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Long Period Average

Context:

The country 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.

  • Rainfall in the four-month June-September southwest monsoon season was in the normal range in 2019, 2020, and 2021 as well.

Relevance:

GS I- Geographical Features & Their Location

Dimensions of the Article:

  1. About Long Period Average
  2. Why LPA is needed
  3. Range of normal rainfall

About Long Period Average

  • The IMD predicts a “normal”, “below normal”, or “above normal” monsoon in relation to a benchmark “long period average” (LPA).
  • According to the IMD, 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”.
  • Recently, The IMD’s prediction of a normal monsoon was based on the LPA of the 1971-2020 period, during which India received 87 cm of rain for the entire country on average.
  • The IMD 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 a result of events such as El Nino or La Nina), as well as for the periodic drought years and 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:

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

-Source: Indian Express

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