The Monsoon Mechanism
NCERT Class 9 • Contemporary India I • Chapter "Climate" | NCERT Class 11 • India Physical Environment • Chapter "Climate"
The Monsoon Mechanism
The word monsoon comes from the Arabic mausim, meaning season. It describes the complete reversal of wind direction between summer and winter. In summer moist winds blow from sea to land and bring rain; in winter dry winds blow from land to sea. This reversal is driven by differential heating and by shifts in upper air circulation.
How the Monsoon Forms
- In summer the land heats faster than the sea, creating a low pressure area over north-west India.
- High pressure sits over the southern Indian Ocean near Madagascar.
- Air moves from the high pressure sea towards the low pressure land, picking up moisture on the way.
- The Inter Tropical Convergence Zone (ITCZ) shifts north over the Ganga plain in summer, pulling the winds further.
- The westerly jet stream withdraws north of the Himalaya and the easterly jet appears, which helps the monsoon burst.
- The sudden arrival with heavy rain and thunder is called the burst of the monsoon.
The Two Branches
| Branch | Path | Rain It Gives |
|---|---|---|
| Arabian Sea | Western Ghats, then north | Very heavy on windward Ghats |
| Bay of Bengal | Up the Ganga plain and to the north-east | Heavy in north-east and plains |
The Arabian Sea branch strikes the Western Ghats, so Mumbai and the Konkan get heavy rain while the leeward Deccan stays dry. The Bay of Bengal branch is turned west by the Himalaya and moves up the Ganga plain, so rainfall decreases from Kolkata towards Delhi and Punjab.
Onset, Break and Withdrawal
- The monsoon usually reaches Kerala around the first week of June and covers the country by mid-July.
- A break in the monsoon is a dry spell of some days in the middle of the rainy season.
- Withdrawal or retreat begins from the north-west by about September and moves south.
- During retreat, cyclones often form in the Bay of Bengal and strike the east coast.
- The total quantity and the timing of monsoon rain are both highly variable from year to year.
- The retreat is gradual and takes about two months to clear the whole country.
- Tamil Nadu receives much of its yearly rain during this retreating phase.
Burst of monsoon — the sudden onset of heavy rain when the monsoon current arrives over a region.
Exam me kaise aata hai
- The word monsoon comes from — Arabic mausim
- Monsoon first reaches which state — Kerala
- Which branch gives rain to the Western Ghats — Arabian Sea branch
- A dry spell inside the rainy season is called — break in the monsoon
UPSC/State PSC ke liye
- The Somali jet and the strength of the cross-equatorial flow affect how vigorous the Arabian Sea branch will be.
- A break in the monsoon happens when the monsoon trough moves close to the Himalayan foothills, giving heavy hill rain and dry plains.
Yahan confuse hote hain
✗ Monsoon means only rain | Monsoon means the seasonal reversal of winds | ✓
✗ Monsoon reaches Delhi before Kolkata | It reaches Kolkata first and Delhi later | ✓
✗ Retreating monsoon gives rain to the whole country | It mainly affects the south-east coast | ✓
Ek nazar me
- Monsoon = reversal of wind direction between summer and winter.
- Summer low over north-west India pulls in moist sea winds.
- Two branches: Arabian Sea and Bay of Bengal.
- Onset in Kerala in early June; retreat from the north-west by September.