How the monsoon works: onset, burst, breaks and its uneven nature
Monsoon means a seasonal reversal of winds. Hot land in summer draws moist sea winds from the south-west; they burst on the Kerala coast in early June.
Air pressure in three lines
- Air has weight. The push of this weight on us is air pressure.
- Temperature and air pressure are inversely related: where it is hot, pressure is low; where it is cool, pressure is high.
- Winds blow from high pressure to low pressure.
How the summer monsoon sets in
In summer the jet stream shifts north of the Himalayas, over Central Asia. This helps the monsoon to start.
A complete reversal of winds
In winter, the north-east trade winds blow from land to sea, so they are dry. By mid-June the general movement of air is from the equatorial Indian Ocean towards the subcontinent. These winds blow from the south-west to the north-east, exactly opposite to the winter winds. This complete reversal with the season is the monsoon, and the summer winds are the south-west monsoon. They are strong, blowing at about 30 km per hour on average.
Onset, burst, branches and breaks
- Onset is the arrival of the monsoon winds and rain. They reach the Andaman and Nicobar Islands in the last week of May and the Kerala coast in the first week of June.
- Burst: on arrival the monsoon gives heavy rain for several days, with thunder and lightning. This happens mainly on the Kerala coast.
- Arabian Sea branch: blocked by the Western Ghats, it gives heavy rain on their western side. It reaches Mumbai by about 10 June and the northern plains by about 20 June.
- East of the Ghats, the Deccan and parts of Madhya Pradesh get less rain. This is a rain shadow: the side of a mountain away from the wind.
- Bay of Bengal branch: it strikes the north-east and coastal West Bengal, then moves west along the plains. Rain falls as it goes: about 120 cm at Kolkata, 91 cm at Allahabad and 56 cm at Delhi.
- The monsoon covers the whole country by about 15 July, as your lesson says; the India Meteorological Department's newer normal date is 8 July.
- Breaks: rain falls for a few days at a time, with rainless dry spells in between. These wet and dry spells are the breaks in the monsoon.
- The rain cools the land: temperatures fall by 5 °C to 8 °C between mid-June and mid-July.
Why the monsoon is uneven
- Monsoon winds are not steady. They are irregular and change with local conditions.
- Their arrival can be early or late, and the breaks can be long or short.
- The rain is not spread equally: some areas get very heavy rain, others very little.
- So the same season can bring floods to some areas and droughts to others. Poor management of water makes floods worse.
On the map
- Kerala coast: the south-west coast, where the monsoon arrives and bursts first.
- Andaman and Nicobar Islands: in the Bay of Bengal; touched in the last week of May.
- Mumbai: on the west coast; reached by about 10 June.
- Monsoonal trough: an imaginary line from Jaisalmer in western Rajasthan to Balasore on the northern Odisha coast.
- Rain shadow: the interior Deccan, east of the Western Ghats, in Maharashtra and Karnataka.
Every year news channels report the day the monsoon reaches Kerala. Farmers across India time the sowing of rice, cotton and soybean by its arrival.
2 marks: “What is meant by the burst of monsoon? Where does it mainly occur?” The sudden onset of heavy rain lasting several days when the monsoon winds arrive. It occurs mainly on the Kerala coast, in the first week of June.
3 marks: “Using the terms reversal, burst and breaks, explain three features of the Indian monsoon.” (1) Reversal: winds blow from the south-west in summer and from the north-east in winter. (2) Burst: heavy rain for several days on arrival, mainly on the Kerala coast. (3) Breaks: wet spells with rainless dry spells between them. Add that the monsoon is uneven, bringing floods to some areas and droughts to others.
1 marks: “From which word does monsoon come, and what does it mean?” From the Arabic word 'mausim', which means season.
Mausim means season. Summer winds come sea to land from the south-west; winter winds go land to sea from the north-east.