Long Period Average (LPA) and IMD Rainfall Categories

The Long Period Average, or LPA, is the average monsoon rainfall that India received over a long base period of many years. The India Meteorological Department (IMD) uses it as the "yardstick" to judge each year's monsoon. For the whole country, the LPA of the June to September monsoon is about 87 cm (868.6 mm), based on the 50 years from 1971 to 2020. When you read "the monsoon was 87% of LPA" or "rainfall was 12% deficient", the comparison is always with this average.

Why do we need an average at all?

Rainfall in India changes a lot from year to year. A number like "760 mm of rain" means nothing on its own. Is that good or bad? We can only tell by comparing it with what is usual. Think of a student's marks: 70 out of 100 is good if the class average is 55, but weak if the class average is 85.

The LPA is the "class average" of monsoon rain. It gives farmers, planners, the Reserve Bank of India and the government one common way to describe how good or bad a season was.

Why is a 50-year average used?

One bad decade or one lucky decade should not shift the yardstick. So the IMD takes a long period, usually about 50 years. This smooths out short ups and downs. Scientists say monsoon rain also moves through long dry and wet phases, called epochs, that each last a few decades. A long base period covers parts of both.

How has the LPA changed over time?

The IMD updates the all-India normal about once every 10 years, so that it reflects recent climate.

• Base period 1951 to 2000: LPA of about 89 cm.
• Base period 1961 to 2010: LPA of about 88 cm (880.6 mm).
• Base period 1971 to 2020: LPA of about 87 cm (868.6 mm). The IMD announced this new normal in April 2022 and used it from the 2022 monsoon season.

The new LPA is about 12 mm lower than the old one. This happened because the 1971 to 2020 period includes many years from a relatively dry phase of the monsoon. It does not mean that one single year became drier.

How does the IMD classify the whole season?

At the end of the season, the IMD compares the all-India rainfall with the LPA and puts it in one of five bands:

• Deficient: less than 90% of LPA
• Below normal: 90% to 95% of LPA
• Normal: 96% to 104% of LPA
• Above normal: 105% to 110% of LPA
• Excess: more than 110% of LPA

So in a season where rainfall is 87% of LPA (a 13% shortfall), the monsoon is called "deficient". At 94% of LPA it is "below normal". A small difference in rain can move the season into a different band.

How are states and districts classified?

For a smaller area, such as a state, a meteorological subdivision or a district, the IMD looks at the departure from that area's own normal. The "departure" is how far actual rain is above or below normal, in per cent. The bands here are wider, because rain in a small area naturally varies more:

• Large excess: +60% or more
• Excess: +20% to +59%
• Normal: −19% to +19%
• Deficient: −20% to −59%
• Large deficient: −60% to −99%
• No rain: −100%

This is why one state can be "normal" in a year when the country as a whole is "deficient". For example, Meghalaya, normally one of the rainiest places on Earth, received about 60% less rain than normal in the 2026 monsoon, which puts it in the "large deficient" band.

How does the IMD forecast the monsoon against the LPA?

The IMD gives its season forecast as a percentage of the LPA, in two stages:

1. First stage, in April: a forecast for the whole country, usually with a model error of about ±5%.
2. Second stage, by the end of May: an updated forecast with a smaller error (about ±4% in recent years), plus forecasts for regions and for the month of June.

The forecast uses a Multi-Model Ensemble (MME). This means the IMD combines the results of several computer climate models, including its own Monsoon Mission Climate Forecast System (MMCFS), instead of trusting just one. For forecasting and reporting, the IMD also divides the country into four broad regions: Northwest India, Central India, South Peninsula, and East and Northeast India, and a Monsoon Core Zone in central India where most farming depends on rain.

What happened to "drought years"?

Earlier, the IMD called a year an "all-India drought year" when the country's monsoon rain was more than 10% below LPA and 20% to 40% of the country's area faced drought. If more than 40% of the area was hit, it was an "all-India severe drought year". In 2016, the IMD stopped using the word "drought" for the whole country.

It now uses "deficient year" and "large deficient year" for these two situations. One reason: declaring a drought, which triggers relief for farmers, is the job of state governments, using many signs such as crop condition, soil moisture and reservoir levels. The IMD's job is to measure rainfall and report the shortfall. The Manual for Drought Management (2016) of the Ministry of Agriculture guides states on how to declare a drought.

India's position: why the LPA matters so much here

The June to September monsoon gives India about three-fourths of its yearly rainfall. About half of India's farmland has no irrigation and depends directly on this rain. Reservoirs, groundwater, hydropower and drinking water in summer also depend on it. When the season ends "deficient", the effects can show up as lower kharif crop output, higher food prices (inflation), weaker rural demand and pressure on water supply in the next summer. That is why the RBI, the Finance Ministry and markets track the "percentage of LPA" closely.

Commonly confused concepts

• LPA vs "normal" rainfall: The LPA is one number (about 87 cm for the country). "Normal" is a band around it (96% to 104% of LPA). Rain can be lower than the LPA and still be "normal", for example at 97%.
• Seasonal "deficient" vs district "deficient": For the whole country, "deficient" means below 90% of LPA (a shortfall of more than 10%). For a district or subdivision, "deficient" means a shortfall of 20% to 59%. Same word, different scale.
• "Below normal" vs "deficient": Both mean less rain than usual. Below normal is 90% to 95% of LPA; deficient is below 90%.
• Southwest monsoon vs northeast monsoon: The LPA of 87 cm is for the southwest monsoon (June to September). The northeast monsoon (October to December) mainly brings rain to Tamil Nadu, Puducherry, coastal Andhra Pradesh, Rayalaseema and South Interior Karnataka, and has its own separate LPA.
• Meteorological drought vs agricultural drought vs hydrological drought: Meteorological drought is a shortage of rainfall. Agricultural drought is when soil moisture is too low for crops. Hydrological drought is when rivers, reservoirs and groundwater fall low. One can happen without the others, for example if irrigation saves the crops.

Issues, criticism and the way forward

• An all-India average hides local pain: A "normal" national season can still have floods in some districts and severe shortage in others. Experts suggest giving more weight to district and subdivision data when planning relief.
• Timing matters, not only the total: A season can end close to normal because of heavy rain in a few days, after long dry spells that already harmed crops. The total percentage of LPA does not show this. Scientists suggest tracking dry spells, rainy days and extreme-rain days as well.
• A moving yardstick: Every time the base period changes, "normal" changes too. A season that was below normal under the old LPA may look normal under the new one. Comparisons across decades must use the same base.
• Forecast errors: Even with a ±4% or ±5% error, the forecast can sometimes miss. For example, in 2022 the actual rain (106% of LPA) was well above the April forecast (99%). The IMD has been investing in better models, more observation stations and higher-resolution forecasts, and now gives monthly and regional forecasts as well.
• Climate change: Warming is making heavy rain events more frequent while also lengthening dry spells in some regions. A single long-term average may not capture these changes, so experts call for regular updating and more detailed indicators.

Concepts to Know

• Monsoon: A seasonal change in wind direction that brings most of India's rain. From June to September, moist winds blow from the sea (southwest) over India; from October to December, winds blow from the land side (northeast).
• Base period: The set of years used to calculate an average. For the current LPA, it is 1971 to 2020.
• Meteorological subdivision: An area the IMD uses to report weather, usually a state or part of a state. India has 36 of them.
• Ensemble forecast: A forecast made by running several models (or one model many times) and combining the results. It is like asking several experts and taking their combined view.
• Kharif crops: Crops sown at the start of the monsoon (June–July) and harvested in autumn, such as rice, maize, cotton and soybean. They depend heavily on monsoon rain.