Long-Period Average Rainfall

The long-period average (LPA) is the average rainfall received over a defined region and period during a long reference span. In India's monsoon assessment, it serves as the climatological benchmark against which a season's observed or forecast rainfall is compared, rather than representing the rainfall expected at every location.

How the benchmark is constructed

The IMD averages rainfall recorded over a specified geographical area and time interval, such as a month or the southwest monsoon season, across a long reference period. Thus, separate LPAs may exist for all-India rainfall, regions, months and seasons; the benchmark also changes when the reference period is updated.

• A multi-decadal average reduces the influence of exceptionally wet or dry individual years.
• An all-India LPA is an area-weighted national average, so it does not mean that every place normally receives the same rainfall.

Use in monsoon assessment

Observed or forecast rainfall is commonly stated as a percentage of LPA: rainfall as percentage of LPA = actual or forecast rainfall ÷ LPA × 100. Departure from LPA indicates the relative surplus or deficit against the climatological benchmark.

• Rainfall equal to 100 per cent of LPA matches the reference-period average for that area and interval.
• LPA provides a common base for classifying and comparing monsoon performance across years.

Interpretive limitations

An all-India seasonal value can conceal large differences in the spatial distribution and timing of rainfall. A season close to LPA may still contain prolonged dry spells, intense short-duration rain or serious regional deficits.

• LPA is a statistical baseline, not a forecast of rainfall in any particular year.
• Assessment of agricultural and hydrological effects also requires rainfall distribution, onset, withdrawal and intra-seasonal variation.