In Focus: Quantification of monsoon rainfall
Calculation of the ‘LPA Normal’
  • To categorize monsoon, the weather department benchmarks compares that season’s rainfall against a ‘normal’ derived from long period average (LPA) of 50 years.

  • IMD changes the ‘normal’ roughly every 10 years and it starts using it whenever the data of all the meteorological stations get updated using new 50-year period.

  • In late 2019, the LPA was updated, based on the average monsoon (June-September) rainfall over the country as a whole during 1961-2010 period, to better reflect the average rainfall in India.

  • Based on this data, the new 'normal' figure of LPA was arrived at 88 cm, indicating a decline in the average yearly rainfall in India.

    • Before that, till 2018 monsoon, the IMD used 'normal' of 89 cm based on during 1951-2000 period.



Categories of Indian Monsoon:
  • The status of the monsoon in India is arrived at from the amount of rain received when compared to the average ‘normal’ rainfall (LPA) during the June-September period.

  • The IMD has five categories to classify the ‘status’ of the monsoon:

    • Rainfall range of less than 90% of the LPA normal is considered ‘deficient’ (category-wise) monsoon

    • 90-96% of the LPA as ‘below normal’

    • 96-104% of the LPA as ‘normal’

    • 104-110% of the LPA as ‘above normal’ and

    • More than 110% as ‘excess’



 
News Summary:
  • This year, an average of 95.8cm of rain was recorded during the monsoon season in the country, as against the LPA of 88cm.

  • Thus, this year’s season ended with countrywide rainfall at 109% of LPA (9% above the long period average), officially categorized as 'above normal' monsoon.

  • This means, for the first time in 61 years, India recorded back-to-back “above normal” monsoon years. 

  • Monsoon rains this year were also the second highest in 26 years after 2019, when rainfall across the country was 10% above the LPA.

  • The last time India had two consecutive years of above normal monsoon was in 1958 (110% of LPA) and 1959 (114% of LPA).

 
Surplus rains in June, Aug & Sept
  • However, the distribution of seasonal rainfall during the June-September period was not uniform.

  • The country recorded the highest 127% of LPA rainfall in August while July was a deficit month with 90% of LPA. 

  • Though India had recorded deficit rainfall in July, the surplus rains in June, August and September helped in the country recording an all-time high acreage in crop sowing.

    • Based on it, the agriculture ministry has set a record target of 301 million tonnes of foodgrains for the 2020-21 crop year.



Monsoon retreat:
  • The monsoon started retreating from western parts of north-west India on September 28 against the normal withdrawal date of September 17.

  • As on 1st of October, the south-west monsoon had withdrawn from Punjab, western Himalayan region, Haryana, Chandigarh, Delhi and parts of Rajasthan and some parts of Uttar Pradesh.

 Environment & Ecology