Variability of rainfall

Also called: Rainfall variability · Topic: Climate of India and the Monsoon · NCERT: Class 11, Ch 4 "Climate"; Class 11, Ch 6 "Natural Hazards and Disasters: Causes, Consequences and Management"

Meaning

Variability of rainfall is how much the rain of a place changes from one year to the next, compared with its long-term average. It is measured by the coefficient of variation (CV). A higher CV means the rain is less reliable.

It matters because the variability is highest where rainfall is lowest. So some part of India faces a drought or a flood every year. This affects crops, rural incomes and disaster planning.

Explanation

How it is measured

• Coefficient of variation (CV): the standard measure of variability.
• CV = (standard deviation ÷ mean) × 100.
• Standard deviation shows how far each year's rainfall usually moves away from the average.

• Dividing by the mean turns this into a percentage. Wet and dry places can then be compared fairly.

• High CV (more than 50%): the rain changes a lot from year to year. Farmers cannot depend on it.

• Low CV (less than 25%): the rain arrives in fairly similar amounts every year. It is reliable.
• Long Period Average (LPA): the long-term "normal" that IMD uses to judge one season.
• The LPA for south-west monsoon rain (June–September) over all of India is 87 cm, based on 1971–2020 [2].
• IMD calls a season deficient below 90% of LPA, normal at 96–104% and excess above 110% [2].

Why dry areas have the highest variability (the inverse rule)

• Variability is highest where rainfall is lowest.
• Dry areas get their rain from a few weak, irregular rain systems.
• With so few rainy events, one storm more or one storm less in a year changes the total a lot.

• So the CV goes above 50% in west Rajasthan, Kachchh and the Thar.

• Wet areas have low variability.

• On the west coast and in the North-East, moist monsoon winds are pushed up by mountains every year. This is orographic rain (rain caused when mountains force moist air to rise).
• Heavy rain comes in season after season, so one year's total stays close to the average. The CV is below 25%.

Variability in timing, not only in totals

• A "normal" year can still bring crop failure.
• A late onset (start) of the monsoon, an early withdrawal (end), or a long break (dry spell) in July–August can ruin kharif crops.

• This can happen even when the season's 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 rose by about 75% between 1950 and 2015 [4].
• Result: more droughts and more flash floods, sometimes in the same season.

Variability across regions in the same year

• The all-India total can hide large regional gaps.
• The 2024 monsoon brought 934.8 mm, or 108% of the LPA (868.6 mm) [4].

• But Central India got 119%, the South Peninsula 114%, North-west India 107%, and East & North-East India only 86% [4].

• 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].

• Ocean links that shift the monsoon from year to year:

• El Niño (a warming of sea-surface temperature in the equatorial Pacific, part of ENSO) usually weakens the Indian monsoon.
• A positive IOD (Indian Ocean Dipole: the western Indian Ocean warmer than the eastern part) usually helps the monsoon.
• IMD's first-stage forecast for 2026 (April 2026) was 92% of LPA (below normal), with a model error of ±5%. It warned that El Niño could develop during the season and that a positive IOD could form towards its end [2].

Where it occurs

CV of annual rainfall	Regions in India	Rainfall zone
< 25% (most reliable)	West coast, Western Ghats, north-eastern Peninsula, eastern Ganga plain, North-East India	Mostly high (> 200 cm) to medium (100–200 cm)
25–50%	Most of the interior Peninsula and the central plains	Mostly medium to low
> 50% (least reliable)	West Rajasthan, Kachchh (Gujarat), the Thar Desert	Inadequate (< 50 cm) to low

• South Peninsula, north-east monsoon (October–December):
• 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].

• This matters for Tamil Nadu, which gets 48% of its annual rain (about 44 cm of its 92 cm total) in October–December [3].

• Rain-shadow interior Deccan: the interior of Maharashtra, Karnataka and Rayalaseema (Andhra Pradesh) gets little rain that does not come reliably. This makes it a drought-prone, semi-arid belt.

• World: the same inverse rule holds in dry lands everywhere. Hot deserts and semi-arid steppes have the least reliable rain.

Don't confuse with

• Distribution of rainfall: distribution is how rain changes from place to place (spatial). Variability is how rain changes from year to year at one place (temporal). The west coast is high on rain and low on variability. The Thar is low on rain and high on variability.
• Average annual rainfall (about 125 cm) vs LPA (87 cm): 125 cm is India's total rain for the whole year. 87 cm is the LPA for the four monsoon months only (June–September, 1971–2020) [2]. Variability is judged as a departure from these averages.
• Drought: drought is an event, a shortage of rain in a given year or season. Variability is the long-term tendency of a place to swing between wet and dry years. High variability makes drought more likely.
• South-west monsoon variability vs north-east monsoon variability: over the south Peninsula, the CV of north-east monsoon (Oct–Dec) rain (30–50%) is higher than that of south-west monsoon rain [3].

Prelims Hooks

• CV = (standard deviation ÷ mean) × 100. A higher CV means less reliable rain.
• CV > 50%: west Rajasthan, Kachchh, Thar Desert. CV < 25%: west coast, Western Ghats, north-eastern Peninsula, eastern Ganga plain, North-East India.
• Inverse rule: variability is highest where rainfall is lowest. So some part of India faces a drought or a flood every year (NCERT Class 11).
• IMD "normal" monsoon = 96–104% of LPA. "Deficient" = below 90%. LPA = 87 cm (1971–2020) [2].
• Trap: for the south Peninsula, the CV of north-east monsoon rain (30–50%) is higher than that of the south-west monsoon. The east coast has a lower CV than the interior [3].
• Dry spells rose 27% (1981–2011 vs 1951–1980). 150 mm+ rain events in central India rose about 75% (1950–2015) [4].

Mains Points

• Causes and pattern (GS-I): rainfall variability follows India's relief and wind systems.
• Orographic rain on the west coast and in the North-East gives heavy rain that comes reliably (CV < 25%).
• Rain-shadow areas and the Thar get rain from a few weak, irregular systems. The Aravalli runs parallel to the Arabian Sea branch, so it does not lift the winds. The result is CV > 50%.

• Ocean links such as El Niño and the IOD shift the monsoon from year to year. An example is the April 2026 forecast of 92% of LPA [2].

• Variability as a disaster driver (GS-III): dry spells are 27% more frequent and 150 mm+ events in central India are up about 75% [4].

• Drought and flash floods now strike more often, sometimes in the same season and region.
• Even in 2024, a 108% year, East & North-East India got only 86% of normal rain [4].

• Responses: district-level contingency crop plans, watershed development, and better early warning and long-range forecasts from IMD.

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

• So unreliable rain affects kharif output, rural incomes, food inflation and hydropower.
• Responses: micro-irrigation, rainwater harvesting, tanks to store north-east monsoon rain in Tamil Nadu, dryland farming in the high-CV Thar and Kachchh, and crop diversification away from water-hungry rice in low-rainfall Punjab and Haryana.

Related concepts

• Annual rainfall of India
• Climatic regions of India

Read more

• Distribution and variability of rainfall; climatic regions of India