New Delhi: The weather office Thursday introduced a new all-India rainfall normal – 868.6 mm – for the south-west monsoon based on the data from 1971-2021, which will be used as the benchmark to measure rainfall in the country.
The new rainfall normal, rounded off at 87 cm for the southwest monsoon season, is a marginal decline from the previous “normal” of 88 cm which was calculated on the basis of rainfall data from 1961-2010, India Meteorological Department (IMD) Director General Mrutyunjay Mohapatra said here.
The 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.
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.
South West 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.
PTI