Distribution and variability of rainfall; climatic regions of India

Detail

1. Annual rainfall of India: the averages

• Average annual rainfall of India: about 125 cm. The amount changes a great deal from place to place (spatial variation: how a value differs across the map).
• About half of India gets less than 75 cm in a year. Because of this, the national average hides very wet strips and very dry areas.
• Long Period Average (LPA): the average rainfall over a long base period. IMD uses it as the "normal" for judging a season.
• The LPA for south-west monsoon rain (June–September) over the whole country is 87 cm, based on 1971–2020 [2].

• The 87 cm is for the four monsoon months only. The 125 cm is the total for the whole year. Do not mix up the two.

• About 75% of India's yearly rainfall comes in the south-west monsoon season (June–September) [4].

• IMD categories for the all-India monsoon (as % of LPA) [2]:

Category	Rainfall (% of LPA)
Deficient	< 90
Below normal	90–95
Normal	96–104
Above normal	105–110
Excess	> 110

• Recent figures:
• The 2024 monsoon brought 934.8 mm, which was 108% of the LPA (868.6 mm). The regions varied: Central India got 119%, the South Peninsula 114%, North-west India 107% and East & North-East India only 86% [4].
• IMD's first-stage forecast for 2026 (April 2026) was 92% of LPA (below normal), with a model error of ±5%. It said El Niño could develop during the season and a positive IOD could form towards its end [2].
• ENSO (El Niño–Southern Oscillation): a warm or cool swing in sea-surface temperature over the equatorial Pacific. El Niño usually weakens the Indian monsoon.
• IOD (Indian Ocean Dipole): a temperature difference between the western and eastern Indian Ocean. A positive IOD usually helps the monsoon.

2. Distribution of rainfall: the four zones (NCERT Class 11, Fig. 4.4)

Zone	Annual rainfall	Areas	Main reason
High	> 200 cm	West coast, Western Ghats, sub-Himalayan North-East, Meghalaya hills (> 1,000 cm in parts of the Khasi and Jaintia hills)	Moist winds are forced up against mountains
Medium	100–200 cm	South Gujarat, east Tamil Nadu, Odisha, Jharkhand, Bihar, east Madhya Pradesh, northern Ganga plain along the sub-Himalaya, Cachar valley (Assam), Manipur	Bay of Bengal branch, depressions, and NE monsoon on the Coromandel coast
Low	50–100 cm	West Uttar Pradesh, Delhi, Haryana, Punjab, Jammu and Kashmir, east Rajasthan, Gujarat, Deccan plateau	Far from the sea, or in rain shadow
Inadequate	< 50 cm	Parts of Andhra Pradesh, Karnataka and Maharashtra; Ladakh; most of west Rajasthan	Deep rain shadow, or no mountain barrier to lift the winds

• Snowfall is limited to the Himalaya.
• The Brahmaputra valley gets less than 200 cm, even though the hills around it get much more. This is a common exam trap.

Why the west coast and Western Ghats are wet (orographic rain):

• The Arabian Sea branch of the south-west monsoon blows in from the south-west and hits the Western Ghats almost at a right angle.
• The air is pushed up. Rising air cools, its water vapour condenses, and heavy rain falls on the windward side (the side facing the wind).
• Orographic rain: rain caused when mountains force moist air to rise.

Why the interior Deccan is dry (rain shadow):

• The air comes down the eastern, leeward side (the side away from the wind).
• Air that sinks gets warmer and drier, so little rain falls.
• Rain shadow: the dry area behind a mountain. Examples are the interior of Maharashtra, Karnataka and Rayalaseema (Andhra Pradesh), which fall in the "inadequate" belt.

Why Meghalaya is the wettest:

• The Bay of Bengal branch moves north and enters the funnel-shaped Garo, Khasi and Jaintia hills.
• The air is trapped and forced to rise sharply, which gives extreme rain.
• Mawsynram (East Khasi Hills, Meghalaya) holds the WMO record for the greatest average annual rainfall in Asia (WMO Region II): 11,872 mm (about 1,187 cm), averaged over 38 years (1941–1979) [5][6].
• Cherrapunji (Sohra) is close by and is also among the wettest places on Earth [6].

Why rain decreases westward along the Ganga plain:

• The Bay of Bengal branch turns west along the Himalaya. It drops rain as it goes, so it holds less moisture further west.
• Rain therefore falls from the medium zone (Bihar, east UP) to the low zone (west UP, Delhi, Punjab).

Why west Rajasthan is a desert:

• The Aravalli range runs south-west to north-east, nearly parallel to the Arabian Sea branch.
• The winds pass along the range without being forced up, so they give little rain.
• Hot, dry land also makes the air less likely to drop rain.

The Coromandel coast exception:

• Tamil Nadu gets more rain in the north-east monsoon (October–December) than in the south-west monsoon.
• Its October–December rain is about 44 cm, which is 48% of its 92 cm annual total (1971–2020) [3].
• The Karaikal–Vedaranyam belt gets 900–1,000 mm in this season. Vedaranyam gets 103 cm [3].
• Tuticorin gets only 40–45 cm of normal rainfall, because the Sri Lanka landmass shelters that part of the coast [3].

3. The 100 cm isohyet: the rice–wheat divide

• Isohyet: a line on a map that joins places with equal rainfall.
• The 100 cm isohyet roughly separates rice lands from wheat lands (link: crops-agriculture-india).
• East and south of the line (more than 100 cm): rice, which needs standing water. Examples are West Bengal, Odisha, Assam and coastal areas.

• North-west of the line (less than 100 cm): wheat, which grows in the drier winter season. Examples are Punjab, Haryana and west UP.

• Course of the line (NCERT project): it runs roughly from south Gujarat, east of the Western Ghats rain shadow, through east Madhya Pradesh and along the middle Ganga plain towards the sub-Himalaya.

• In Punjab and Haryana, irrigation has pushed rice beyond its rainfall limit. This is one reason those states depend heavily on groundwater.

4. Variability of rainfall

• Variability of rainfall: how much the rain of a place changes from one year to the next.
• Coefficient of variation (CV): the measure used for variability.
• CV = (standard deviation ÷ mean) × 100.
• A higher CV means rain is less reliable.

CV of annual rainfall	Regions (NCERT, earlier Class 11 edition map)
< 25% (most reliable)	West coast, Western Ghats, north-eastern Peninsula, eastern Ganga plain, North-East India
25–50%	Most of the interior Peninsula and the central plains (in between)
> 50% (least reliable)	West Rajasthan, Kachchh (Gujarat), the Thar Desert

The inverse rule: variability is highest where rainfall is lowest.

• Dry areas get rain from a few weak, irregular systems.
• So one storm more or less in a year changes the total a lot.
• As a result, some part of India faces a drought or a flood every year (NCERT Class 11, effect on economic life, point iv).

North-east monsoon variability:

• Over the south Peninsula, the CV of October–December rain is 30–50%. This is generally higher than the CV of south-west monsoon rain [3].
• The east coast has smaller CVs than the interior [3].

Timing matters as much as totals:

• A late onset, an early withdrawal or a long break (dry spell) in July–August can ruin kharif crops even when the yearly total is "normal".
• Dry spells in the summer monsoon became 27% more common in 1981–2011 than in 1951–1980 [4].
• In central India, extreme daily rain events above 150 mm increased by about 75% between 1950 and 2015 [4].
• The result is more drought and more flash floods at the same time.
• IMD's sub-divisional scale:
• Deficient = 20–59% below normal; large deficient = 60–99% below normal [4].
• 16 sub-divisions were deficient or large deficient in 2015, the highest in a decade [4].
• 78% of districts got normal or excess rain in 2024, compared with 51% in 2015 [4].

5. Climatic regions of India: the simple seven-type scheme (NCERT Class 7)

Type	Where	Key features / crops
Alpine	High Himalaya	Very cold; snow for much of the year
Temperate	Lower Himalaya and hills	Cool summers, cold winters
Subtropical	Northern plains	Hot summers, cool winters; wheat
Arid	Thar (west Rajasthan)	Very low, uncertain rain; very hot days
Tropical wet	West coast	Heavy monsoon rain; rice and spices
Semi-arid	Deccan plateau (rain shadow)	Moderate, unreliable rain
Tropical	Eastern India and south Peninsula	Distinct wet and dry periods

6. Climatic regions of India: Koeppen scheme (NCERT, earlier Class 11 edition)

• Koeppen classification: it divides climates using monthly temperature and rainfall, matched to the natural vegetation (the full scheme is in world-climate-koeppen).
• Main letters:
• A = tropical: every month warmer than 18 °C.
• B = dry: more water evaporates than falls as rain.
• C = warm temperate: coldest month between 18 °C and −3 °C.
• D = cold snow forest: coldest month below −3 °C.

• E = polar or ice: warmest month below 10 °C.

• Small letters:

• m = monsoon with a short dry season.
• s = dry summer.
• w = dry winter.
• h = hot (dry climates).
• f = no dry season.
• c = cool, short summer.
• g = Ganges type: the hottest month comes before the summer solstice (June), because the rains cool July and August.

Code	Type	Region
Amw	Monsoon with short dry season	West coast south of Goa
As	Monsoon with dry summer, rain in winter (Oct–Dec)	Coromandel coast (Tamil Nadu, south Andhra Pradesh)
Aw	Tropical savanna	Most of the Peninsular plateau (south of the Tropic of Cancer)
BShw	Semi-arid steppe	North-west Gujarat, parts of west Rajasthan and Punjab
BWhw	Hot desert	Extreme west Rajasthan
Cwg	Monsoon with dry winter	Ganga plain, east Rajasthan, north Madhya Pradesh, most of the North-East
Dfc	Cold humid winter, short summer	Arunachal Pradesh
E	Polar type	Jammu and Kashmir, Himachal Pradesh, Uttarakhand (high Himalaya)

• How "As" fits the Coromandel coast:
• In summer, this coast lies in the rain shadow of the Western Ghats, so its summer is dry.
• In October–December, retreating monsoon and north-east monsoon winds pick up moisture over the Bay of Bengal.
• They bring the main rain, along with depressions and cyclones.

• This is why Tamil Nadu gets 48% of its annual rain in this season [3].

• How the two schemes match:

Class 7 type	Koeppen code
Tropical wet	Amw
Arid	BWhw
Semi-arid	BShw, and the drier Aw areas
Subtropical	Cwg
Alpine / temperate	E / Dfc

Prelims Hooks

• Mawsynram (Meghalaya) holds the WMO record for the greatest average annual rainfall in Asia: 11,872 mm, 1941–1979 [5]. The Brahmaputra valley itself gets less than 200 cm.
• LPA of south-west monsoon rain = 87 cm (1971–2020) [2]. India's average annual rainfall = about 125 cm. "Normal" monsoon = 96–104% of LPA [2].
• About 75% of India's yearly rain falls in June–September [4]. About half of India gets less than 75 cm.
• The 100 cm isohyet divides rice lands (wetter side) from wheat lands (drier side).
• CV of rainfall: < 25% on the west coast and in the North-East; > 50% in west Rajasthan, Kachchh and the Thar. Variability is highest where rainfall is lowest.
• Ladakh is in the < 50 cm zone, while Jammu and Kashmir as a whole is in the 50–100 cm zone. Cachar and Manipur are in the 100–200 cm zone.
• Koeppen pairs:
• As → Coromandel coast
• Amw → west coast south of Goa
• Dfc → Arunachal Pradesh
• BWhw → extreme west Rajasthan
• Cwg → Ganga plain

• E → J&K, HP, Uttarakhand

• Trap: Tamil Nadu gets more rain in Oct–Dec (48% of its annual total) than in June–September [3]. Its CV for north-east monsoon rain (30–50%) is higher than for south-west monsoon rain [3].

• Class 7 types: the tropical wet type has rice and spices (west coast). The subtropical type has wheat (northern plains).

Mains Points

• Pattern and causes (GS-I): India's rain follows its relief.
• Orographic rain makes the west coast and Meghalaya very wet (> 200 cm; Mawsynram 1,187 cm [5]).
• The rain shadow of the Western Ghats leaves the interior Deccan dry.
• The Aravalli runs parallel to the winds, so the Thar stays dry.
• Rain falls steadily westward along the Ganga plain as the Bay branch loses moisture.

• This pattern shapes the crop map (the 100 cm isohyet splits rice and wheat), settlement density and water stress.

• Variability as a disaster driver (GS-III): variability is highest where rainfall is lowest (CV > 50% in the Thar and Kachchh).

• As a result, drought and flood hit some part of India every year.
• Dry spells rose 27% (1981–2011 compared with 1951–1980), and 150 mm+ rain events in central India rose about 75% (1950–2015) [4]. So drought and flash floods now strike more often, even together.

• This calls for district-level contingency crop plans, watershed development and better early warning.

• Agriculture and economy (GS-III): about 64% of Indians depend on agriculture, and only about 55% of net sown area is irrigated [4].

• A below-normal monsoon (e.g. the April 2026 forecast of 92% of LPA, linked to a possible El Niño [2]) therefore affects rural incomes, food inflation and hydropower.

• Policy responses: micro-irrigation, rainwater harvesting, crop diversification away from water-hungry rice in low-rainfall Punjab and Haryana, and IMD's improved long-range forecasting.

• Regional climatic zoning (GS-I): the Koeppen and Class 7 schemes help in planning.

• Examples: Amw/tropical-wet areas suit plantations and spices. BWhw/BShw areas need dryland farming and pastoralism. As (Coromandel) areas need tanks to store north-east monsoon water. E/Dfc Himalayan areas need cold-climate horticulture and protection from snow-related hazards.