What is Long Period Average (LPA) of Rainfall?

About Long Period Average (LPA) of Rainfall:

• It is the rainfall recorded over a particular region for a given interval (like a month or season) averaged over a long period of time, like 30 years or 50 years. 
• The value is used as a benchmark while forecasting the quantitative rainfall for that region for a specific month or season. 
• IMD, which monitors the rainfall in the country, has prepared this rainfall normal based on the data for the period of 50 years and is updated periodically once every decade by incorporating the latest data from rain gauge stations. 
• The current LPAof all India south-west monsoon rainfall based on the average rainfall over the period 1971-2020 is 868 cm.
• 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.
• Similarly, at the national level, the annual LPA has been reduced from 117 cm to 116 cm.
• Why LPA is needed?
  • 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.
• 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 the 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 the departure of actual rainfall is less than 90% of LPA; and
  • Excess, when the departure of actual rainfall is more than 110% of LPA.

Q1: What is Monsoon?

A monsoon is a seasonal reversing wind accompanied by corresponding changes in precipitation and associated with annual latitudinal oscillation of the Intertropical Convergence Zone (ITCZ). ITCZ is the region that circles the Earth and where the trade winds of the Northern and Southern Hemispheres converge - creating a broad trough of low pressure. Seasonal shifts (between equator and tropic of cancer) in the location of the ITCZ with the apparent movement of the sun results in the wet and dry seasons of the tropics. Usually, the term monsoon is used to refer to the rainy phase of a seasonally changing pattern.

Source: Monsoon to be ‘above normal’, predicts IMD