The Met Office issues forecasts and summaries in terms of departures from normal.
New Delhi:
The Met Office today introduced a new all-India rainfall normal – 868.6 mm – for the southwest monsoon based on data from 1971-2021, which will be used as a benchmark to measure rainfall over the country.
The new rainfall normal for the southwest monsoon season, rounded off at 87 cm, is a slight drop from the previous “normal” of 88 cm, the director of the India Meteorological Department (IMD), calculated based on the 1961–2010 rainfall data was done. General Mrityunjay Mohapatra said.
The IMD issues weather forecasts and summaries in terms of departure from normal, which is the Long Period Average (LPA) of rainfall received over a 50-year period. ‘Normal’ rainfall or LPA is updated every 10 years.
The last update of the LPA was delayed and was done only in 2018. By then the Met Office had used the 1951-2001 LPA, which was 89 cm, as the benchmark to measure rainfall.
Mr. Mohapatra attributed the gradual decrease in average rainfall to the natural multi-decade epoch-making variability of the dry and wet ages of all-India rainfall.
“At present, the southwest monsoon is passing through a dry age that started in the 1971-80s,” he said.
According to Mr. Mohapatra, the decadal average of all-India southwest monsoon rainfall for the decade 2011-20 is minus 3.8 per cent below the long-term average.
“The next decade i.e. 2021-30 will come closer to normal and the southwest monsoon is likely to enter a humid era from 2031-40s,” he said.
The All India Annual Rainfall Normal based on 1971-2021 data has been fixed at 1160.1 mm as against the earlier normal of 1176.9 mm based on 1961-2010 data.
The southwest monsoon rainfall, spread over the months of June-September, contributes to 74.9 per cent of the annual rainfall, while the pre-monsoon rainfall – March-April-May – contributes 11.3 per cent.
Post-monsoon rainfall – October, November, December – contributes 10.4 per cent to annual rainfall, while winter rains in January and February contribute 3.4 per cent of annual rainfall.
The new calculation of normal rainfall has been done using rainfall data of 4132 rain-gauge stations distributed in 703 districts of the country.
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