Long period average rainfall

Source-This post on Long period average rainfall is based on the article “Monsoon to be ‘above normal’, predicts IMD” published in “Hindustan Times” on 16th April 2024.

Why in the News?

India Meteorological Department (IMD) has predicted that India will have an ‘above normal’ monsoon season with 106% of the long-period average.

About Long period average rainfall

1. About: LPA refers to the average amount of rainfall recorded over a specific region during a given interval which can be a month or a season.

2. This average is calculated over an extended period, typically spanning 30 years or 50 years.

3. Purpose:

a) Since annual rainfall can significantly differ across regions, months, and even years within the same region or month, the Long Period Average (LPA) is essentially calculated to even out these fluctuations to enable relatively accurate predictions.

b) A 50-year LPA compensates for major deviations due to exceptional years of abnormally high or low rainfall, such as those caused by El Niño or La Niña events, recurring droughts etc.

4. IMD Categories: The IMD classifies rainfall deviations based on the LPA into the following categories:

a) Normal or Near Normal: Actual rainfall within +/-10% of LPA (96-104% of LPA).

b) Below Normal: Actual rainfall less than 10% of LPA (90-96% of LPA)

c) Above Normal: Actual rainfall between 104% and 110% of LPA.

d) Deficient: Actual rainfall less than 90% of LPA.

e) Excess: Actual rainfall more than 110% of LPA.

5. The current Long Period Average (LPA) for all India south-west monsoon rainfall, calculated from the 1971-2020 data, is 86.8 cm.

Previously, the India Meteorological Department (IMD) set the LPA at 88 cm based on the 1961-2010 data and at 89 cm for the 1951-2000 period. The national annual LPA has decreased from 117 cm to 116 cm.

UPSC Syllabus: Environment & Ecology