The monsoon mechanism, the seasons, rainfall distribution and ENSO on one page. For explanations, go back to the [chapter](/geography/climate/).

{{< callout type="tip" title="How to use this sheet" >}}
Scan it top to bottom in the week before Prelims. **Bold** entries are the facts examiners return to most often.
{{< /callout >}}

## Mechanism and Dates

| Fact | Detail |
|------|--------|
| Monsoon | Arabic *mausim* = season; **seasonal reversal of winds** |
| Climate type | Tropical monsoon; Tropic of Cancer crosses the middle of India, including northern Chhattisgarh |
| Thermal concept | Halley (1686): land–sea heating contrast |
| Dynamic concept | **ITCZ** shifts to 20°–25° N = **monsoon trough** (Flohn) |
| Westerly jet | Winter, south of the Himalaya (western disturbances); **moves north of the Himalaya before onset** |
| Tropical easterly jet | Summer, upper air over the peninsula |
| Somali (Findlater) jet | Low-level cross-equatorial jet off East Africa |
| Mascarene High | High pressure near Madagascar |
| **Kerala onset** | **1 June** (± about a week) |
| **Whole country** | **8 July** (revised 2020; old value about 15 July) |
| **Withdrawal begins** | **About 17 September**, west Rajasthan (old value 1 September) |
| **Withdrawal complete** | **About 15 October** |
| Burst | Sudden, heavy-rain onset |
| Break | Monsoon trough shifts to the **Himalayan foothills**; rain there and in the north-east, dry elsewhere |

## Branches

| Branch | Key facts |
|--------|-----------|
| **Arabian Sea** (stronger) | Western Ghats (heavy rain, rain shadow on the Deccan); Narmada–Tapi corridor to central India; Saurashtra–Kachchh–Rajasthan (**parallel to the Aravalli**, little rain) |
| **Bay of Bengal** | Myanmar and Bangladesh; Garo–Khasi hills (Mawsynram, Cherrapunji); turned west by the Himalaya, so rain **decreases east to west** in the Ganga plain; depressions over **Odisha–Chhattisgarh–MP** |
| Tamil Nadu | Rain shadow in June–September; gets rain in **October–December** (north-east monsoon) |

## Seasons and Local Weather

| Season / phenomenon | Detail |
|---------------------|--------|
| Cold weather | December–February; **western disturbances** (Mediterranean; winter rain for **wheat**) |
| Hot weather | March–May; May hottest in north and central India |
| **Loo** | Hot, dry afternoon winds, north-west India |
| **Mango showers** | **Kerala and coastal Karnataka** |
| Blossom showers | **Coffee** areas of Kerala and Karnataka |
| **Kalbaisakhi / nor'westers** | **West Bengal**, Odisha, Assam; in Assam **Bardoli Chheerha** (tea showers) |
| Highest recorded temperature | 51.0 °C, Phalodi (Rajasthan), May 2016 |
| Retreating monsoon | October–November; **October heat** (hot and humid) |
| Cyclone seasons | **April–May and October–November**; Bay of Bengal more active |
| IMD statistical seasons | Winter Jan–Feb; pre-monsoon Mar–May; SW monsoon Jun–Sep; post-monsoon Oct–Dec |
| Six ritus | Vasant, Grishma, Varsha, Sharad, Hemant, Shishir |

## Rainfall Numbers

| Fact | Detail |
|------|--------|
| **Annual LPA (1971–2020)** | **1,160.1 mm** |
| **June–September LPA (1971–2020)** | **868.6 mm (87 cm)**; old (1961–2010) about 88 cm |
| Monsoon share | About **75%** of annual |
| IMD categories (all-India) | **<90% deficient**; 90–95 below normal; **96–104 normal**; 105–110 above normal; **>110 excess** |
| District / subdivision normal | Within **±19%** |
| 2023 | About 94% of LPA (El Niño + positive IOD) |
| 2025 | About 108% of LPA |
| **2026** | **About 87% of LPA (759.4 mm)**: deficient, lowest since 2015 |
| Wettest | **Mawsynram** (average); Cherrapunji (historical records) |
| Driest | **Leh**, western Rajasthan |
| Variability >50% | Western Rajasthan, Ladakh, interior Deccan |
| Rule | **Low rainfall → high variability** |

## Koeppen Types (NCERT)

| Type | Region |
|------|--------|
| Amw | West coast south of Goa |
| **As** | **Coromandel coast** |
| **Aw** | Most of the peninsular plateau (most of **Chhattisgarh**) |
| BShw | Parts of Rajasthan, Haryana, Gujarat |
| BWhw | Extreme western Rajasthan |
| **Cwg** | **Ganga plain**, eastern Rajasthan, northern MP, most of the north-east |
| Dfc | Arunachal Pradesh |
| E | High Himalaya (J&K, Ladakh, HP, Uttarakhand) |

## ENSO and IOD

| Fact | Detail |
|------|--------|
| **El Niño** | Warm central/eastern Pacific; weak trades; Peru upwelling suppressed; **usually weak Indian monsoon** |
| **La Niña** | Cool central/eastern Pacific; strong trades; **usually good monsoon** |
| Southern Oscillation | **Gilbert Walker**, 1920s; SOI = Tahiti − Darwin; **negative SOI = El Niño** |
| ENSO coupling | **Bjerknes, 1969** |
| Niño 3.4 | 5° N–5° S, 170° W–120° W; El Niño at +0.5 °C or more |
| **RONI** | NOAA's official ENSO index from **February 2026** (anomaly relative to the tropical mean) |
| El Niño Modoki | Central-Pacific warming |
| **Positive IOD** | **Western Indian Ocean warmer**; helps the monsoon (Saji–Yamagata, 1999) |
| MJO | Eastward pulse, 30–60 days; onset and active/break spells |
| 1997 | Very strong El Niño, positive IOD, near-normal monsoon |
| 2002, 2009 | El Niño droughts (deficits of about 19% and 22%) |
| 2020–2022 | Triple-dip La Niña |
| 2026 | NOAA El Niño Advisory (June 2026); monsoon about 87% of LPA |

## Chhattisgarh Angle

| Fact | Detail |
|------|--------|
| Annual rainfall | About 1,200–1,300 mm; mostly June–September |
| Main rain source | **Bay of Bengal branch** and its depressions |
| Wettest | **Bastar plateau** |
| Driest | **Western plain** (Kabirdham, Bemetara), in the lee of the **Maikal range** |
| Monsoon arrival | Around mid-June, Bastar first |
| Risk | Concentration and July–August dry spells for rain-fed paddy |

## Don't Confuse

| This | vs | That |
|------|----|------|
| Westerly jet (winter, western disturbances) | vs | Easterly jet (summer, monsoon depressions) |
| Mango showers (Kerala, Karnataka) | vs | Kalbaisakhi (Bengal) |
| Mawsynram (highest average) | vs | Cherrapunji (historical records) |
| LPA 87 cm (June–September) | vs | About 116 cm (annual) |
| Positive IOD (west warm, good) | vs | Negative IOD (east warm, bad) |
| Negative SOI (El Niño) | vs | Positive SOI (La Niña) |
| Old normal: whole country covered by 15 July | vs | Current normal: **8 July** (since 2020) |

{{< callout type="warning" title="Favourite MCQ trap" >}}
Old textbooks give **15 July** for full coverage, **1 September** for the start of withdrawal and **88 cm** for the LPA. Current values are **8 July**, **17 September** and **87 cm (1971–2020)**. And remember: **not every El Niño means drought** (1997).
{{< /callout >}}

---
Source: https://cgpscnotes.in/geography/climate/quick-revision/

Geography Climate and the Indian Monsoon

Climate and the Indian Monsoon — Quick Revision

One-page CGPSC quick revision for Climate and the Indian Monsoon: dates, LPA figures, seasons, local winds, Koeppen types, El Niño, La Niña and IOD.

4 min read Updated Oct 2026 #quick revision#cgpsc#monsoon

The monsoon mechanism, the seasons, rainfall distribution and ENSO on one page. For explanations, go back to the chapter.

How to use this sheet

Scan it top to bottom in the week before Prelims. Bold entries are the facts examiners return to most often.

Mechanism and Dates

Fact	Detail
Monsoon	Arabic mausim = season; seasonal reversal of winds
Climate type	Tropical monsoon; Tropic of Cancer crosses the middle of India, including northern Chhattisgarh
Thermal concept	Halley (1686): land–sea heating contrast
Dynamic concept	ITCZ shifts to 20°–25° N = monsoon trough (Flohn)
Westerly jet	Winter, south of the Himalaya (western disturbances); moves north of the Himalaya before onset
Tropical easterly jet	Summer, upper air over the peninsula
Somali (Findlater) jet	Low-level cross-equatorial jet off East Africa
Mascarene High	High pressure near Madagascar
Kerala onset	1 June (± about a week)
Whole country	8 July (revised 2020; old value about 15 July)
Withdrawal begins	About 17 September, west Rajasthan (old value 1 September)
Withdrawal complete	About 15 October
Burst	Sudden, heavy-rain onset
Break	Monsoon trough shifts to the Himalayan foothills; rain there and in the north-east, dry elsewhere

Branches

Branch	Key facts
Arabian Sea (stronger)	Western Ghats (heavy rain, rain shadow on the Deccan); Narmada–Tapi corridor to central India; Saurashtra–Kachchh–Rajasthan (parallel to the Aravalli, little rain)
Bay of Bengal	Myanmar and Bangladesh; Garo–Khasi hills (Mawsynram, Cherrapunji); turned west by the Himalaya, so rain decreases east to west in the Ganga plain; depressions over Odisha–Chhattisgarh–MP
Tamil Nadu	Rain shadow in June–September; gets rain in October–December (north-east monsoon)

Seasons and Local Weather

Season / phenomenon	Detail
Cold weather	December–February; western disturbances (Mediterranean; winter rain for wheat)
Hot weather	March–May; May hottest in north and central India
Loo	Hot, dry afternoon winds, north-west India
Mango showers	Kerala and coastal Karnataka
Blossom showers	Coffee areas of Kerala and Karnataka
Kalbaisakhi / nor’westers	West Bengal, Odisha, Assam; in Assam Bardoli Chheerha (tea showers)
Highest recorded temperature	51.0 °C, Phalodi (Rajasthan), May 2016
Retreating monsoon	October–November; October heat (hot and humid)
Cyclone seasons	April–May and October–November; Bay of Bengal more active
IMD statistical seasons	Winter Jan–Feb; pre-monsoon Mar–May; SW monsoon Jun–Sep; post-monsoon Oct–Dec
Six ritus	Vasant, Grishma, Varsha, Sharad, Hemant, Shishir

Rainfall Numbers

Fact	Detail
Annual LPA (1971–2020)	1,160.1 mm
June–September LPA (1971–2020)	868.6 mm (87 cm); old (1961–2010) about 88 cm
Monsoon share	About 75% of annual
IMD categories (all-India)	<90% deficient; 90–95 below normal; 96–104 normal; 105–110 above normal; >110 excess
District / subdivision normal	Within ±19%
2023	About 94% of LPA (El Niño + positive IOD)
2025	About 108% of LPA
2026	About 87% of LPA (759.4 mm): deficient, lowest since 2015
Wettest	Mawsynram (average); Cherrapunji (historical records)
Driest	Leh, western Rajasthan
Variability >50%	Western Rajasthan, Ladakh, interior Deccan
Rule	Low rainfall → high variability

Koeppen Types (NCERT)

Type	Region
Amw	West coast south of Goa
As	Coromandel coast
Aw	Most of the peninsular plateau (most of Chhattisgarh)
BShw	Parts of Rajasthan, Haryana, Gujarat
BWhw	Extreme western Rajasthan
Cwg	Ganga plain, eastern Rajasthan, northern MP, most of the north-east
Dfc	Arunachal Pradesh
E	High Himalaya (J&K, Ladakh, HP, Uttarakhand)

ENSO and IOD

Fact	Detail
El Niño	Warm central/eastern Pacific; weak trades; Peru upwelling suppressed; usually weak Indian monsoon
La Niña	Cool central/eastern Pacific; strong trades; usually good monsoon
Southern Oscillation	Gilbert Walker, 1920s; SOI = Tahiti − Darwin; negative SOI = El Niño
ENSO coupling	Bjerknes, 1969
Niño 3.4	5° N–5° S, 170° W–120° W; El Niño at +0.5 °C or more
RONI	NOAA’s official ENSO index from February 2026 (anomaly relative to the tropical mean)
El Niño Modoki	Central-Pacific warming
Positive IOD	Western Indian Ocean warmer; helps the monsoon (Saji–Yamagata, 1999)
MJO	Eastward pulse, 30–60 days; onset and active/break spells
1997	Very strong El Niño, positive IOD, near-normal monsoon
2002, 2009	El Niño droughts (deficits of about 19% and 22%)
2020–2022	Triple-dip La Niña
2026	NOAA El Niño Advisory (June 2026); monsoon about 87% of LPA

Chhattisgarh Angle

Fact	Detail
Annual rainfall	About 1,200–1,300 mm; mostly June–September
Main rain source	Bay of Bengal branch and its depressions
Wettest	Bastar plateau
Driest	Western plain (Kabirdham, Bemetara), in the lee of the Maikal range
Monsoon arrival	Around mid-June, Bastar first
Risk	Concentration and July–August dry spells for rain-fed paddy

Don’t Confuse

This	vs	That
Westerly jet (winter, western disturbances)	vs	Easterly jet (summer, monsoon depressions)
Mango showers (Kerala, Karnataka)	vs	Kalbaisakhi (Bengal)
Mawsynram (highest average)	vs	Cherrapunji (historical records)
LPA 87 cm (June–September)	vs	About 116 cm (annual)
Positive IOD (west warm, good)	vs	Negative IOD (east warm, bad)
Negative SOI (El Niño)	vs	Positive SOI (La Niña)
Old normal: whole country covered by 15 July	vs	Current normal: 8 July (since 2020)

Favourite MCQ trap

Old textbooks give 15 July for full coverage, 1 September for the start of withdrawal and 88 cm for the LPA. Current values are 8 July, 17 September and 87 cm (1971–2020). And remember: not every El Niño means drought (1997).

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