Long Period Average: The IMD yardstick for determining rainfall

Mains Paper 1 : Geographical Features & Their Location |

News

  • While releasing its monsoon forecast, the IMD expressed the projected rainfall in terms of Long Period Average (LPA), saying that it was expected to be 96% of LPA.

What is LPA of Monsoon

  • The LPA for the season is calculated on the basis of the mean rainfall during the four-month monsoon season over the 50-year period from 1951-2010.
  • It works out to an average of 89 cm for the country as a whole.
  • This is the average rainfall recorded during the months from June to September, calculated during the 50-year period.
  • It is kept as a benchmark while forecasting the quantitative rainfall for the monsoon season every year.
  • When IMD forecasts the category of rainfall, be it for country, region or month, the forecast is based on these standardised figures calculated for a period of 50 years.
  • As per the outputs obtained from the weather models, the rainfall is categorised as normal, below normal, or above normal.

Five Rainfall Distribution categories

  1. Normal or Near Normal: When per cent 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
  5. Excess: When departure of actual rainfall is more than 110% of LPA

Region-wise LPA

  • 83 cm for East and Northeast India,
  • 55 cm for Central India,
  • 61 cm for South Peninsular India, and
  • 50 for Northwest India, which put together, bring the all-India figure to 88.75 cm.
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