · Fact-checked

The south-west monsoon sets in when intense heating of north-west India forms a low that draws in the south-east trades across the equator, helped by the northward shift of the ITCZ (the monsoon trough at about 20 to 25 degrees N in July) and the easterly jet stream along 15 degrees N. It reaches Kerala by 1 June and covers India by mid-July, with breaks, and El Nino and the Southern Oscillation assist forecasting.

Understand The Indian Monsoon: Onset, ITCZ, Jet Streams, Breaks, El Nino and Forecasting

Why this topic matters. Mechanism questions (differential heating, ITCZ, jet streams), arrival dates and the meaning of a break are standard Board items.

1. The puzzle. No single theory explains the monsoon fully; a breakthrough came from studying it at the global level. Two aspects matter: onset and break.

2. Differential heating. In April-May the sun is vertically over the Tropic of Cancer, the land north of the Indian Ocean heats up and an intense low forms over north-west India. The Indian Ocean stays at relatively higher pressure because water heats slowly. The low attracts the south-east trades across the equator, and the south-west monsoon is their continuation deflected towards India after crossing the equator between 40 degrees E and 60 degrees E.

3. ITCZ. A low-pressure zone at the equator where trades converge and air rises. In July it lies around 20 to 25 degrees N over the Gangetic plain: the monsoon trough, with a thermal low over north and north-west India. In winter the ITCZ shifts south and the winds reverse to the north-east monsoon.

4. Jet streams. The shift of the ITCZ is linked to the withdrawal of the westerly jet stream from its position south of the Himalayas. Only then does the easterly jet stream set in along 15 degrees N, which is held responsible for the burst of the monsoon.

5. Arrival. Over Kerala by 1 June; Mumbai and Kolkata between 10 and 13 June; the whole subcontinent by mid-July.

6. Break. After a few rainy days, if rain fails for one or more weeks, it is a break. In northern India it occurs when rain-bearing storms are not frequent along the monsoon trough; on the west coast when winds blow parallel to the coast.

7. El Nino and the Southern Oscillation. El Nino is a weather system appearing every 3 to 7 years in which warm water appears off Peru, replacing the cold Peruvian current for a time, with effects on circulation and fisheries; the name means Child Christ because it appears around Christmas. The monsoon's strength is measured by the pressure difference between Tahiti and Port Darwin. The IMD uses 16 indicators in forecasting.

The Indian Monsoon: Onset, ITCZ, Jet Streams, Breaks, El Nino and Forecasting: key facts

Trigger: April-May: sun over Tropic of Cancer
Intense low over north-west India
Pulls south-east trades across the equator
Crossing between 40 degrees E and 60 degrees E; South-west monsoon.
ITCZ: July: about 20 to 25 degrees N over the Gangetic plain
Called the monsoon trough
Moves south in winter; Low-pressure zone at the equator.
Arrival dates: Kerala by 1 June
Mumbai and Kolkata 10 to 13 June
Whole country by mid-July; Approximate dates.
El Nino: Every 3 to 7 years
Warm water off Peru
Tahiti and Port Darwin pressure difference
IMD uses 16 indicators; Forecasting aids.

How to answer The Indian Monsoon: Onset, ITCZ, Jet Streams, Breaks, El Nino and Forecasting questions

Mechanism answers must follow the sequence of events.

1Start with heating and the low pressure over north-west India.
2Add the pull on the south-east trades and the Coriolis deflection.
3Bring in the ITCZ shift and the jet streams.
4End with the arrival at Kerala and the spread by mid-July.

The Indian Monsoon: Onset, ITCZ, Jet Streams, Breaks, El Nino and Forecasting in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Dry spell in July

Suppose rains lash a town for a week in July and then stop for ten days even though it is the monsoon months.

What is this called?

Try it first, then show the solutionSolution

Rain fails for one or more weeks after a few rainy days.
Such dry spells are common in the monsoon.

Answer: It is a break in the monsoon.

Memory hook: Wet days, then a gap.

Forecast puzzle

Suppose a forecaster sees unusually high pressure at Tahiti and low at Darwin and wants to judge the coming monsoon.

Which aspect is being used?

Try it first, then show the solutionSolution

The Southern Oscillation is measured by the Tahiti-Darwin pressure difference.
It helps forecast the south-west monsoon.

Answer: The Southern Oscillation indicator.

Memory hook: Tahiti minus Darwin.

Tricks to remember The Indian Monsoon: Onset, ITCZ, Jet Streams, Breaks, El Nino and Forecasting

Short memory aids. Say each one aloud once, then use it on a question the same day.

Heat, low, pull
Heating, low pressure, trades pulled across the equator
Use it when: Onset sequence.
1-10-15
Kerala 1 June, Mumbai and Kolkata 10-13 June, all India by 15 July
Use it when: Dates.
Easterly at 15
Easterly jet along 15 degrees N bursts the monsoon
Use it when: Jet stream.

The Indian Monsoon: Onset, ITCZ, Jet Streams, Breaks, El Nino and Forecasting: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: ITCZ in July · 1 mark · MCQBoard level

In July the ITCZ lies over the:
(A) Deccan plateau
(B) Gangetic plain
(C) Thar desert only
(D) Coromandel coast

Try it first, then show the solutionSolution

It lies around 20 to 25 degrees N over the Gangetic plain (1 mark).

Answer: (B)

Example 2: Meaning of a break · 2 marks · Very short answerBoard level

What is a break in the monsoon? Give one cause for northern India. (2 marks)

Try it first, then show the solutionSolution

Rain fails for one or more weeks after a few rainy days (1 mark).
Rain-bearing storms are not frequent along the monsoon trough (1 mark).

Answer: A break is a dry spell of one or more weeks within the monsoon. In northern India it occurs when rain-bearing storms are infrequent along the monsoon trough (ITCZ).

Example 3: Why the monsoon blows from the south-west · 3 marks · Short answerHarder board level

The south-east trades blow towards the equator from the southern hemisphere. Explain how they become south-west winds over India. (3 marks)

Try it first, then show the solutionSolution

Intense low over north-west India attracts the trades across the equator (1 mark).
Crossing between 40 degrees E and 60 degrees E (1 mark).
Coriolis force deflects them to the right in the northern hemisphere so they blow south-west to north-east (1 mark).

Answer: The intense low over north-west India pulls the south-east trades across the equator between 40 degrees E and 60 degrees E. After crossing, the Coriolis force deflects them to the right, so they blow from the south-west towards the north-east as the south-west monsoon.

Example 4: Assertion and reason: ITCZ · 1 mark · Assertion-reasonHarder board level

Assertion (A): In winter the winds over India reverse and blow as the north-east monsoon.
Reason (R): The ITCZ moves south of the equator in winter.
(A) Both true, R explains A
(B) Both true, R does not explain A
(C) A true, R false
(D) A false, R true

Try it first, then show the solutionSolution

The shift of the ITCZ to the south goes with the reversal of winds (1 mark).

Answer: (A)

Example 5: HOTS: late monsoon · 5 marks · Long answerHOTS / competency

A monsoon year shows a delayed onset at Kerala, a long break in July and strong El Nino conditions. Explain how the mechanisms in this chapter could connect these, and what the IMD uses to forecast. (5 marks)

Try it first, then show the solutionSolution

The monsoon depends on heating, the low over north-west India and the shift of the ITCZ (1 mark).
The easterly jet along 15 degrees N is linked to its burst (1 mark).
El Nino distorts equatorial circulation and is used in long-range forecasting (1 mark).
Breaks occur when storms are infrequent along the monsoon trough (1 mark).
IMD forecasts using 16 indicators including the Southern Oscillation (1 mark).

Answer: The monsoon arrives through the northward shift of the ITCZ and the pull of the low over north-west India, with the easterly jet stream along 15 degrees N tied to its burst. El Nino warm waters off Peru distort equatorial circulation and are used in long-range forecasting, and the Southern Oscillation (Tahiti-Darwin pressure) measures monsoon strength. A long break occurs when rain-bearing storms are infrequent along the monsoon trough. The IMD combines 16 indicators for its forecasts. (The chapter does not state that El Nino causes a specific delay; it is an aid to forecasting.)

Where students lose marks in The Indian Monsoon: Onset, ITCZ, Jet Streams, Breaks, El Nino and Forecasting

The slips examiners see most often, and what to write instead.

Mistake: Saying the monsoon is simply sea breeze on a large scale
Over-simplification.
Wrong working
Monsoon arises only from heating of land.
Right working
Heating and the shifting ITCZ, jet streams and trades are all involved.
Do this: Mention ITCZ and jet streams.
Usually costs: 1 mark.
Mistake: Wrong direction of trades
Hemisphere confusion.
Wrong working
Southwest monsoon comes from the north-east trades.
Right working
South-east trades cross the equator and are deflected.
Do this: Name them as south-east trades.
Usually costs: 1 mark.
Mistake: Confusing break with the withdrawal
Both mean no rain.
Wrong working
A break is the end of the monsoon.
Right working
A break is a dry spell within the season.
Do this: Say "one or more weeks".
Usually costs: 1 mark.

Exam tips

Draw the ITCZ position for July and January on a sketch map if asked.
Always give 40 degrees E to 60 degrees E as the crossing zone.
Link El Nino to forecasting rather than claiming it always fails the monsoon.

Practice questions on The Indian Monsoon: Onset, ITCZ, Jet Streams, Breaks, El Nino and Forecasting

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
The south-west monsoon sets in over Kerala by:
(A) 1 May
(B) 1 June
(C) 1 July
(D) 15 June
Hint
Date.
Answer
(B)
Q2 · 2 marks · Very short answerBoard level
What is the ITCZ? (2 marks)
Hint
Equator.
Answer
The ITCZ is a low-pressure zone at the equator where the trade winds converge and air ascends. In July it lies at about 20 to 25 degrees N over the Gangetic plain.
Q3 · 3 marks · Short answerHarder board level
Explain the role of the jet streams in the onset of the monsoon. (3 marks)
Hint
Westerly withdraws.
Answer
The shift of the ITCZ is linked to the withdrawal of the westerly jet stream from its position over the northern plain, south of the Himalayas. Only then does the easterly jet stream establish itself along 15 degrees N, and it is held responsible for the burst of the monsoon.
Q4 · 3 marks · Short answerHarder board level
Describe differential heating as the cause of the monsoon in April-May. (3 marks)
Hint
Low over north-west.
Answer
In April-May the sun is vertically over the Tropic of Cancer and the land north of the Indian Ocean heats intensely, forming an intense low over north-west India. The Indian Ocean stays relatively high in pressure because water heats slowly, so air is drawn from the south across the equator.
Q5 · 4 marks · Short answerHOTS / competency
What is El Nino, and how does it help with monsoon forecasting in India? (4 marks)
Hint
Peru, 3-7 years.
Answer
El Nino is a weather system appearing every 3 to 7 years in which warm water appears off Peru and temporarily replaces the cold Peruvian current, distorting equatorial circulation. It is used in India for long-range monsoon forecasting, along with the Southern Oscillation measured as the Tahiti-Darwin pressure difference.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Where does the south-west monsoon cross the equator?Show answer
Between 40 degrees E and 60 degrees E.
2.Where is the easterly jet stream in the burst?Show answer
Along 15 degrees N.
3.What does El Nino mean?Show answer
Child Christ.
4.Which agency forecasts the monsoon?Show answer
IMD.

The Indian Monsoon: Onset, ITCZ, Jet Streams, Breaks, El Nino and Forecasting: frequently asked questions

What is the monsoon trough?

The ITCZ lying over the Gangetic plain in July.

Why is it called the south-west monsoon?

The deflected winds blow from the south-west to the north-east.

Do breaks happen every year?

Such dry spells are common during the south-west monsoon.