Monsoon Updates

What is the ‘Long Period Average’, IMD’s benchmark for monsoon prediction?


From UPSC perspective, the following things are important :

Prelims level : LPA, Indian Monsoon

Mains level : Not Much

India is likely to receive a normal monsoon for the fourth consecutive year, the India Meteorological Department (IMD) said in its first Long Range Forecast (LRF) for this year.

What is Long Period Average (LPA)?

  • The IMD predicts a “normal”, “below normal”, or “above normal” monsoon in relation to a benchmark “long period average” (LPA).
  • The LPA of rainfall is the rainfall recorded over a particular region for a given interval (like month or season) average over a long period like 30 years, 50 years, etc.
  • LPA refers to the average rainfall recorded from June to September for the entire country, the amount of rain that falls every year varies from region to region and from month to month.
  • The IMD’s prediction of a normal monsoon is based on the LPA of the 1971-2020 period, during which India received 87 cm of rain for the entire country on average.
  • It has in the past calculated the LPA at 88 cm for the 1961-2010 period, and at 89 cm for the period 1951-2000.

Why LPA is needed?

  • The IMD records rainfall data at more than 2,400 locations and 3,500 rain-gauge stations.
  • Because annual rainfall can vary greatly not just from region to region and from month to month, but also from year to year within a particular region or month.
  • An LPA is needed to smooth out trends so that a reasonably accurate prediction can be made.
  • A 50-year LPA covers for large variations in either direction caused by freak years of unusually high or low rainfall, as well as for the periodic drought years.
  • It also takes into account the increasingly common extreme weather events caused by climate change.

Range of normal rainfall

The IMD maintains five rainfall distribution categories on an all-India scale. These are:

  1. Normal or near normal, when the percentage departure of actual rainfall is +/-10% of LPA, that is, between 96-104% of LPA;
  2. Below normal, when departure of actual rainfall is less than 10% of LPA, that is 90-96% of LPA;
  3. Above normal, when actual rainfall is 104-110% of LPA;
  4. Deficient, when departure of actual rainfall is less than 90% of LPA; and
  5. Excess, when the departure of actual rainfall is more than 110% of LPA.

Also read:

Various terms related to Indian Monsoon


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