The weather office issues forecasts and summaries in terms of departures from the normal.
The weather office on Thursday reduced the benchmark to define normal Southwest monsoon rainfall to 868.6 mm from the previous 880.6 mm on the basis of availability of fresh data from its network of rain guages across the country.
The new all-India rainfall normal has been calculated on the basis of rainfall data over a 50-year period from 1971-2020 and will be used as the benchmark to measure rainfall in the country.
The India Meteorological Department (IMD) issues weather forecasts and summaries in terms of departures from the normal, which is a long period average (LPA) of rainfall received over a 50-year period. The 'normal' rainfall or the LPA is updated every 10 years.
The last update of the LPA was delayed and done only in 2018. Till then the weather office used the LPA of 1951-2001, which was 89 cm, as the benchmark to measure rainfall.
IMD Director General Mrutyunjay Mohapatra said the definition of the LPA has to be updated after every decade, as per the international practice.
"We take into consideration various aspects. One of them is the climate variability that changes over the period of time. Second, the number of rain gauge stations increases over the period of time giving us more data that is uniformly distributed. So, it (forecast) becomes more realistic and also caters to requirements of smaller regions and specific locations," he said.
The previous update had taken place in 2019, after a delay of seven years as gathering of data and analysing it took time, he said.
"We have been able to update the new normals within two years as we now have introduced automated processes of data reception, data delegation and calibration of instruments," Mohapatra said.
He said the new rainfall normal has been computed using rainfall data from 4,132 rain gauge stations well distributed over the country representing 703 districts of India.
Mohapatra attributed the gradual decrease in the average rainfall to natural multi-decadal epochal variability of dry and wet epochs of all India rainfall.
"Presently the south-west monsoon is passing through a dry epoch which started in the decade of 1971-80," he said.
According to Mohapatra, the decadal average of all India south-west monsoon rainfall for the 2011-20 decade is minus 3.8 per cent from the long-term mean.
"The next decade i.e. 2021-30 will come closer to normal and the south-west monsoon is likely to enter into the wet epoch from the decade of 2031-40," he said.
The new all India annual rainfall normal, based on the 1971-2021 data, has been fixed at 1160.1 mm compared to the earlier normal of 1176.9 mm based on the 1961-2010 data.
Southwest monsoon rainfall, spread across the months of June-September, contributes 74.9 per cent to the annual rainfall, while the pre-monsoon rains - March-April-May - contributes 11.3 per cent.
Post monsoon rainfall - October, November, December - contributes 10.4 per cent to the annual rainfall, while winter rains in January and February contribute 3.4 per cent to the yearly rainfall.
The new rainfall normal has been computed using rainfall data 4132 raingauge stations distributed across 703 districts of the country.
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